Why will reducing the boarding time reduce the costs? There are quite a number of publications about airplane boarding which all usually begin with something like:

  Boarding is an expensive process that costs so much, that by reducing the boarding time airlines could save a lot of money.

However, none of the publications I've read says explicitly why the costs are reduced. Of course there is at least one obvious reason:

A shorter ground time reduces delays and thus the number of missed connecting flights, leading to fewer liabilities.

Further there is the possibility that a plane can do an additional flight during the day. But if a plane does 5 flights a day and saves 5 minutes per boarding, then the resulting 25 minutes would not suffice (at least in countries with start restrictions over the night).

So therefore the question: What factors play a role in saving money by reducing boarding time?

It's not as much boarding as much as standing around does not generate any revenue.

Use Your Capital Assets: 25 minutes a day is a lot, and in some cases is equivalent to a domestic leg. Rather than arriving at 23:00, you could arrive at 22:30 and still squeeze in an additional short flight before midnight. If you're flying 10 flights a day and you can now fly 11, you're doing 10% more work with the same amount of assets. Estimates for a flight hour of a B737 are ≅ \$3000-\$3500 in the US, for an aircraft worth perhaps $60 million.
Fixed Costs: You're looking at a bit of fixed costs, such as insurance. The more flying time you can squeeze out of the plane, the cheaper each hour will become on average.
Less Staff, More Product: You need a certain number of ground staff. If you can cut the time required for an aircraft to be handled by ground staff by 20% (50 → 40 minutes), that potentially means that they could handle an additional aircraft in their shift. This principle applies for airline flight crew, as well. It's a lot cheaper to let the your pilot fly a few hours more than needing to hire another one.
Standing around is not always cheap either: Depending on airport cost/fee structure, you may be charged less for the ground time. This is not only standing around; for instance, Munich airport charges you €44 for every 30 minutes of using a ground power unit.


Why does afterburner exhaust have "pulses"? Instead of being consistently-dense, the afterburner exhaust looks "pulsed" in these pictures below.  Is this a trick of the camera, a natural result of interaction with the air, or is there some design science in the propulsion system that causes this effect?

Those are known as "Mach diamonds" or "Shock diamonds" (and a number of other names) and are not a characteristic of the afterburner per se but are formed by standing waves in the exhaust when the pressure of the expanded exhaust gases do not exactly match external ambient pressure. They can be formed by either over-expansion or under-expansion of the exhaust gases. The diamonds form at the transition from sonic to supersonic flow which occurs immediately on the "downstream" side of a rocket or jet engine nozzle.
Photo from the link below:
SR-71 Blackbird taking off.

In rockets, flow through such a nozzle is always sonic at the throat (in all except trivially low pressure systems where it can be subsonic - but these are not usually encountered in practice.
In jet engines, whether flow is sonic or supersonic at the exhaust depends on the design which in turn tends to depend on the application. "Pure jet" engines which use a combustion driven compressor to compress input air and which develop their thrust from the jet exhaust will usually have supersonic exhausts. Ongoing development has led to a range of "jet" engine types with somewhat differing capabilities - turbojet ("pure" jet), turbofan (jet driven internal "fan" produces air which partially bypasses jet proper), turboprop (jet drives external propeller),propfan (turboprop on steroids optimised for close to Mach 1 cruise) and some fellow travelers such as turboshaft - like a turboprop but with shaft output - useful if you want to power eg a helicopter.
In a turbo-fan engine the amount of air which bypasses the jet engine proper is governed by the "bypass ratio". High bypass engines send most air past the jet proper to be combined with the jet exhaust and form the actual engine exhaust. Pure jet or some "low bypass ratio" turbo fan engines may have supersonic exhausts while "high bypass ratio" turbofan are always subsonic. Modern long distance aircraft (especially airliners) which aim for maximum fuel economy tend to have high bypass ratios. As Mach diamonds are only associated with trans-sonic operation you will not see Mach diamonds from eg a 747.
So, low bypass fanjets may have supersonic exhausts for part of their operation. A turbojet may be sub or super sonic and afterburner will almost always (but not necessarily in theory) produce supersonic outlet flow.
In systems which do have supersonic exit exhaust velocities, as the flow changes from sonic at the nozzle throat to supersonic in the expansion system the aim is to make the exit pressure exactly match ambient. "Exactly" never exactly happens in practice. Where the mismatch occurs there will be a shockwave - an "impedance mismatch in another perspective - and the visible evidence of this are the Mach Diamonds.
As both exhaust pressure and ambient pressure change with conditions there will essentially always be some degree of such shock waves present in "qualifying" systems, but more so when the engine operation departs more significantly from the ideal. Smaller versions may not be obvious without careful examination. As the overall system is usually optimised for "normal" conditions, afterburner operation is more likely to cause a major pressure mismatch as it deviates from normal use.
An exception would occur if the designers decided they wanted absolutely maximum performance from afterburner under given conditions (so that eg an interceptor could achieve absolute maximum velocity when needed) - in which case the Mach diamonds could be minimised on afterburner and would be more pronounced in normal operation.

See this page for some good photos and the diagrams below -

Aviation Aerospace - Shock Diamonds and Mach Disks

Below: creation of Mach Diamonds from under-expanded flow.

Below: - creation of Mach Diamonds from over-expanded flow.

Good explanation with pictures here - Wikipedia - Shock Diamond
Numerous relevant images - each linked to a related web page.

Related:
Wikipedia - Jet Engine excellent.
He178 0- the very first Jet aircraft
NASA visual Turbojet simulation


What is a blind encoder? As I'm studying, I found that GPS has to have 5 satellites or 4 satellites with "blind encoders" to use RAIM.

What is a blind encoder and what is its purpose?

A blind encoder is an altimeter that encodes pressure altitude and outputs it on a databus. They are "blind" in that they do not display data to the pilot. Most general aviation units use a parallel bus with Gillham encoding. More expensive units and air data computers use serial bus output, commonly ARINC 429.

Historically, the blind encoder existed to provide pressure altitude data to the transponder to enable Mode C reporting.

When it was proposed to use GPS as a primary means of navigation in the early 1990's, the FAA deemed that a means of ensuring the integrity of the signal was essential. The longer term solution was to establish WAAS which provided both augmentation and an integrity channel. The short term solution was to have the receivers include an internal monitor, which took on the name RAIM for Receiver Autonomous Integrity Monitor.

RAIM algorithms have evolved several times over the years. Since early GPS receivers were often single or 3 channel receivers, the use of altitude as a substitute for an additional satellite was common. (Altitude is just another range in the rho-rho computation.)

Later GPS receivers moved to 6 and 12 channel receivers, thus enabling RAIM algorithms that don't need altitude data except in extreme cases.

So yes, the number of satellites needed for a RAIM calculation can be reduced by one when pressure altitude is available from a blind encoder.


How much impact can be taken when flying in formation? This recently published 360-degree video shows four F/A-18 Hornets performing impressive maneuvers what appears to be only a few inches apart.

Considering the risk of impact I find it hard to believe that a very slight contact would lead to immediate disaster.

Is this true? Assuming no damage is caused to the aircraft, how much impact can be handled (somewhat) safely?

@Lnafziger is correct that absolutely ZERO contact between aircraft flying in formation is acceptable, with Aerial Refueling (AR) being the sole exception to this -- but in that case the AR equipment is designed to allow for what's going on.

The problem with contact IS the significant risk of damage to the aircraft involved, so the assumption in "assuming no damage is caused, how much impact..." is outside of typical experience.  IF you could assure no damage, then all sorts of things might be possible, but even pretty stoutly built fighter jets still aren't "bumper cars" and shouldn't be treated as such.  Even if a collision didn't cause an immediate structural failure, the risk of damage to the internal structure of the wing/tail/whatever would be enough that the aircraft would have to be taken apart & xray'd (or another non-destructive inspection method) to ensure that there wasn't damage that would later result in structural failure.

The one exception to all of this that I can recall was a technique that the RAF used in WW II to defeat the V-1 "buzz bomb" cruise missiles.  While the Spitfires could shoot them down, the small size of the missile meant that you had to get so close that you were probably flying through a cloud of debris right after you killed it (i.e. the warhead would detonate and the shrapnel might bring down the Spitfire as well).  They discovered that the gyroscopes in the V-1 guidance (i.e. the primitive autopilot) would tumble if they were inverted, so some pilots would fly formation with the V-1 then intentionally make contact with its wing with their own, flipping the buzz bomb onto its back.  At this point, the autopilot lost its bearings and the V-1 went down into (presumably!) empty countryside, crashing far from its London target.

Other than this, formation flight is carefully planned out so as to minimize all risk of contact between the aircraft.  Even in the (very cool!) video of the Blue Angels, you can see that there is always space between the wingtips -- more space for the more dynamic maneuvers, with the least space during a fairly brief and reasonably stable pass.  Remember, these pilots are extremely good & highly experienced in this sort of formation flying, and while there IS oscillation in pitch & roll, the formation is designed so that this can safely go on.  There just isn't much risk there of a force that would move one aircraft directly to the left or right of its position. Turbulence might produce a rolling or pitching motion that could LEAD TO a collision, but the pilots have enough control authority to correct these sorts of forces.

That's why they're able to do what they do, safely.

Fly Navy!


Which commercial aircraft can fly Micro-SLOP procedures? "Strategic Lateral Offset Procedures (SLOP) are approved procedures
  that allow aircraft to fly on a parallel track to the right of the
  centre line relative to the direction of flight to mitigate the
  lateral overlap probability due to increased navigation accuracy and
  wake turbulence encounters."

ICAO 4444 Pans ATM 16th Edition section 16.5 Note 1.

SLOP of one or two nautical miles has been recommended for use in oceanic and
remote continental airspace for a long time. For example, the North Atlantic Track (NAT) system has permitted SLOP since 2004 and has been a mandatory requirement to fly SLOP over the North Atlantic since 2017.

In the 16th Edition of ICAO 4444, published in 2015, ICAO extended the use of
SLOP to en-route airspace in tenths of a nautical mile up to a maximum of 0.5NM,
i.e. Micro-SLOP.

This change was originally proposed in ICAO Circular 331 to counter the effect of the navigation paradox after the mid air collision over Brazil in 2006, see Gol Transportes Aéreos Flight 1907.

