"Fundamental of Aerodynamics" 5th editon, example about oblique shock. Is the author's solution wrong? I am reading "Fundamentals of Aerodynamics" 5th edition, J.D.Anderson. If you have the book, go to chapter 9: Oblique shock and expansion waves. Please look at example 9.2, page 618:

Consider a supersonic flow with M = 2, p = 1 atm, and T = 288 K. This flow is
deflected at a compression corner through 20◦. Calculate M, p, T, $p_0$, and $T_0$ behind the resulting oblique shock wave.(I only have problem with $p_0$).

Before see the solution, try to solve this example by yourself, you might need to see the appendices, here it is: https://drive.google.com/file/d/1MX6MASeP8pU-u_4MDjInfKIzPJfUfMaB/view?usp=sharing

And this is the solution from author:

The interesting thing here is: I was reading the note carefully, I still see something wrong here:

And this is my solution for $p_0,2$:

$p_2$ = 2.82 atm

From $M_2$ = 1.21 $\Rightarrow$ $p_0,2$/$p_2$ = 2.489 (isentropic flow, appendix A)

So $p_{0,2}$ = 2.82 x 2.489 = 7.019  atm

You may think the result is the same as the author's solution, but this is just coincident, look at example 9.5, page 620:

Consider a Mach 3 flow. It is desired to slow this flow to a subsonic speed. Consider two separate ways of achieving this: (1) the Mach 3 flow is slowed by passing directly through a normal shock wave; (2) the Mach 3 flow first passes through an oblique shock with a 40◦ wave angle, and then subsequently through a normal shock. These two cases are sketched in Figure 9.14. Calculate the ratio of the final total pressure values for the two cases, that is, the total pressure behind the normal shock for case 2 divided by the total pressure behind the normal shock for case 1. Comment on the significance of the result.

Again, try to solve this example by your own. Author's solution:

My solution:

Case 1: same as author's solution

Case 2:

$M_{n,1}$ = 1.93 $\Rightarrow$ $p_2$/$p_1$ = 4.224 (1)

$M_1 = 3$ $\Rightarrow$ $p_{0,1}$/$p_1$ = 36.73  (2)

(1) and (2) $\Rightarrow$ $p_{0,1}$/$p_2$ = 36.73/4.224 (3)

$M_2 = 1.9$ $\Rightarrow$ $p_{0,2}$/$p_2$ = 6.701 (4)

(3) and (4) $\Rightarrow$ $p_{0,2}$/$p_{0,1}$ = 6.701x4.224/36.73 = 0.771 (different from author's solution $p_{0,2}$/$p_{0,1}$ = 0.7535

$p_{0,3}$/$p_{0,2}$ is same as author's solution

In both examples, you can see what is going on. Am I wrong or Anderson wrong ??
You might think: "Oh, Anderson said that changes across an oblique shock wave are
governed only by the component of velocity normal to the wave." But this holds only for p, T, $\rho$, $M_{n,1}$, $M_{n,2}$. For $p_{0,1}$ and $p_{0,2}$, we must consider the tangential components of velocities.

Continue reading, I have found the answer. Look at problem 9.3, page 663:

And here is the author's solution:


Why are planes allowed to fly with just one pilot? I chanced on this article which raises the titled question: Why aren't two pilots mandated for all planes esp. military? What happens if the lone pilot becomes incapacitated, esp. if the plane has passengers who aren't pilots?

Two captains of Cathay Pacific Airways lost sight during two separate flights, according to the Hong Kong Civil Aviation Department. In both cases, the first officer managed to land the aircraft safely.
[...]
On January 28, 2016, a pilot of the British Royal Air Force had lost sight during a training flight in a Hawk jet in North Yorkshire. Another jet had been scrambled with an instructor on board that helped the impaired pilot to land at RAF Leeming air base. Sources quoted by the Telegraph had said at the time that his vision was affected by the sudden deterioration of an eye infection.