Currently, Micro-SLOP may only be applied on routes promulgated in AIPs by aircraft equipped with the appropriate automatic offset tracking capability.

Most ANSPs do not permit Micro-SLOP in their AIPs. However, the NAT recommendations have recently changed to recommend that pilots fly Micro-SLOP over the North Atlantic, see Two is not enough.

My pilot friends tell me that small commercial aircraft fitted with the Universal Avionics SCN 1000 and 1100 FMS's can fly 0.1NM offsets but that Airbus A320's can only fly offsets in 1NM increments.

The question is: which commercial aircraft currently have the capability to fly Micro-SLOP procedures with route offsets of only 0.1NM?

According to the below references, these commercial aircraft are capable of doing the Micro-SLOP procedures with 0.1 NM increments:

INTERNATIONAL FEDERATION OF AIR TRAFFIC CONTROLLERS' ASSOCIATIONS 58TH ANNUAL CONFERENCE – Conchal, Costa Rica, 20-24 May 2019

  2.6.2 Most flight management systems in service today can execute an automatic offset track in whole nautical miles. Unfortunately, as offsetting by tenths of a nautical mile was only introduced in ICAO provisions in 2014, most flight management systems are currently incapable of executing such offsets. Of the major aircraft and flight management system combinations in production, only the Boeing BBJ, 737 and 787 are currently capable of offsetting in tenths of a nautical mile. Depending on the aircraft mix in the airspace, even if a State permits offsetting in tenths of a nautical mile, the majority of aircraft may not be able to participate in this SLOP, and the full benefits may not be realised.

Cir 331 A /192N Implementation of Strategic Lateral Offset Procedures ICAO

  1.3.8 The idea quickly emerged to employ the capability of some modern aircraft to offset in fractions of a mile for this purpose. Both Boeing and Airbus announced that all their future aircraft would have the capability to offset in tenths of a mile and it was therefore clear that using such “micro-offsets” to mitigate the collision risk would be viable in the future.

IFALPA (International Federation of Air Line Pilots’ Associations) Saftey Bulletin (NAT Lateral ASEPS)

  Micro-SLOP is currently authorized throughout the NAT region. The latest information available to ALPA is that the most recent Boeing B737s (NG, Max), B787s, and the B777X (when it enters service) have Micro-SLOP capability. Airbus is planning an enhancement across their product offerings in the 2022+ timeframe. There is no hazard introduced by this mixed implementation of Micro-SLOP.


What are the restrictions preventing more intelligent autopilot systems? Current autopilot system is integrated with Flight Management System (FMS) for navigation such as LNAV and VNAV; both of them are based on preset parameters given by the pilot prior to departure (IRS alignment, total fuel/pax, waypoints/airways, flight level, etc) or in short, expert system

What are the restrictions for intelligent system autopilot in which the autopilot shall assist the pilot for making decision (or the autopilot decides per se)? For example, during flight under certain condition where some sensors are failed and flight control system reverts back to direct law (in case of FBW) and the intelligent system autopilot recognizes this case so it helps pilot to decide what to do or controls the aircraft thoroughly

Under the FAA there are not really regulations that limit the ability of an autopilot and shy of takeoff most modern systems are capable of performing most of a full flight with no pilot input (aside from route updates etc). Ultimately any aircraft system will have to get certified and this involves rigorous testing as well as demonstrations the system works under all stated cases.

There is however a bit of a logic issue in your statement:

  For example, during flight under certain condition where some sensors
  are failed and flight control system reverts back to direct law (in
  case of FBW) and the intelligent system autopilot recognizes this case
  so it helps pilot to decide what to do or controls the aircraft
  thoroughly

The reason the autopilot disengages or disconnects is because, due to the sensor failure, it no longer has the information it needs to properly control the aircraft. No matter how smart the system is, without proper sensor input, positive control is not possible or would be logically unsound. This is even true for human pilots and was in part the issue that lead to the Air France 447 disaster.

There is a broader question here of autopilots potentially handling emergencies which is a bit different than a sensor failure (read pitot static failure). This is already in place in part on some aircraft and as time moves we see more and more of it. Aircraft are now capable of detecting a depressurization and automatically descending to a safe altitude; even new small GA planes have this feature. Autopilots also now have forms of automatic level mode which will bring the aircraft back to a straight and level attitude at the push of a button.

This also comes down in part to design philosophy. This is highly opinionated so I'll stay away from it on this site, but there is hard data to support both cases: in the event of an emergency, is a pilot or computer system better equipped to handle the situation?


Why are aircraft designed so that the wing stalls before the tail? I read somewhere that conventional aircraft are designed so that the main wing always stalls before the tailplane. Why is that?

My concern is that if the wing stalls, suddenly the tail will push the aircraft into a higher AoA, and maybe stall itself, though the pilot would still have control for some time to avoid that. On the other hand, if the tail stalls, the plane will (violently?) pitch down, but would be able to recover afterward.

So what am I missing?

Edit: Assume the center of gravity of the plane is in front of the main wing

You are missing the weight force. The wing should stall first because then it will produce less lift and the weight will make the aircraft pitch down.

In attached flow, the lift from wing and tail is balanced such that the combined resulting force is acting exactly at the longitudinal position of the center of gravity. If the wing stalls, the balance of lift is shifted backwards (regardless of the tail producing lift or a downforce), because now the forward part of the wing-tail-combination will produce proportionally less lift than it did in attached flow. The center of gravity will then be ahead of the resulting lift force and will pull the nose of the aircraft down.

A tail stall is bad news for the pilot and should be avoided:

A stalled tail will have much less elevator effectivity, reducing the control power available to the pilot
A stalled tail at high angle of attack is stalling at maximum lift, so it produced positive lift before. If it stalls, its lift will shrink and make the aircraft pitch up.

In a conventional configuration the tail flies in the flow field of the wing. Because the wing's downwash increases with angle of attack, the angle of attack variations at the tail are reduced. This helps to keep the flow at the tail attached over the whole useable angle of attack range of the wing. A tail stall is either caused by a wrong location of the center of gravity (CG) or by a very poor transsonic design which causes shock-induced stalling of the tail when the wing is still doing fine. Very rarely can you stall the tail by a wrong trim setting when a moveable stabilizer is used for trimming, or in a deep stall.

Some supersonic aircraft compensate for the backward shift of the center of lift in supersonic flow by pumping fuel from forward to rear. If an airplane with such a rear CG location slows down to subsonic speed, it will need to produce proportionally more lift with the tail than with the wing. If it now pitches up (say, for a tight turn) it will risk a tail stall. Note that such a configuration is aerodynamically unstable at subsonic speed.

EDIT:

Now you define the center of gravity as ahead of the wing, which makes for a very stable configuration. The tail is producing a downforce to compensate the pitch-down action of the wing's lift.

If the wing stalls first, lift will become less than weight and the aircraft will accelerate downwards. Now the angle of attack will increase at both the tail and the wing; at the tail even for two reasons:

The downward acceleration will change the local flow direction to a larger angle of attack, and
The diminished downwash from the wing will cause an angle of attack increase of its own at the tail. This effect takes a little longer to manifest itself: The delay is the distance between the quarter points of tail and wing divided by the airspeed.

Both effects will quickly create a dominant pitch-down moment for the aircraft because the tail will contribute much less downforce than before the stall, and the diminished pitch-down moment of the wing will become insignificant.


Does the FAA allow a jet bridge to approach an aircraft that has engine #2 running? Does the FAA allow a jet bridge to approach an aircraft that has engine #2 running?

On a two engine airplane, Engine #2 will be on the opposite side from the jetway, and keeping the engine running while the bridge is connected on the other side is not an issue.

(see related question: How are engines numbered?)

On a 4 engine airplane, Engine #2 will be directly behind the jet bridge, and should be shut down before the bridge is extended.  This is for the safety of the engine, to avoid ingesting any debris from the jet bridge, or debris dropped by people, rather than for the safety of the people (who are unlikely to be sucked in by an engine at low power).


Is it possible to estimate airspeed on top and bottom sides of a wing at zero angle of attack? Assuming a flat bottom wing moving through the air at say 50km/h and at zero degree angle of attack, what will be the likely airspeed above and below the wing?

I know this will be affected by camber and maybe wing chord too. But is there like a proportional estimate for specific wing camber?

I haven't got much knowledge about wing maths, but a formula would be appreciated.

Applying Bernoulli's equation requires the relative airflow above and below the wing. The things is, I'm trying to experiment with the fact that given atmospheric pressure, the two different airflows, and the wing area, one can actually determine the pressure difference between the top and bottom surface using Bernoulli's equation and also the total force holding the plane up by getting the product of the pressure difference and the wing area. So what I'm trying to do is estimate the lift force on my theoretical airplane model at a particular wind speed. I want to use this to estimate the minimum airspeed required for normal flight 'without flaps'.

The short answer is that it is possible, by way of knowing the pressure distribution and applying the Bernoulli equation to relate the pressure and the velocity.

More specifically to your question, I'm not aware of any published correlation between camber and air velocity (nor do I think it'd have much practical application), but you could come up with your own by running a variety of geometries through XFOIL for a given airfoil geometry (maybe a Clark Y of varying camber for a flat-bottomed design) and applying Bernoulli on the calculated pressure distribution for whatever freestream atmospheric conditions and chord length you want.

Another note: You asked specifically about a wing, not an airfoil. A wing will have lots of other effects contributing to variations in velocity over its surface; there is no easy way to account for all of them.


How frequently are airline pilots switching between aircraft types (e.g. Airbus, Boeing, etc)? Are pilots at Airlines typically assigned to fly a single Type in a given month / year or do they get switched routinely between multiple types of aircraft as per scheduling convenience?

i.e. Is it common to, say, fly a B-747 & then return flying a B-777 etc.

How about more extreme changes like switching between Boeings & Airbuses or Turbojet / Turboprop etc.?

I guess that begs the question whether pilots typically are certified & current across multiple Types? How common is this?

I'm looking more for comments about what is operating practice across airlines rather than a hard rule, which I guess does not exist.

Airline pilots are typically only certified and current on one type. It is rare for a pilot to be certified and current on multiple types because they must do re-current training and check rides for each type in order to remain current and legal. The extra cost for training and check rides would not make financial sense to an airline trying to be competitive with other airlines.