Well I guess there are multiple angles to this. Perhaps someone can chime in with reference to regulations, but I can offer a more pragmatic answer.

Like it or not, aviation, like many other things, is a trade-off between risk and cost and I think that's what it boils down to. So what is the risk of single-pilot incapacitation and what is the potential consequences versus what is the cost to have all aircraft have at least two pilots aboard (not withstanding the impracticability of fitting two people in most single-seater aircraft.

So, I guess, we as a society have accepted the risk of single-pilot incapacitation and it seems that mostly the consequences are acceptable compared to the cost. It would be unlikely that we would mandate against single-pilot cockpits, but where to draw the line between single-pilot and multi-crew aircraft is certainly an on-going and active regulatory discourse.

In the end, I could ask you why don't we mandate two people with two steering wheels in cars? It would certainly prevent some accidents (hell, maybe many!), but it could also cause some too. And, practically, it just isn't going to happen is it?


What are the limits on myopia for a pre-eye refraction surgery for a class 1 medical certificate? Are there any limitations for myopia (-number) for a pre-eye refraction surgery for a class 1 medical certificate? I live in India & I have -6.5 & -7.5 in the left & right eye respectively.

It looks like there are no limits for your pre-surgery eyesight:

  Flight crew having undergone refractive surgery will be considered for
  medical fitness for flying on a case-to-case basis. Such cases will be
  examined only after a minimum period of six months after the
  procedure. Medical fitness for initial issue of licence may also be
  considered for such cases, if the visual requirements for the license
  category are met with normal corneal topography, no post surgical
  complications like corneal opacity interfering with vision and
  unstable refraction.

That makes sense: LASIK physically changes your eyes, so whatever your values were before the surgery are completely irrelevant after it. The general class I eyesight requirements in India are taken from ICAO Annex 1, which isn't freely available online. However, ICAO's Manual of Civil Aviation Medicine says:

  Distant visual acuity with or without correction shall be 6/9 or
  better in each eye separately, and binocular visual acuity shall be
  6/6 or better. No limits apply to uncorrected visual acuity.


How do I know how to describe my position on the airport when I contact ATC? When asking for taxi clearance, a pilot must include his location when contacting ATC. However, I'm not quite sure exactly how to state your position in the airport.

For example, I've heard some pilots say something such as "Ground, N123DB is southside by the tower, requesting taxi clearance," but I've also heard something like "Ground, N123DB is at transient parking, requesting taxi clearance." How do you know your location in the airport?

Obviously, I can look at an airport diagram and find out where I am, but how do I communicate this using proper terminology to ATC? Is it standardized?

There isn't really an official protocol, beyond the conventions everybody uses out of habit.  Just pick the most prominent landmark, building or chart-labeled zone you can find or think of.  The controller knows all of them so as long as it's not something like "next to a light pole", he/she will figure it out, and if not, they'll ask for clarification.

If the airport has an area designated "transient parking" on the airport chart, use that.  If you're on a ramp area close to the entrance to taxiway Alpha, you could say "xxxx on the ramp near Alpha".  If you're on a ramp in the hinterland somewhere near some hangars, pick the nearest building with a business name associated with it, like "xxxx by Standard Aviation...", or "xxxx by the Shell fuel pumps", whatever.  Use common sense and don't overthink it.


Where can I find airport taxi instruction "code names"? At larger airports, common taxi instructions are sometimes "named" for brevity as they are repeated often. For (an entirely contrived) example,

  Taxi to terminal via alpha, tango, foxtrot, mike four, sierra

could be shortened to something like

  Taxi via red-flower

Where can these common taxi route definitions be found?

Excerpt from the FAA Publication "Instrument Procedures Handbook":

  Standard taxi routes improve ground management at highdensity
  airports, namely those that have airline service. At
  these airports, typical taxiway traffic patterns used to move
  aircraft between gate and runway are laid out and coded.
  The ATC specialist (ATCS) can reduce radio communication
  time and eliminate taxi instruction misinterpretation by
  simply clearing the pilot to taxi via a specific, named route.
  An example of this would be Los Angeles International
  Airport (KLAX), where North Route is used to transition
  to Runway 24L. [Figure 1-7] These routes are issued by
  ground control, and if unable to comply, pilots must
  advise ground control on initial contact. If for any reason
  the pilot becomes uncertain as to the correct taxi route, a
  request should be made for progressive taxi instructions.
  These step-by-step routing directions are also issued if the
  controller deems it necessary due to traffic, closed taxiways,
  airport construction, etc. It is the pilot’s responsibility to
  know if a particular airport has preplanned taxi routes, to
  be familiar with them, and to have the taxi descriptions
  in their possession. Specific information about airports
  that use coded taxiway routes is included in the Notice to
  Airmen Publication (NTAP).

NTAP can be found on: https://www.faa.gov/air_traffic/publications/notices/

At the time of writing the KLAX North Route example can be found on page 4-SW-11, and reads:

  North Route
  Taxi towards taxilane Sierra (S) taxi northbound on taxilane Sierra (S), and at Check-point-1 contact Ground
  Control on frequency 121.65, hold short of taxiway Delta (D).
  Taxilane Sierra (S) is not visible from the ATCT

There's also FAA Advisory Circular 7110.116 regulating Standard Taxi Routes (STR's) as bilateral agreements (LOA's - Letter of Agreement) between each ATC unit and their users (airlines). The document contains information on what the LOA needs to contain.

Outside of the US, standard taxi instructions, if any, could be expected to be found in the AD-section (Aerodromes) of that respective country's AIP (Aeronautical Information Publication).

As an airline pilot, this information might be integrated into your company's nav data solution, ie. information about standard taxi routes available to you would be written/drawn out bundled with your airport diagrams etc.


Are the jet blast fences positioned other way around in this video? I was watching the following video, showing a 757 landing on Skiathos, Greece.

If you take a look at the 8:26 mark, you'll clearly see that all jet blast fences are positioned with their support toward the apron.

Can someone who knows better than I do confirm for me that this is the wrong way to position jet blast fences?

Here's an image, for those who can't/won't watch the video:

They're being used in a different way than normal in that case.  That's just a taxiway, not an engine run-up pad, so they aren't there to deflect max thrust jet blast.  They've been arrayed "backwards" to kind of trap debris that gets blown toward the building as aircraft taxi by.  That's my guess anyway.


Are there any characteristics of a hypoxic flight? Any patterns that investigators look for? For example, what happens when the engine flames out? is it guaranteed to enter a high speed dive? And would this affect certain parts of the aircraft, causing them
to separate under torsion and not compression?

If a "ghost" airplane with passed out pilots is unattended and the engines flame out, what is likely to happen is it will fly along on autopilot until one engine flames out a few minutes before the other. This will impose a pretty large amount of yaw, which the airplane's yaw damper may or may not have enough authority to counteract.  Yaw dampers generally can move the rudder somewhere around 1/3rd of normal travel and aren't meant to cope with asymmetric thrust (I recall from recurrent training in the CRJ that a flameout in cruise still required more rudder than could be applied by the YD to keep it flying straight and you still had to get your feet on the pedals post haste).
With the one engine dead, the yaw damper will be at full travel trying to "center the brick" (the little box shaped skid indicator on the Primary Flight Display), and if that's not enough, the airplane will be skidding along trying to roll into the dead engine, and the autopilot will be applying aileron to try to keep the wings level, and the airplane will fly along cockeyed, like happens when a new pilot on the type course experiences his/her first engine cut and forgets about their feet.
The amount of aileron may cause the autopilot to kick off at some point, at which point the airplane will immediately roll off toward the dead engine into a spiral dive (they won't just dive straight down, they have to roll off to one side into a spiral where the airplane's natural speed seeking pitching tendency just causes it to wind up the turn tighter and tighter as it goes steeper and steeper and faster and faster).
Or, if the autopilot is able to hang on, eventually the other engine flames out, the ram air turbine drops, most electrical services are terminated except emergency ones and the autopilot is likely to kick off then.
At that point, if the airplane is trimmed nicely and the air is smooth, it will likely simply descend more or less straight at its existing trim speed in a straight ahead glide... until something disturbs it, like a bump that makes it bank a bit more than dihedral effect can accommodate.
At that point most airplanes will drift off slowly into a descending spiral. It will turn steeper and steeper and descend faster and faster, either until bits start to come off the airplane from overspeed/over G, or it hits the ground (bits raining from the sky following spiral dives is what usually happens in GA disorientation events in instrument conditions - think Kennedy's crash).
So in the end, one of two things will happen.  If the airplane was trimmed at 240kt indicated, it will fly into the ground straight ahead descending at 240kt, or, if it got bumped into a spiral, it will come nearly straight down at close to Mach, making a small crater, or come down in individual pieces after breaking up on the way down.
And ending like the Lady Be Good is unlikely with a jet.