Why is there no Maltese Cross in this non-precision approach chart which has a FAF? I am wondering why this chart does not show the FAF with a maltese cross as it is used in non-precision approaches to depict FAF.
The chart’s briefing strip refers to D6.0 as the FAF and on the plan view FD23 code can be seen(which also indicates that this the FAF).

It could be a charting issue.  It is also the same for the NDB Z Rwy 23 approach into LTAY.    Feel free to ask Jeppesen and they will tell you if the chart is in error.

https://support.jeppesen.com/


Are 0g pushovers in a C172 safe/legal? I see YouTube pilots do it all the time, but I’m wondering how safe this maneuver really is given the fuel is a gravity fed system. Also, is it legal?

When doing "bunts", it's about the oil, not the fuel. The airframe doesn't care; it's in a ballistic arc and is completely unloaded, like some trash you launched into a neighbor's yard with your Yard-A-Pult.
Overall, the main risk is of a temporary drop in engine oil pressure when oil in the sump turns to globs and floats around, potentially unporting the oil pump suction pickup at the bottom of the sump.
The effects of this are less severe in a carbureted engine because the engine will also quit temporarily while this is happening (the carb will flood), and without the torque related forces acting on the main bearings, it's unlikely to do damage to the main bearings for the couple of seconds you can sustain the bunt.
Oil pressure loss is more likely to happen if you push too hard and go negative G. All the garbage that was on the floor, and is now on the ceiling of the cabin, also tells you that the oil in the engine is up against the top of the sump and running backwards through the drain path into the main crankcase, and the oil pickup is sucking air for sure.
If the engine is fuel injected, the risk of doing real bearing damage is higher; it'll still produce power during the time you can sustain the maneuver (until fuel in the lines is consumed if the fuel in the tanks unports, which will take longer than you can sustain the bunt) and if the oil pressure drops while it's making power, that's a very bad thing, as it's likely the main bearings will achieve metal to metal contact, and it's downhill from there.
If I was an FBO who learned that my renters were doing zero G bunts in my planes, I'd probably be not too happy about it.


What is the procedure to replace carpets in airliners? Are all the seats removed, as well as trim and baseboard, before the carpet is replaced? If so, how is reinstalling the seats laid out; do they feel for the boltholes? Are the seats bolted on top of the carpet or directly to the floor?

The seats are installed on rails, so they can be easily removed (or moved to a different seat pitch). The bolts are in plain sight (random source for example photo).

The carpets are laid around the rails. I suspect the carpet does not protrude underneath the base board, so they'd only have to remove the seats to get the carpet out.

This is the inside of an airliner with the seats and carpet removed:


How to fly LOC approach when IAF is a specific DME from LOC, but LOC has no DME? I was looking at this approach plate and noticed that SNOWL is an initial approach fix, and that it's charted to be 6.4 NM from the localizer I-IWA.

The only problem with starting my approach from the northwest and crossing over SNOWL is that the localizer does not have DME.

First Question:

How can I fly this approach assuming that I'm coming in from the northwest and want to start from the initial approach fix SNOWL?

Second Question:

If SNOWL is 6.4 NM from, why does the profile view show that it's 5.8 (1.9 + 3.9) NM away from the localizer?

You have a few different points here but the short version is: IWA is a VORTAC and I-IWA is a localizer, i.e. they're different navaids. DME is available from IWA and it's optional but not required for the I-IWA localizer approach. If you do have it, you can use it to identify the ORIYE fix and use the lower minimums at the bottom of the plate, e.g. 1800-1 for category A. If you can't identify ORIYE, you can only go to the regular minimums of 1940-1.

First, I suspect you're confusing I-IWA (a localizer) with IWA (a VORTAC) but they're different navaids that use different frequencies, as shown on the chart. The I-IWA localizer provides lateral and vertical (glideslope) guidance but not DME, which you get from the IWA VORTAC. If you don't have a DME receiver, you could use an IFR-certified GPS instead.

Second, if I understood your question correctly about approaching from the northwest, you're asking how to identify SNOWL without using DME. SNOWL is an intersection ("SNOWL INT") and you can identify it using VOR only, by finding the intersection of any two of these radials:

IWA radial 122
PXR radial 109
TFD radial 028

Third, as far as I can see from the plate, DME is the only way to identify ORIYE (remember that you could use GPS instead of an actual DME unit). When you look at the minimums at the bottom of the plate you can see that the regular category A localizer minimums are 1940-1 but there's an extra set of minimums that you can use if you can identify ORIYE and they let you get lower, to 1800-1. In other words, if you have DME then you can use it to identify ORIYE; if you don't, you ignore ORIYE. Either way, you still have to use the correct set of minimums.

Finally, SNOWL is 6.4 DME from the IWA VORTAC, but only 5.8 DME from the "airport profile" (see the chart legend), i.e. the runway. That means, the runway end is 0.6 DME from IWA (that distance is also shown on the profile).


Velocity induced by a straight vortex filament Consider a straight vortex filament as shown below. At each point, there's a point vortex of strength $\it T$. Consider that a point $P$ is there on the outermost circle of flow induced by the vortex about the origin $O$. That is the point $P$ lies in the plane containing the flow induced by the vortex at $O$. So, the only point on the vortex filament that will induce a flow at point $P$ is $O$.
Therefore, the vortex at point $O^{'}$ will not induce any flow at point $P$. Vortex at point $O^{'}$ can only induce flow at those points that are in the plane containing the flow induced by the vortex at $O^{'}$.

So, my question is how Biot-Savart Law is applied for a straight filament vortex? How can the segment $dl$ induce a flow at a point $P$ that is not in the plane? Only the vortex centered at the point of intersection of line and perpendicular from point $P$ to the line can induce the flow at $P$. Please clear my query.

You need to think in 3D. A vortex will induce a flow speed everywhere.

That is the point  lies in the plane containing the flow induced by the vortex at

Why would you think that vorticity induces a speed only in one plane? You start from the wrong presupposition.
Now consider the rings around the vortex filament in your first drawing. All around those rings a flow speed is induced. Its magnitude drops with the inverse of the radius of one particular ring but is constant along the circumference of that ring. Actually, that ring is only a section through an infinitesimal tube, just as infinitesimal as that vortex filament.

Irrotational vortex. © Silver Spoon, picture source.
Also, the symbol for vortex strength is a capital Greek letter Gamma ($\Gamma$), not $T$.


Did any Boeing 787 operator achieve the 20% fuel efficiency improvement over the 767? Is there any airline which are able to get the complete 20% fuel efficiency on their 787 with respect to 767 it is going to replace, as told by Boeing.

I have read that one of the early customer like Air India are not able to get even 15% of fuel efficiency on their Dreamliners.

You have not defined your benchmark/reference value, but a three-year-old article suggests this was possible depending on definition even at a very early stage for some airlines:

  ANA earlier this month was the first to reveal that its aircraft were producing around 21% lower fuel burn on international flights compared to the 767-300ER that the 787 is designed to replace.

They also say however:

  After its first six months of service, ANA says the 787 efficiency levels are slightly up on the 20% savings originally expected at the beginning of the program. On domestic routes, the saving is 15-20%, which meets expectations, say ANA officials.


What is the working principle of g-suits? Pilots since world wars have been wearing g-suits in fighter planes to minimize the effect of high g encountered by them during close maneuvers.

How do g-suits help in minimizing the effect of high g's on the pilots?
What is the maximum g that a g-suit could handle properly?

How do g-suits help in minimizing the effect of high g's on the pilots?

G-suit create extra external pressure on the limbs (legs in particular) to prevent the blood to stagnate far from the heart. In normal conditions specific biological systems automatically return the blood from the extremities to the heart, but when subjected to high G loads these systems fail, as they are not designed for such conditions.

By creating pressure on the body the G-suit alleviates the G load perceived by the body.

  What is the maximum g that a g-suit could handle properly?

The main limitation is not only the amount of Gs, but for how long the human body can sustain them (spoiler alert: not much)

Image source

As you can see the Gs usually experienced in a fighter aircraft (blue line, see diagram on top right) cannot be sustained much beyond 3 seconds if they are of even moderate intensity. The G-suits help the body resist slightly longer.


Can an aircraft continue flying and land safely following an engine failure? Is it possible that an aircraft could continue to fly and then land safely on a single engine if another stopped working?

The answer to your question strongly depends on what kind of airplane you're talking about (in particular, what set of rules it was certificated under), as well as the phase of flight you are in (takeoff vs. cruise vs. approach/landing) and whether you are in a jet or a (turbine or piston) propeller aircraft.  (I'm going to exclude single-engine airplanes from this discussion, as well.)

Cruise Flight

In general, an engine failure in cruise flight is a manageable emergency unless you are in extremely high terrain in a light twin or some other corner case.  The general procedure is to "drift down" in a slow-ish descent to your single-engine cruise altitude -- if you don't already have a driftdown airspeed worked out, you can use Vyse as a substitute to get a slow driftdown -- and then divert to a nearby suitable airport to get the busted engine looked at, as you'll be flying lower and less fuel-efficiently as a result.

Takeoff

Takeoff is the most troublesome time for an engine failure to happen, bar none: you are at high gross weight and don't have altitude under you to turn into airspeed and landing options.  What you want to do is get to the pattern altitude, fly a pattern, and land in a reasonably quick, but orderly fashion -- you don't need to ultra-hurry things even if you have an engine on fire, but you do want to get back on the ground in a reasonable timeframe to get things looked at.

Light Twins (cabin class, commuter, pre-FAR 25 airliners)

These aircraft, unfortunately, struggle in single-engine situations on the best of days, as the certification requirements for single-engine performance in FAR 23 are minimal at best.  The common adage you will hear about light twins, even, is "the second engine will carry you to the scene of the crash" -- it is possible for a light twin to be type certificated with nil or negative worst case single-engine climb performance in the case of an engine failure at or shortly after takeoff.

Business Jets (certificated under FAR 23)

Corporate or business-type light jet aircraft (Citations, Gulfstreams, LearJets, Beechjets, etal) and their turboprop counterparts (such as KingAirs), while often certificated under Part 23, will generally have superior single-engine performance to light twins, although this is by no means guaranteed.  You'll likely get up into the pattern and be able to make a reasonably normal single-engine landing, barring any aggravating factors at least; this is because even the smallest turbine engines are large enough that the aircraft will have some degree of excess thrust as a twin, leaving it with a thrust reserve in a single engine situation.  These aircraft are also flown by pilots with type ratings and a CPL or ATPL, which means they will be more familiar with the performance of that specific aircraft and any aircraft-specific engine-out procedures needed to eke out maximum single-engine climb rates.