Must Part 91 pilots comply with runway departures marked as NA? I answered a recent question about the meaning of Takeoff Minimum NA and indicated that they do not apply to Part 91 pilots, but after thinking about it, I’m not so sure. I can’t find anything specifically saying that you can’t use runways marked as NA, although often the reason that the runway is not available would make using it a very unwise decision. e.g. terrain or obstacles. On the other hand an unmonitored ground facility when the tower is closed wouldn’t be an issue if you were using GPS to fly the departure.

The AIM 5-2-8 just says,

Pilots operating under 14 CFR Part 91 are strongly encouraged to file and fly a DP at night, during marginal Visual Meteorological Conditions (VMC) and Instrument Meteorological Conditions (IMC), when one is available.

Part 91 §91.175   Takeoff and landing under IFR. (f) Civil airport takeoff minimums. excludes Part 91 operators by giving limitations that only apply to Part 121, 129, and 135 operators.

A a towered field with an operating control tower, they are unlikely to allow you to to takeoff under IFR on a runway marked as Takeoff Minimum NA but is there any restriction when the tower is closed or there is no tower?

Other than flying a published SID or ODP with charted takeoff minimums, there are no other takeoff minimum requirements for pilots operating under Part 91.

If a Part 91 pilot receives and accepts a published departure procedure from ATC then they must comply with any minimums published on the procedure.

For example, some SID's may show takeoff minimums with a runway listed as "NA" (the Aspen Seven Departure as KASE lists "Rwy 15: NA - Terrain"). If you accepted this SID then you wouldn't be able to use runway 15 even operating under Part 91.

But just because you can doesn't mean you should. Every pilot flying Part 91 should have a set of personal minimums.

Just a pilot interpreting the regulations though, not a lawyer :)


If an accident were to occur at a cross-border airport, who would investigate? In the Wikipedia article Canada–United States border, there are several airports mentioned where the airport straddles the US-Canada border, in some cases the runway even being partially in the United States and partially in Canada.

If there was an accident at one of those airports, what agency would investigate the accident (i.e. the NTSB or the Canadian equivalent (the TSB?))

I'm specifically thinking of an accident that might have begun on one side (such as hitting the runway too hard on touchdown, or a botched approach), and led to something happening on the other side (such as an overrun, or a loss of control on the runway).

By ICAO conventions, the lead investigator is the country in which the accident/incident occurs, unless otherwise delegated. This is fairly common: for example an engine fails in Canadian airspace but they divert to Iceland, it's initially Canada's responsibility.

So you have an argument for whatever country the airplane hit the ground first in your example of a hard landing.

Additionally, ICAO Annex 13 recommends that if the country can't be decided, the state of registry should conduct the investigation:

  5.3 When the location of the accident or the serious incident cannot definitely be established as being in the territory of any State, the State of Registry shall institute and conduct any necessary investigation of the accident or serious incident. However, it may delegate the whole or any part of the investigation to another State by mutual arrangement and consent.

Practically, I would imagine they would delegate the investigation to the country of registry of the aircraft, since that's where the tax dollars were paid.


Does a conversion for wheel to floats require an STC or Form 337? When you add floats to a tricycle gear (e.g. Beaver) do you need an STC to install the TSO'd floats? Or can this installation be approved with a Form 337 for a single aircraft?

This sounds like a major mod, but I will have to see the design data to really able to come to a conclusion. If its a major mod, it has to be done via an STC.


Why do MP and RPM adjustments have to be made in a specific order in constant-speed props? Can anybody explain why do I have to pull back RPM lever first and then pull back MP after climbout when setting up my cruising speed MP/RPM combination. When I make an approach to land I do vice versa - firstly adjust MP (pull back to around 15) and then RPM (set to full).

My training aircraft is Diamond DV20 with automatic mixture control. It has only Throttle and RPM levers.

When setting cruise power, if you were to set the manifold pressure first, it would not remain constant as the RPM is reduced. It would have to be re-adjusted after lowering the RPM.

When adjusting the RPM lever you are actually controlling a governor which maintains a constant RPM. Any change in manifold pressure will not cause a change in RPM.

So adjusting RPM first just makes it a little simpler to set your power.


Yaw rate of a multi-rotor Is there any type of relation that limits yaw rate of a multi-rotor (i.e quadcopter)?
Something like this relation for fixed wing aircraft: $R = \frac{v^2}{g \tan(b)}$, where $R$ is the minimum turning radius and $b$ is the maximum bank angle.

A conventional quadcopter yaws by speeding up two rotors and slowing down the other two.  Once those two have slowed down to zero RPM, it can't yaw any faster.
A formula for this would involve the rotational moments of inertia of each motor+rotor and of the quadcopter's chassis, and the distance from the quadcopter's center of mass to each rotor's axis.  (If the rotors aren't coplanar, it gets messy.)
If those two rotors can be reversed, then they are (inefficiently) thrusting up instead of down.  The limit then may be when they overpower the thrust of the first two, forcing the quadcopter to descend.  Or, if the rotors are spectacularly inefficient when reversed, the limit is when the rotors reach maximum RPM.


What was the first fighter jet that utilised thrust vectoring nozzles? What is the first fighter jet that utilised thrust vectoring nozzles?

Hawker harrier... (actual plane that got used)
https://en.wikipedia.org/wiki/Hawker_Siddeley_Harrier


What is the certification process for a new propeller blade design? I am wondering whether a propeller blade design (say from Hatzell) is submitted to a specific agency for testing and approval, and if these designs are then registered. Looking through Hatzell's product list, I would assume that there are requirements on specifications (material and form), but there must be some allowance in this?
I am also wondering if those documents are then publicly accessible, like a patent would be?

The airworthiness regulations specify functional requirements. They do not specify materials, form, etc. Thefore, the regulations in themselves do not prohibit innovation using new materials or construction techniques, so long as the final product meets the same functional safety requirements as any other design.
It is the manufacturer's responsibility to submit the required proof that a design meets the requirements. I'm not a "propeller guy" but for engines, there is usually a round of pre-certification discussion with the certifying authorities (e.g. the FAA or the equivalent in other countries) to agree exactly how the demonstration of will be carried out - for example whether by modelling and computer simulation, or by testing the finished product. In the case of computer simulation, this will include the method by which the software was verified (i.e. "how do you know that the software actually does what you think it does") and how the model was validated (i.e "how do you know if the model results are consistent with real life.")
The US certification requirements for propellers are Part 35 of the airworthiness regulations, and publicly available here.
Of course the entire process is carried out by humans, who are fallible. The regulations are often updated as a consequence of their failure - and not just for newsworthy major failures like the B787 battery fires of the B737-MAX problems.