Twin-Engine Airliners (transport class twin turboprops and twin jets certified under FAR 25)

The FAR 25 requirements for engine-out takeoff performance are much more stringent than FAR 23, and practically guarantee that the aircraft is capable of positive climb in a V1 cut (engine failure right after decision speed) at maximum takeoff weight.  In addition, V1 cuts are a classic airline training scenario that every airline flight crew on the planet runs in the sim on a regular basis (I wouldn't be surprised if a V1 cut came up every few sim sessions or so, if not every sim session).

In a practical sense, this means that unless the aircraft is overloaded or is suffering from something beyond a simple engine failure (I'll talk about that later), an airline flight crew will handle engine failure on takeoff in an airliner as a routine event.

3 and 4 engine airliners

A single engine failure on a three or four engine aircraft is by no means critical, especially on three- and four-engined jet transports.  This doesn't mean that 747 crews don't practice V1 cuts any longer, though!

Props vs Jets

Engine failure in a jet, whether it be a T-37 or an A380, is simple: you put appropriate rudder in, ID which engine died (between your instruments and "dead foot, dead engine", you should have enough data to go on), add power as needed on the working engine (say, if you are doing an assumed/FLEX temp takeoff), and kill the dead engine, then run the rest of the checklist at a leisurely pace now that safe flight is more-or-less assured.

Propeller aircraft are a bit more complicated though in that you need to feather the dead engine's propeller to keep it from dragging the airplane down -- older constant-speed prop designs such as those on warbirds and piston-engine airliners had FEATHER buttons for this job, one per engine; newer aircraft with constant-speed props have a gate on either the power or the condition levers that serves in place of the feathering button.  You also have more levers to manipulate when adding power to the working engine(s) and killing the dead engine, and may have more difficulty figuring out which engine died without positive flight control responses (I can only imagine how baffling a center engine failure on a Trislander would be!).

As a result of the critical nature of the feathering process, modern (and even some not-so-modern) transport-class (and some other) propeller aircraft have an autofeather or NTS (Negative Torque Sensing) system that will automatically drive the blades of a propeller towards (or even all the way to) the feathered position if it is armed and senses that the propeller is trying to drive the engine.  However, feathering the propeller on an engine that isn't shut down is also a bad idea as it causes the propeller and engine to overspeed due to the airloads the engine was driving going away.

Of course, if you are in a light twin with fixed-pitch propellers, none of this feathering talk applies.  Be prepared for the unexpected emission of foul language during engine-out procedures, though!

Approach/Landing

An engine failure during the early phases of approach is generally unproblematic -- it's similar to a failure at cruise, just minus the driftdown.  However, an engine failure on short final can be troublesome due to the high workload of an approach, combined with the thrust asymmetry induced during a full-power, single-engine go-around.  Prompt rudder inputs are essential in order to maintain aircraft track and control during such a maneuver, and engine failure checklists are best deferred until stable flight.

Engine failures during the landing roll are rare, by the way, and typically a non-event.

Worse than Engine Failure

There are engine damage scenarios that represent a greater hazard to the aircraft than a simple engine failure, or even a fire in an airliner engine.  These fall into three categories: uncontained turbine engine failures, engine detachments, and propeller failures.

Uncontained Turbine Engine Failure

Turbine engine failures are divided into two types: "contained" failures where the busted parts fly out the tailpipe, and "uncontained" failures where a turbine or compressor disk basically explodes radially, sending parts flying in a 360° radius around the engine; at this point, further damage is a game of aerial roulette, and can range from a few holes in the cowl to near-catastrophic levels of damage that pose a threat to the entire aircraft.

Engine Detachment

Engines, even when pod-mounted, rarely fall off airplanes, and much of the time when they do, they depart quietly, a yanked-back thrust lever and nil engine readings the only signs of their absence visible to the flight crew.  However, a detached engine is somewhat unpredictable in its behavior, and can strike another engine on the same wing or another part of the aircraft, leading to further damage that can compromise aircraft systems or impair the operation of that neighboring engine to the point where the thrust asymmetry becomes uncontrollable.  Nacelle-mounted engines are actually worse in this regard -- a nacelle-mounted engine departing its nacelle leaves behind a big, draggy air scoop that will severely hamper single-engine flying, and on a light twin, this is simply insult atop injury as you have gone from "barely making it to your site of choice" to "we need to pick the nearest open spot NOW because we're trying to fall out of the sky in a hurry"

Propeller failure

Propellers do not fail nearly as often as engines do, but when a propeller fails in flight, the results can be catastrophic; an overspeeding prop can fly apart violently or rip its mated engine apart while producing asymmetric forces that have the potential to overwhelm all flight control, while the inability to feather a failed engine's propeller turns it into a drag brake on that side of the aircraft, crippling any attempt to obtain reasonable single-engine climb performance.


Do you need type ratings for private flying? This question popped into my head when I learned some Citation X series have shared (?) type ratings.

So a theoretical situation:

I am a happy person with PPL and IFR ratings and then I decide to rent a 737-800 to give a birthday party to a close friend.

Can I just charter a private jet (assuming I am lucky enough to find a company that will give one to me) and fly? Do civilians need type ratings? I have heard you need a type rating to be the pilot in command. (It wouldn't be hard to find a pilot if that is the case.)

In addition to a type rating, you cannot fly a 737 without a commercial pilot's license and 1,200 flight hours. Further, your flight and maintenance operation must be certificated by the FAA. You'll need FAA approved opspecs, flight manuals, pilot training and testing, a drug test program, hazmat program, a designated safety manager, and more.

Aircraft with 20 or more passenger seats, or 6,000 lbs payload capacity must be either flown under Part 121 air carrier rules or else they fall under Part 125 regulation. Except for limited circumstances, they cannot be operated under Part 91 (exceptions fall under Subpart F). Because your party isn't a training flight, you don't own the plane, it isn't a sports team, etc., you are under Part 125.

Once the aircraft is large enough, there is no longer a distinction between private and commercial operation, Part 125 covers commercial operations short of common carriage. Even if you're operating an aircraft privately, you have to comply with commercial level safety requirements.


What is this aeroplane - not a Blackbird? What is the aeroplane in the first image?

I noticed this photo in a youtube clip, and at first glance thought it was a three-engine Blackbird SR-71, but it appears to be a single-seater.

The text and audio does not reveal what the photo is of.

  youtube.com/watch?v=nagVeYYt13s#t=10m41s

Some googling suggests that is a D-21 drone atop a M-21, and wikipedia has this somewhat-unhelpful photo.

The large aircraft is the Lockheed M-21 (which was developed from the Lockheed A-12, the same aircraft that the SR-71 was developed from), and the smaller aircraft is the D-21 drone.

The wikipedia article on the D-21 has a lot of information.


How does the debris rejection system in the CFM LEAP engines work? How does the debris rejection system in the CFM LEAP engines work?

LEAP - The debris rejection system

It's an inertial particle separator system incorporated into the compressor inlet.  It forces intake air to follow a curved passage that makes particles with mass sling to the outside of the duct due to the sudden change of direction, where, when required, those bypass ducts open into the stream to dump the particle laden air overboard.

Turboprops have used inertial particle separators that can be opened or closed for eons; this is the first time I've seen the concept applied to a jet.  It's desirable for turbojets and turbofans to have an inlet that's good at "catching" air for a good ram effect; better for efficiency and a strong ram effect makes the engine windmill well (important for re-light capability).  Problem is, strong ram effect also means a lot of particles being ingested.  While large objects like birds are an obvious factor, with jet engines there is actually a more subtle long term problem, sand particle ingestion that literally sandblasts the compressor blades over time and shortens the life of the engine.  This is a huge problem in desert operating environments.

Some engines like early versions of the GE CF-34 used a compressor inlet profile that was "flush" to the airflow downstream of the fan, which tended to encourage particles to pass on by in the fan discharge. This was a design goal for the A-10 Warthog, for which the engine was originally designed, and made it fairly FOD resistant.  Similar variants were used in the Challenger business jet and the CRJ200 Regional Jet.

Problem was the ram effect was low, which made it windmill poorly (you have to maintain a fairly high speed if it flames out, to keep the core of the engine from stopping - the dreaded "corelock").  On later commercial variants, the compressor inlet profile was revised to present a more direct into-the-flow inlet for the compressor, improving efficiency and making the engine able to windmill at low speeds.  The price of this was the FOD resistance was reduced and in sandy environments the erosion wear problems went way up.

This design appears to be an attempt to keep a compressor inlet profile with a good ram effect while providing the ability to minimize sand ingestion when required.  The benefit is decent engine life for operators in sandy environments.  Middle eastern airlines would be very keen to have this feature because they tend to suffer from short TBOs from their engines getting sandblasted.


What kind of aircraft are currently being used by Boulder County to fight wildfires? I was wondering, what kind of aircraft are currently being used by Boulder County to fight wildfires? For example, see photo attached of a plane dropping flame retardant on a wildfire over Left-Hand Canyon in Boulder, Colorado.

I would appreciate any insight anyone can share on this. Thank you.

It's a McDonnell Douglas MD-87, specifically one from Erickson Aero Tanker.

Source: eatanker.com


Why does flying not have any age restrictions like driving does? I read an autobiography about Vicki Van Meter, who, with her instructor, flew across the United States at age 11 in 1993. Why could someone of that age not drive with the instructor?
Does it have to do with a different set of principles that flying has as opposed to driving?

Short answer: the US government agreed with you 3 years after her record flight.

In the FAA reauthorization act of 1996, the following clause was added to 49 U.S. Code § 44724 - Manipulation of flight controls:

  (a)Prohibition.—No pilot in command of an aircraft may allow an individual who does not hold—

  (1) a valid private pilots certificate issued by the Administrator of the Federal Aviation Administration under part 61 of title 14, Code of Federal Regulations; and
  (2) the appropriate medical certificate issued by the Administrator under part 67 of such title,
  to manipulate the controls of an aircraft if the pilot knows or should have known that the individual is attempting to set a record or engage in an aeronautical competition or aeronautical feat, as defined by the Administrator.

Per 14 CFR 91 (can't find the exact text right now, but I know this is true from my flight training), you cannot obtain a student pilot's license until age 14 (so, no soloing until then, let alone a long-distance trip) and a private pilot's license until age 16 (for gliders).  You can't get a private pilot's license for your typical single engine aircraft until age 18.  A sport pilot's license is possible by age 17, but is limited on the aircraft you can fly (that said, a coast-to-coast trip has been flown in a Piper J-3 Cub, which fits under the sport pilot license) and conditions you can fly in.

Hence, it would not presently be legal to do what Vicki did in 1993.  According to a NY Times article covering another young pilot, Jessica Dubroff (aged 7 at the time of her fatally unsuccessful attempt at a coast-to-coast flight):

  Jessica was breaking no aviation rules when she flew.  The plane had dual sets of controls, meaning that the flight instructor was legally considered in charge and had the ability to take control instantly, though the plans called for Mr. Reid to take over only in an emergency.  But within hours of the crash, Federal authorities announced that they would re-examine existing regulations on underage pilots.

This would have preceded the addition to the USC quoted above.


What is the purpose of a dorsal fin such as on the Boeing 737? I've noticed that the 737 has a rather unusual vertical stabilizer, with an extension at the front. Is the 737 the only plane equipped with such a vertical stabilizer, and what is its purpose?

The "extension" at the front is called "dorsal fin". Its main purpose is to improve directional stability in high side-slip situations (asymmetric flight due to engine failure, crosswind landings, etc). The double-sweep of the leading edge of the vertical stabilizer helps the vertical stabilizer to be effective at a larger range of side-slip angles (high lateral angles of attack).

The stabilizer is an airfoil, much like a wing. The amount of lift (or in this case, sideways force) it generates, varies with its angle of attack (AoA, or $\alpha$), or in this case, sideslip-angle ($\beta$) something like this:

Lift coefficient ($C_L$) increases with increased $\alpha$, until it reaches $C_{L_{MAX}}$. This angle is called $\alpha_{crit}$.

Generally, the more sweepback an airfoil has, the lower its $C_{L_{MAX}}$, BUT the higher its $\alpha_{crit}$. This means that the non-dorsal-fin-part of the stabilizer (low sweepback) will be able to generate its $C_{L_{MAX}}$ (sideways lift) at a lower $\beta$ than the dorsal fin. Increasing the side-slip angle beyond its $\beta_{crit}$ decrease the amount of sideways force it generates. By using a secondary airfoil (the dorsal fin), with a higher sweep angle, (ie, higher $\beta_{crit}$), the net loss in sideways force with increased side-slip may be avoided, or at least mitigated. Further, at high $\alpha$, (or in this case, with a vertical stabilizer $\beta$), a delta with high sweepback will generate a vortex, which will follow the upper side of the surface down-stream of it, adding kinetic energy to its airflow, thus enhancing its effectiveness.

The current version of the dorsal fin was added to the Boeing 737 with the Classic (−300 and onwards), but the design is not unique for the Boeing 737. Look the Cessna 172 for instance (Wikimedia Commons):

The ATR 72 doesn't have a "pronounced" dorsal fin, but you can still see clearly how the sweepback of the vertical stabilizer changes along its length (Wikimedia Commons):

Or, on the other side of the speed spectrum (Wikimedia Commons):


Why don't the noses have the same de-icing metal surface like that of the wing's leading edge, engines, tail, etc.? For reference, let's take the A320. The nose is painted like the rest of the body.

Image Source

Areas on the airplane with ice protection have it for important reasons:

Ice on the wing/empennage disrupts airflow, adding drag and making it more susceptible to stalling.
Ice on an engine cowl lip can disrupt flow into the engine, causing stability issues. If it dislodges it can damage the engine.
Ice in the pitot/static system prevents the air data system from reading airspeed or altitude properly.
Ice on the windshield can make it harder to see through.
Ice in drain lines can cause them to clog and not drain properly.

The nose is not as important. Ice will add drag, and has some risk of breaking off and damaging something. But this is not as critical as with other areas. There is also an added challenge in adding anti-ice measures, as it must interfere as little as possible with the weather radar inside. It's made from composite material, which doesn't conduct or cope with heat very well, so using either bleed air or electric heating would be an issue.


Which language is used to develop new aircrafts' avionic systems AI? As an airframe and powerplant maintenance student, I also want to get an avionics license. My main goal is working on artificial intelligence of aircrafts avionics. However I'm concerned about the right language which I should work on. I've been learning Python with OOP for a long time (it was someones advice) but I think I should switch to C++ or Ada.
Honestly, I want to continue with Python but nowadays I have almost no motivation about it. Any advice for my roadmap? Thanks a lot!

Aviation is a very verbose world. Pilots do checklists. They use standard phraseology. Each part in an aircraft has a written history like an ancient sword. There are two communicating pilots in the airliner.
And the main selling point of Python has always been its extremely low level of verbosity. Python ommitts lots of declarative code other langages require. This makes the programming fast and easy but does not give context to see the errors early. An error that would prevent Eiffel or C++ code from even being built would often pass ok for Python, and the program will crash during the run instead, and maybe not always, and maybe after it passes the testing.
As a result, I am convinced that Python is not a choice for something that costs lives when it fails. Such cases have they own priorities and "working prototype in two days" is not between them. I think the role of Python in serious aviation will be rather limited.


Does lift-to-drag ratio asymptote to 4 (or 6) at high Mach numbers? The Kuchemann equations for L/D at high mach numbers, approximately verified by wind tunnel tests, were:
$$\left(\frac{L}{D}\right)_{max} = \frac{4\cdot(M+3)}{M}$$
and, for wave riders
$$\left(\frac{L}{D}\right)_{max} = \frac{6\cdot(M+2)}{M}$$
As you can see, these equations asymptote to 4 and 6 respectively.
If my understanding of the matter is correct, at ultra high Mach numbers, air begins to behave more like rays of radiation. Hence, L/D would simply be:
L/D = 1/tanA

where A is angle of attack.
Hence, the corresponding angles of attack for the two asymptotes would be 14 degrees and 7.5 degrees, respectively.
Is there any physical evidence for the above asymptotes (and perhaps the AoA values)?

My source is STUDIES OF HIGH LIFT/DRAG RATIO HYPERSONIC CONFIGURATIONS by John V. Becker, to be found online here. It is from 1964, so more than 50 years old, but since a lot of research had been performed already before that date, it might still be relevant.
In short: It depends. On thickness ratio and Reynolds number, for example, as can be seen in this plot for flat plates from the above linked report:

Note the grey areas noting the maximum for glide rsp. reentry vehicles. Their performance is roughly half as good as that of a flat plate.
Most of the report uses the volumetric efficiency parameter $\frac{V^⅔}{S}$ for comparisons. This is the volume $V$ to the power of ⅔ divided by the reference area $S$. Below is a carpet plot for a variety of wedge shapes, valid for Mach 6.8 (same source as above).

An infinitely thin wing will achieve an L/D of 6.5, irrespective of its leading edge sweep, and a more realistic t/c of 0.09 will not get above an L/D of 5.5 at 85° sweep, dropping below 5 for 70° sweep.
It seems the Küchemann approximation holds up well for single-digit Mach numbers but becomes too optimistic above this speed. And impact theory alone is not sufficient to predict L/D with any confidence.
The plot below explains why the Shuttle is a low wing configuration: Above Mach 10 it is cleary advantageous to put the flat side on the bottom:

The index FB represents the flat bottom version of the vehicle.


What does the solid black star symbol on this VNC chart mean? On the Canadian VNC there are symbols, the meaning of I can't quite figure out. In the Nanaimo aerodrome there are several "star" symbols. The unfilled one shows the location of the aerodrome beacon. What do the other solid black star symbols mean?

The one circled in red reads "S2 Fl R 209". Based on my knowledge of marine lights I thought that it could describe a red flashing light. The star symbol, being in an aerodrome could mean "limited hours" but even then, I'm not able to make sense of the rest of the description.

What does the solid black star and "S2 Fl R 209" mean?

EDIT: According to the CFS, the solid star is a hazard beacon light. Not sure how to decode the text.

Here's how to decode S2 Fl R 209:

S1 - Identifier. Bearing relative to an aerodrome (N, W, NE). If there are multiple with the same bearing, they are numbered

Fl - Light pattern

R - Color

209 - Elevation

Source


What has Airbus done to address the dual input issue? There have been a couple of fatal incidents involving Airbus aircraft, namely Indonesia AirAsia Flight 8501 and Air France Flight 447, that could have been prevented had the captains orders / input been the only accepted one. In both cases the captain was commanding push down to recover from a stall, while the FO was pulling up. Because of the dual-input system they cancelled each-other out and the stalls were not recovered from.

Has Airbus done anything to address this? It seems to me that if the aircraft is in alternate law, as it was in both cases, perhaps it should only accept input from the left seat.

Nothing was changed in the stick design or logic.
The side stick design and logic are not addressed by the final recommendations of both accident reports. Of the issues were the training and lack of crew resource management (CRM). Recommendations toward the manufacturer, for example, are with regard to the barrage of warnings the crew of Indonesia AirAsia had, even though Airbus aircraft already have an EMER CANC button to mute spurious alerts:

The KNKT recommends that Airbus to consider in developing a means for flight crews to effectively manage multiple and repetitive Master Caution alarms to reduce distraction. (PK-AXC accident report)

(There are already buttons to take priority, as well as aural and visual cues as to which seat currently has priority.)
Active sticks
Coupled digital side sticks weren't there when Airbus first introduced the A320, as a technology they became feasible in the early 90s. And were first introduced in c. 2015 on the Gulfstream 500 and 600 business jets.
Imagine panicked, under-trained, and uncommunicative pilots in an active sticks cockpit; they might think their sticks got jammed or the coupling motors are acting up. And seeing how long it took for the active sticks to make it into a business jet already hints at the snail pace (and cost) of aviation hardware certification and approval in general.
As for the captain's / left seat inputs being correct, as others have commented, there have been numerous accidents where the captain was in control and the first officer's recommendations were not heard or they kept silent, again, bad CRM.

Re comments on visually noticing the other yoke (not really a side stick design/logic issue):
That did not help Flash Airlines Flight 604 and Atlas Air Flight 3591. Tunnel vision and/or not speaking up is the main problem. Same with the thrust levers; despite moving automatically on Boeing planes -- showing the current setting -- they did not help Asiana Airlines Flight 214 and Turkish Airlines Flight 1951.


How is fuel weight calculated? I was having a conversation concerning airplanes and the calculated fuel. We were having a conversation about the Airbus A380. Based on this site:

http://www.modernairliners.com/airbus-a380/airbus-a380-specs/

It shows a capacity of 320,000 Litres. After some conversion (I am in the US)

320,000 Litres = 84,535 gallons

84,535 gallons = 710,000 lbs

Now that is based on water density. I've heard that jet fuel isn't exactly comparable to water? Is this true? I am trying to confirm because 700 thousand pounds of fuel sounds like a lot? Is there something I am missing? An average Olympic sized swimming pool carries about 660,430 gallons of water. How does this plane carry that much weight? And what would be the easiest way to explain this to a layman like myself?

Fuel doesn't have the same density as water. We're not planning a real flight, so a quick and dirty fuel density figure from Wikipedia tells us:

  In performance calculations, airliner manufacturers use a density of jet fuel around 6.7 lb/USgal or 0.8 kg/l.

So if we take your figure of 320,000 litres (it's a bit unclear quite how much of that is actually usable for flight, but that's a subject for another question), that's about 560,000lbs of fuel or around 254,000kgs.

Which, indeed, is a lot. The fuel is spread throughout the wings (and the trim tank in the tail), which are designed to carry the weight. If you look at What is the weight budget of a fully loaded A380?, you'll see how the fuel capacity interacts with the other weight specifications for the aircraft:

  The maximum fuel weight is the difference between maximum ramp weight and zero fuel weight (which is 562,000−361,000=201,000kg)

Max take-off weight is a bit less than maximum ramp weight, but we'll handwave over taxi fuel for simplicity. The point here is that if you can fill the aircraft with cargo, it would be too heavy if you also filled it with fuel. If you want to take a full load of fuel, you'd have to reduce your payload to keep the total weight low enough. Working out the weight/fuel/range trade-offs for a given flight is what dispatchers and load planners do, using specialized software.


Is flow attached at fixed propeller blade at every RPM / speed conditions? Propeller blade AoA depends on tangential veloctiy caused by rotation and forward speed "head wind".
If propeller has fixed pitch is flow attached  on blade at every RPM / speed conditions?
For example when plane has full throttle when staying on ground at same place,forward veloctiy is very small,that mean AoA is higher,does this cause flow separation at low pressure side of  blade  at this RPM/speed condition..etc

No, flow separation far away from the intended operating point is common.
Witness racing planes. Before Ratier made the first useable variable-pitch propellers in 1932, high speed aircraft had abysmally low propeller efficiencies at low speed, so much that world records were set with floatplanes (which had virtually unlimited "airfields" at their disposal). Same with small racing boats: To get over the hump (transition from buoyancy to planing), drivers need to lean forward and to patiently gain speed with their high-pitch propellers providing only marginal thrust due to flow separation.
The rotating movement of propeller blades helps to accelerate the boundary layer outwards, so flow separation on propellers happens much later (at more adverse pressure gradients) than on wings. Regardless, if blade angle and flow direction at the blade don't match well enough, separation is unavoidable.


Why did a circular paper aeroplane of larger diameter fly further than one of a smaller diameter? http://www.10paperairplanes.com/how-to-make-paper-airplanes/07-the-ring.html
I am not an expert at this- just a high school student, so please explain this in fairly simple terms. Thank-you very much in advance!

The larger glider flies with a larger Reynolds number, so friction drag will be lower relative to lift.

Using paper of the same grammage (weight per area) would result in the same wing loading and the same flight speed, and the Reynolds number will grow linearly with glider size. If you look up in the linked answer how friction drag changes with Reynolds number, you will see that friction drag drops with increasing Reynolds numbers.


What is the best prop location for distributed propulsion - pusher, tractor or blown flap for an ultralight? What is the best prop location for distributed propulsion - pusher, tractor or mid top wing ( blown flap) for an ultralight, max speed about 60mph, for MINIMUM takeoff distance ( 20-40') and about 15mph stall speed?

I would assume Blown flap, as the wing and prop would see clean air, and the flap sees the highest air speed increase due to exit speed of the prop, assuming 100% coverage.

My assumptions are:

Tractor configuration: entire wings sees prop exit speed but air is turbulent, so less lift, prop is in clean air, so max thrust.
Blown flap (prop on top of wing): wing is in clean air, so max lift, flap sees highest increase in air speed in this configuration, so max lift, prop is in clean air, so max thrust.
Pusher configuration: wing and flap in clean air, so good lift, prop in dirty air, so less thrust

Is this correct?

You are not wrong to go with tractor config.

In my opinion the best location of the propeller is pusher. The reason is that there is no airframe behind to interact with a heavily turbulent propeller wash. The thrust is developed on propeller blades alone and the airflow on the wing is the result of wing being thrusted forward in stationary air. It has nothing to do with propeller wash. The wing benefits from the most undisturbed air possible but the propeller gets the wash off the wing and airframe. For ultralight it may not be a big deal because of good thrust to weight ratio.

The tractor solution puts the engine ahead of the wing and is more common. The propeller gets the soft air and the wing gets the propeller wash. Maximum thrust, but not the best lift or drag.

Distributed propulsion means you have more contributors to propulsion.

15mph stall is similar to paramotor. You may go even lower if you’ll consider a gyro plane.


Does the CF6 use Pneumatic Reverse Thrust from the APU's bleed air? Does the A330's engine (the GE CF6) use only pneumatic power from the APU for reverse thrust? Please explain detailed. Sorry for my English.

Thrust reversers on high-bypass engines are airframe parts

The thrust reversers on a high-bypass turbofan such as the CF6 are treated as part of the nacelle, which is made by the airframer, as opposed to being part of the engine.  This is unlike target-type thrust reversers, which are treated as engine parts by and large.

Airbus uses hydraulic reversers by and large

The reversers on current Airbii (including the A330 that you ask about) are hydraulic.  This is in contrast to the B747 and the A300/310 which also use the CF6 engine and use pneumatic reversers (driven from high stage engine bleed air directly, not the general bleed ducts).


What is the formula for induced drag? I am looking to create one of these graphs for a simple model I tested in a wind tunnel.

I have managed to define the lift coefficient and thus I've been able to create the parasitic drag line. The problem is that I am having issues with finding the correct formula to use for the induced drag. I have been looking a while now and the only formula's I have found had the induced drag increase with a higher velocity.

the problem i'm running into is that the formula i see around is

$D_i = \frac{L^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$

this formula has Lift squared above the divider and lift itself is dependant on velocity so whenever I use that formula the graph I get is of one that increases with speed instead of decreases.

What I'm asking for is the correct formula to use here

It seem your graph of induced lift is not decreasing because you assume that the lift increases with velocity. This is generally not the case.

Typically, a drag vs velocity graph is made for unaccelerated level flight. Under these conditions the lift is equal to the weight of the aircraft.

$L = W =  \frac{1}{2}\rho V^2 c_L S$

From this we can obtain the lift coefficient as a function of velocity:

$c_L =  \frac{W }{\frac{1}{2} \rho V^2 S}$

The drag of the  aircraft is the sum of the parasite drag and the induced drag:

$D = D_p + D_i$

With the parasite drag:

$D_p = c_{D,0}\frac{1}{2} \rho V^2 S $

And the induced drag:

$D_i = \frac{1}{2}\rho V^2 S \frac{c_L^2}{\pi  AR \epsilon} =
\frac{W^2}{\frac{1}{2}\rho V^2 S \pi AR \epsilon}  = \frac{W^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$

It is important to understand that this only holds when the lift is equal to the weight of the aircraft (e.g. straight & level flight)

Nomenclature:
    $L\:\:\:\:\:$ lift
    $W\:\:\:$ aircraft's weight
    $\rho\:\:\:\:\:$ air density
    $V\:\:\:\:$ velocity
    $S\:\:\:\:$ wing surface area
    $c_L \:\:\:$ lift coefficient
    $c_{D0} \:$  zero-lift drag coefficient
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\epsilon \:\:\:\:\:\:$  the wing's Oswald factor
    $b \:\:\:\:\:\:$  wing span


Do I have to take science in high school to become a pilot? Some articles say we don’t need to take science in high school to become a pilot but some say we do... It’s very confusing and I don’t know which subject to take; Science, commerce or arts.
Hope someone can help.

There are no specific high school or college academic requirements specified for pilots in the U.S. Code of Federal Regulations.
However, basic knowledge in math and the physical sciences is important, and will greatly improve your understanding of subjects that are taught in ground school so that you can successfully pass required written and oral exams.
Furthermore, to be competitive as a professional pilot a college degree definitely helps, and you will likely need some minimum science credits both to get in, and to graduate.  Don't skimp on your education!


Have there been accidents involving flight simulators? Have there been any accidents which have happened to/in flight simulators?
I understand that simulators are used to simulate general accidents for training purposes (engine failure, birdstrikes etc), and also specific accidents for forensic purposes (the first use, in 1972, of a CAE JAT DC9 SIM), but I am wondering if there have been any accidents with the simulator itself? I am thinking of possibly a simulator's motion system failure? or fire? or trainees trapped? etc.
Additionally, if you knew of any source which might keep record of this type of information, that would also be very helpful.
The example that first comes to mind is the tragic 2014 crash of a Beechcraft King Air B200 into a flight simulator in Wichita, Kansas.

Update: To clarify, I am interested in all examples, but those that have received media attention for whatever reason are my primary interest (these stories will be held in the public record and can thus be further researched). Though interested in “work-place incidents”, these would likely have been dealt with internally unless there was a certain severity or sensational aspect.
I have answered my own question for now, will reward answer to someone else if they have any further information.

I have been attempting to find information on this question for quite a while, and judging by the lack of answers, I am assuming that Zeus, in the comments to the question, is right: that accidents involving a flight simulator, to the extent that they are severe or sensational, are very rare. More often, accidents would be considered “workplace incidents” and would go unreported outside the company.
In Larry Milberry’s text The Story of CAE (2015), while discussing safety procedures and accidents in the development of flight simulation motion systems, several accidents did indeed happen, but none which involved injury:

There was a failure of a British Airways simulator where “a
malfunction drove the cockpit at maximum pitch-up velocity into the
end of travel, breaking the upper bearings and sending the simulator
to the floor.”
During the testing of the (never installed) Iranian CH-47 simulator
“there was a motion system failure. The simulators unusual vertical
configuration meant that any uncontrolled loss of hydraulic system
pressure would cause the flight compartment to lurch nose-down…
The potential for Murphy’s law to take over was there in spades, and
one day the interlocks did fail…”

In my pursuit of this question, working through historic newspapers, I have come across several tangentially related, though ultimately unsatisfactory, examples to my question: the 1967 oxygen fire in the Apollo 1 capsule simulator that killed 2 airmen (and 16 rabbits), an eerie repeat of the explosion which killed the 3 astronauts the Friday before; the 1991 B52 crash during the Gulf War which again was an "eerie repeat" of a “crash-in-the-box” during previous training; the 1991 deaths of two pilots “simulating” the attack on Pearl Harbor in Florida; the 2001 attempt by a man to open the emergency door mid-flight saying “it’s OK, we are just on a simulator”; and finally, many instances where flight simulators have been used to familiarize those with intent to hijack a plane.
Again, these are not what I am looking for, but they are the types of things you find when you start searching for this particular information. My sense is that there must be examples of accidents in flight simulators which have been significant but which, likely due to them being workplace accidents, did not receive media attention. Nonetheless, maybe this answer will help provoke the memory of others to contribute a more precise answer.


Does a private pilot certificate require a certain number of ground school hours? To obtain a private pilot certificate in the US you need a minimum of 40 flying hours. Is there a minimum of ground school hours? Or is it just however long you need to pass the test?

For training conducted under 14 CFR 61 there is no requirement for groundschool.  The only requirements are that you take and pass written tests, but how you prepare for them (formal groundschool, self-study, etc) is completely up to you.  You will need a recommendation from a flight instructor to take a written test, so your preparation method and readiness for the test will be evaluated before you are allowed to take it.

The only exception to the above statement is knowledge tests for airline transport pilot certificates with airplane multi-engine class ratings taken after 31 July 2014.  These written tests do require specific groundschool training minimums, which are specified in 14 CFR 61.156

If you are instead training under 14 CFR 141, your school has a specific curriculum and examination authority approved by the FAA.  The specifics of your program will dictate what groundschool is required to take the relevant written exams.  The groundschool aeronautical knowledge baseline requirements are specified in 14 CFR 141 Subpart F.  For a private pilot certificate with an airplane category rating, these requirements are:

  35 hours groundschool training in

  Applicable Federal Aviation Regulations for private pilot privileges, limitations, and flight operations;
  Accident reporting requirements of the National Transportation Safety Board;
  Applicable subjects of the “Aeronautical Information Manual” and the appropriate FAA advisory circulars;
  Aeronautical charts for VFR navigation using pilotage, dead reckoning, and navigation systems;
  Radio communication procedures;
  Recognition of critical weather situations from the ground and in flight, windshear avoidance, and the procurement and use of aeronautical weather reports and forecasts;
  Safe and efficient operation of aircraft, including collision avoidance, and recognition and avoidance of wake turbulence;
  Effects of density altitude on takeoff and climb performance;
  Weight and balance computations;
  Principles of aerodynamics, powerplants, and aircraft systems;
  If the course of training is for an airplane category or glider category rating, stall awareness, spin entry, spins, and spin recovery techniques;
  Aeronautical decision making and judgment; and
  Preflight action that includes—

  How to obtain information on runway lengths at airports of intended use, data on takeoff and landing distances, weather reports and forecasts, and fuel requirements; and
  How to plan for alternatives if the planned flight cannot be completed or delays are encountered.


Does adding more engines to commercial aircraft increase safety? For example, the Boeing 747 has four engines, whereas the Boeing 737 has 2. Does this means that the Boeing 747 would be safer if one of the engines fail?

In the modern day and age, it is very difficult to put a numerical value to this type of question, simply because the chance of something happening is so incredibly remote. This is not a clear-cut equation and is dependant on several factors.

A few thoughts:

The impact of loosing one engine will be less significant on a four-engined aircraft than a two-engined aircraft. You only loose 25% of thrust as compared to 50%. However, two engined jets are built with greater safety margin and are able to handle an engine failure at any phase in the flight. Four engined jets are certified to be able to fly on two engines on one side of the wing.

In theory, you are twice as likely to suffer an engine failure with four engines, since you have twice as many engines. While the lost thrust would be less significant, very rarely damage can propagate to cause damage to other parts (See QF32 and ELY1862). In general, it is easier, cheaper and faster to maintain two engines than four. Should a complete engine swap be necessary for instance, you only need to do it twice instead of four times. Every extra step is an step where something could theoretically could go wrong.

The GE90 (Boeing 777) is certified to handle 'large' birds (4kg) as compared to the CFM56 (Airbus A340) only being able to handle 'medium' birds. Hence, the Boeing 777 engines are likely to be a bit more robust.

From a human perspective, pilot workload will be lower with two engines rather than four. Should something happen in a time-critical emergency, there will be more systems you need to keep an overview of. See CAL006.

Furthermore, not every engine-related incident is spared simply cause you have four engines. BA Flight 9 lost all four engines due to volcanic ash. On BA Flight 38, the engines failed in unison due to ice in the heat exchanger on the engines that were operating in identical conditions.

Despite ETOPS being around for several decades and applying to thousands of aircraft I have not heard about a single accident involving commercial jets operating in that domain.


If an elliptically-shaped aerofoil gives the optimum lift distribution, why aren't propeller blades designed around this ideal? So given an elliptical lift distribution is the most efficient, why are propellers not designed around this? Props aren't affected by the adverse stall characteristics that concern a "main", elliptically-shaped wing, given they (theoretically) should never stall.
The picture of a C208 propeller below illustrates what I mean; the propeller is raked at the tips. Is this related to encouraging a spanwise flow due to the tips approaching Mach 1? I would have thought stubby propeller blades like those wouldn't reach speeds where wave drag becomes an issue?

I suppose this P-51 propeller is closer to that ideal shape but it is still relatively square at the edges. I understand that you couldn't strap a perfectly elliptical wing to a propeller given the high aspect ratio needed of the blades, but nevertheless, I don't understand why propellers are not at least approximated to the ideal shape...

If an elliptically-shaped aerofoil gives the optimum lift distribution …

I interpret this as an elliptically shaped wing, because only then the question makes sense.
Propellers are similar, but not identical to wings.

Dynamic pressure varies along blade span. The outer sections needs less chord for the same lift compared to inner sections.
Propeller blades operate at the start of a helical wake, so induced speeds are different than those on straight wings.

What is required is not an elliptic lift distribution, but minimum induced losses. Albert Betz showed in 1919 that the optimum induced speed is constant over the propeller disk, just like downwash is ideally constant over span for minimum induced losses. Based on this, Larrabee found an efficient computation method and practical propellers have to find a compromise between theory and practical constraints. The most important are

Compressibility effects near the tips (which force thinner airfoil and lower lift coefficients than what the ideal prescribes). An extreme case is the Junkers VS-9 propeller used on the Ta-152.
Structural limits at the root (which force the use of much thicker airfoils).

Betz' theory is meant for lightly loaded props. Once disk loading goes up and the propeller diameter cannot grow proportionally, propeller chord and blade count grows and the main concern shifts to converting all available power to thrust. Concerning fancy blade shapes: They have more to do with marketing or unscientific voodoo beliefs by some hobbyists. Generally, a straight shape works best.


Why don't airliners use light strips on physical controls (yoke/stick, throttle and trim) to indicate who has control - pilot or flight computer? I've watched a lot of Air Disasters episodes about crashes where pilots thought auto-pilot, auto-throttle, auto-trim were "in control" but they weren't (or vice-versa) due to some innocuous pilot action - like bumping the go around setting. See: https://en.wikipedia.org/wiki/China_Airlines_Flight_140
From a Human Factors engineering perspective, it seems that it should always be obvious who has control of the various flight controls. One simple option is to put light strips on each physical control (yoke/stick, throttle, trim) to indicate who "has control" - the pilot or the flight computer.  If the throttle levers light strip is green, auto-throttle has control.  If the light is off, the pilot has control of the throttle. This may seem a bit simplistic for normal aircraft operations but in times of high stress such a simple system might make a difference - preventing a crash.  Lights like this would of course need to adjust to the ambient light so they're not glaring during night yet visible in bright daylight.

They do.
On the 737 there is a light in the button for each autopilot and next to the autothrottle switch. As well as annunciation in green text at the top-center of the Primary Flight Display (PFD).
I'll see if I can find, or take, a photo (that isn't copyrighted) to illustrate this.
The indications are there; the challenge can be getting thru the information overload so that the pilots realize what the aircraft is telling them.


What are the advantages of 3 spool over 2 spool turbofan engines? What are the advantages of 3 spool over 2 spool turbofan engines?

There are two advantages;

It's easier to match the compressor rotational speed with the turbine. Typically, the first stages of the compressor want to rotate slower than the turbine is required to rotate. The turbine needs to spin faster to help obtain as much energy from the airstream as possible, but initial stages of compression prefer to spin slower. With 3 spools instead of 2, less compromise is required.
A greater pressure rise per stage can be achieved. This occurs because each compressor in a 3 spool engine (fan, LPC, HPC) will have fewer stages than each compressor in a 2 spool engine (for the same overall pressure ratio). A 5 stage compressor requires less compromise than an 8 stage compressor. The front stages prefer a high airflow to avoid stall, but the back stages can't always swallow the same mass flow, and can cause a backpressure which can lead to stall. Hence, air flow is often bled off in the middle of the compressor, to avoid stall. This is air that has been partly compressed, absorbing work, but isn't available to generate thrust. Hence, it's a parasitic load. So, the compressors in a 3 spool engine are better optimised than in a 2 spool engine, because this situation isn't as severe. Hence, the designer can be a little more aggressive in the pressure rise per stage the compressor achieves, because the compressor is less prone to stall. Thus, a 3 spool engine, in theory, can achieve the same overall pressure ratio, as a 2 spool, with fewer stages. This means, a lighter weight engine.


How can I determine specific heats of gases for turbojet engine calculations? I'm in a combustion lab and we are performing experiments on a turbojet engine. However, I am having some trouble calculating the exit velocity and compressor/turbine powers of the engine.

Consider the nozzle exit, where the exhaust gases go. Assuming 100% adiabatic efficiency of the nozzle and applying the 1st law of thermodynamics gives
$$
\frac{u_e^2}{2} = h_{0e}-h_e = c_{p,N}(T_{0e}-T_e)=c_{p,N}T_{0e}[1-(p_a/p_{0e})^{(\gamma_n - 1)/\gamma_n}],
$$
assuming $p_e = p_a$, the ambient pressure. In the lab, we've set up thermocouples that measure the stagnation temperature and pressure at the exit, as well as ambient conditions. My problem, however, is that I have no idea where to get $c_{p,N}$, the specific heat of the gases in the nozzle from. I've looked up in a reference book that the average specific heat ratio $\gamma_n = 1.36$, but this still doesn't give me the specific heat that I need because I don't know the specific gas constant $R$ for these gases. For reference, it is an SR-30 turbojet with Jet-A fuel.

In addition, I have the same problem when calculating the power outputs. Assuming that all of the power of the turbine is transmitted to the compressor,
$$
\dot{W_T} = (\dot{m_a} + \dot{m_f})c_{p,T}(T_{04}-T_{05})=\dot{m_a}c_{p,C}(T_{03}-T_{02}) = \dot{W_C}
$$
Again, I have no clue what the specific heats are, only their ratio. Is there some way I can calculate these quantities or look them up somewhere? Or even find the specific gas constants somewhere?

Do you have a copy of the book Gas Turbine Theory by H. Cohen, G.F.C. Rogers and H.I.H. Saravanamuttoo? You may well find the answers in there I think.


What can a new commercial pilot do to build flight time? Most new commercial pilots don't have a lot of flight time, so won't be hired by the airlines or corporate operators.

A lot of operations require an air carrier certificate (even though they are a "commercial" pilot, they can't just start flying people around)!

What can they do to build time while (hopefully) getting paid to fly?

For a commercial pilot (without a CFI rating) you've got some options, but the real golden goose of time building is flight instructing. Most folks I know who are time-building accumulate the minimum amount of time until they have enough to get the CFI rating, then go out and instruct to get to their end goal (time / types).

Without the CFI rating, some things to look in to include:

Banner Tow (if available in your area)
Crop dusting (again, if available in your area)
Glider towing
Skydiving
The FAA has a page on skydiving operations this that's worth a read.
Ferry Piloting (if you don't mind the travel)
Photo work - anything from costal erosion photos to event coverage depending on your location.
News work - Hand in hand with Photo work, but also traffic and the like.
Sightseeing flights
AOPA has a page on this which is worth a read, take care as you almost certainly want to be sure you fall under Part 91 and not Part 135!
Deliveries.
Someone needs a wedding cake moved across the state? You can often do that in a Cessna more efficiently than they can by car, and as a commercial pilot you can even charge for it.
Corporate Flight Jobs
Sightseeing within 25sm of an airport

There are some outfits that will "sell" you SIC in their cargo twins. It's a way to build TT, but it's considered an unscrupulous practice.

Many of these will probably require the very time you're looking to build, or experience in specific types of aircraft, but it can't hurt to investigate.


What is a typical ratio of airline pilots to aircraft? Delta has about 800 aircraft for their current operations; how many pilots does the company have? What is a typical or average number of airline pilots employed per aircraft in the fleet?

A Google search shows United Airlines has 709 aircraft and 12505 pilots for a ratio of 17.6 : 1.

If an aircraft was operated 100% of the time, a month's activity (720 hrs) would need 7 pilots pulling 100 hr/month duties, so it's an interesting related question why they need twice this amount. Perhaps there's significant variation in the hours worked by pilots - not all want to work 100 hrs/month, or can be efficiently scheduled to do so because of their home locations etc.


Would adding a slat and a slotted flap increase the max CL of a multi element airfoil to 6.5? Would adding a slat and a slotted flap increase the  of a multi element foil to 6.5?

I understand, as a general rule, the max $C_L$ for a classic NACA airfoil is as follows:

Base: 1.5
Slat: + 0.7
Slotted flap: + 1.3
Total $C_L$: 3.5

But with a Selig multi-element foil, the starting max $C_L$ is around 4.5.

Would adding a slat and flap to a Selig airfoil, with a 20° deflection of the main foil, increase the  $C_{L_{max}}$ to anything close to 6.5? Assume a Reynolds number in the 600-900k range.

Here is a pic of the foil:

The theoretical maximum lift of a cylinder is 4π. At that point the forward and rear stagnation points coalesce on the bottom of the cylinder. Practical solutions use rotating cylinders (Flettner rotor) but will not reach this theoretical limit.

This should illustrate that 6.5 is not outright impossible but the distance between a cylinder and an airfoil is quite large, so this theoretical limit does not apply to airfoils.

Adding a slat and a slotted flap to a regular airfoil will bring you at most to a c$_{l_{max}}$ of 2.8. Now you want to add them to a multi-element foil. Would that be several slats and flaps in sequence? Practical solutions use a single slat and a multi-slotted flap where the deflection angle of the single flap elements increases along the flow path of the air. A typical example would be the airfoil of the Boeing 727 which had a triple slotted flap which could reach a two-dimensional c$_{l_{max}}$ of 4.2 with 60° deflection and rearward movement of the flaps which increased the effective wing area. Note that wing sweep and ailerons reduced the practical maximum lift coefficient of the full aircraft to 3.0 (see plot below, taken from A. M. O. Smith's excellent article High-Lift Aerodynamics).

With modern CFD this could be improved a bit, but then the preference of aircraft builders has been to reduce the number of flap elements while losing only a little of lift.

Another practical limit is the maximum Mach number in the suction peak of all the airfoil elements. Once this reaches 1.58, no lift growth could be observed in experiments. This translates to a maximum for the product of Mach squared and pressure coefficient of -1.0. In other words: You need to fly very slowly in order to achieve high values of c$_{l_{max}}$.

The only practical design I know which reached that magic limit of 6.5 given as the c$_{l_{max}}$ of its wing airfoil is the Antonov An-70. Here this number is valid for the inner wing which is immersed in the prop wash of the mighty Aerosila SV-27 propellers, driven by the 13,240 hp Progress D-27 turboprop engines. By using the flight speed and clean wing chord for the reference parameters, this looks to me a bit like cheating. But the achieved lift is real!

EDIT:

Now that you show the Selig airfoil, it should be obvious that it is a point design for maximum lift and no practical applicability in real aircraft. Neither will it allow a thick spar nor low drag at lower lift coefficients. Adding a slat will help little, because the limiting part is the pressure rise on the main airfoil. The accelerating flow up to a chord of about 20% is meant to increase the area in the c$_p$-plot and the pressure rise aft of this point is a Stratford distribution which maximizes pressure gain without separation. Reduce pressure in the suction peak only a bit and the flow will separate, and lift collapses.


Is it possible to do linear interpolation on a manual E6-b? Suppose the performance tables for my fictional airplane indicate the following:

    Density             Takeoff
  Altitude (ft)       Distance (ft)
----------------    ----------------
       0                 1,865
      2,000              2,602

Suppose that the density altitude of the day is 1,400 ft and I would like to calculate my takeoff distance.  Obviously I could do it using the linear interpolation equation:

\begin{align}
y &= \frac{(x-x_0)(y_1 - y_0)}{x_1-x_0} + y_0 \\
&= \frac{(1400-0)(2602 - 1865)}{2000-0} + 1865 \\
y &= 2,381 ft
\end{align}

...but that's a lot of numbers.  I suppose some digital E6-b's might have a linear interpolation solver, but is there a clever way to calculate (or estimate) it using a manual E6-b?

Edit:  To be explicit, I could not care less about actual takeoff performance for the purposes of this question.  It's just there so we have a common example to use when discussing the actual question of E6-b manipulation.

My advice in this case would probably be: Once you know the density altitude, don't bother with the flight computer. The exact values are going to depend enough on the specific airplane that a general-purpose device will only get you approximations anyway, no matter what you do.

Rather, consider that for your fictious case, when going from a density altitude of 0 feet to 2,000 feet, takeoff distance increases from 1,865 feet to 2,602 feet.

Split the difference into reasonable, approximately equal-sized chunks. Say density altitude 0 ft, 700 ft, 1,400 ft, 2,000 ft. Calculate take-off distance for those, possibly based on a linear extrapolation (which isn't going to be exact), and note the change in take-off distance per change in density altitude. In your example case, for every 700 ft of density altitude increase in this range, the take-off distance increases by about 250 ft. Alternatively, for every 1,000 ft of density altitude increase in this range, take-off distance increases by about 370 ft. (If you're in a metric country, this happens to work out nicely to a little over 100 meters of runway per 1,000 ft of density altitude increase, so for a first order approximation you could just cross over the final digit in the density altitude in feet.) Note these values.

Now suppose that the density altitude for your airport of choice on the day and time of your flight is 1,370 ft. That's reasonably close to 2,000 ft minus 700 ft, so take the take-off distance for 2,000 ft (2,600 ft, give or take) and subtract 250 ft to get an approximate value of 2,350 ft take-off distance for this density altitude.

Or suppose the density altitude is 491 ft. That's closer to 700 ft than it is to 0 ft, so take the take-off distance for 0 ft (1,865 ft) and add 250 ft. That gives about 2,100 ft take-off distance. If you split into thousands, you could take half of 370 ft or about 200 ft, for a hair over 2,050 ft take-off distance.

For your example case, if the last hundred-ish or so feet of runway are going to be what makes or breaks the take-off, then I suspect you're cutting it too close anyway, so this level of accuracy should be sufficient for general aviation. With this, I'd probably have a rule of my own saying that if the runway length is less than, say, 150% of the calculated take-off distance, it needs a more accurate calculation or manufacturer performance data lookup; if the runway is less than 125% of the take-off distance thus obtained, it's a no-go. Remember that you have to be able to safely abort the takeoff even late during the takeoff roll!


Can non-instrument rated pilots request practice approaches? I'm working on my instrument rating and wondering if I can practice ILS approaches without an instructor? Using a safety pilot, also not instrument rated, could I call approach stating VFR would like ILS into X. Or do I just call the tower and ask for it there? Or can I only do this with an instructor?

Sure you can! There's nothing wrong with shooting practice approaches if you're not instrument rated (in the US, at least) as long as you don't actually accept an IFR clearance.

You would probably be better off asking approach control for practice approaches rather than tower, if you have approach available to you.