Why use thrust reversers? I see a lot of planes using thrust reversers and stopping in the middle of the runway in plane spotting videos, but I don't understand why they do it when they can obviously not use the reversers and still stop safely.

Also, doesn't it waste fuel? Don't airlines prioritize fuel savings over stopping faster? Why are reversers used even when they don't need to be used?

There's a lengthy paper on the NASA web site that investigated this. There's a lot of detail covered in that paper so I'm not going to reproduce any of it here, but the main positive points that emerge are:

Improved safety margins on landing1
Reduced wear on brakes and tyres
Reduced brake temperature leading to faster aircraft turn around.

Even when setting this against additional engine maintenance and fuel consumption, the airlines surveyed overwhelmingly considered that the installation and use of thrust reversers gave a worthwhile return on investment.
1 Thrust reversers are not considered when assessing braking systems for certification.


Why does stall speed decrease when flaps are deployed? I've been told that deploying the flaps on approach enables the following:

Potentially steeper approach
Slows down the plane
Lower stall speed

It's the last one that confuses me.

Given the equation for lift:

$ L = \frac{1}{2}C_l  ρ V^2 S$

with:

$L$ - lift
$C_l$- lift coefficient
$\rho$ - density of the air
$V$ - velocity
$S$ - surface area of the wing

When we deploy the flaps, we automatically change the camber of the wing which does the following:

Increases the surface area of the wing
Increases our current angle of attack

As a result of the change in surface area of the wing, we can decrease our V in the equation and generate the same amount of lift that was needed before flaps. Does that sound correct?

For most practical purposes, your average GA pilot doesn't deal with the AOA because it can't be readily calculated so the POH typically lists the stall speed Vs. However, as I understand it, stall speed is indirectly correlated with our plane's "critical angle of attack".

What I don't quite understand is, why exactly does our stall speed decrease when we deploy the flaps?

Is it because of what I outlined earlier, that we can produce an equivalent amount of lift at a lower speed because of the change in wing surface area?

Your stall speed decreases because the $C_{L_{MAX}}$ of the wing increases with flaps deflected.

Deploying flaps increases wing camber and increases both $C_L$ and $C_D$ at the same AoA and airspeed. Depending on the type of flap, the critical AoA actually reduces: the airfoil with simple flaps deflected stalls at a lower AoA than without flaps.

Slats or leading edge flaps do not change the $C_L$ at the same AoA, but allow for a higher AoA and therefore higher $C_L$ before the airfoil stalls.


Is it possible to modify an AV-8B Harrier II to a supersonic aircraft by applying some modification? Is it possible to convert McDonnell Douglas/BAe Systems AV-8B Harrier II Plus into an effective and usable supersonic aircraft?

May be, by changing the engine or applying some modifications on the air-frame...

There was actually a plan to convert the Harrier into a supersonic aircraft.

The original plan that lead to the development of the Harrier actually had a supersonic variant, the Hawker Siddeley P.1154. The P.1154 was supposed to be a supersonic aircraft using the Bristol Siddeley BS100 engine, which used Plenum Chamber Burning(PCB).

In the BS100 engine, cold bypass and hot core turbine airflows are split between two sets of nozzles, front and rear, like the Rolls Royce Pegasus engine which powered the Harrier. The PCB called for additional fuel and afterburning applied to the front cold air nozzles only, increasing thrust. The PCB increased the thrust from 116 kN to 160 kN.

Source: aeromodelbasic.blogspot.com

However, the P.1154 never saw the light of the day due to various reasons and the subsonic Hawker P.1127 was developed into the AV-8 Harrier.

Later, Hawker and McDonnel Douglas joined to develop the AV-16 concept, basically a plan to double the range and payload of the Harrier. One version of the AV-16 called for supersonic flight too.

Some modifications that were considered in the study for supersonic aircraft were,

A more powerful engine (called Pegasus 15) which used PCB
A bigger fuselage for the bigger engine
A larger 'supercritical' wing
A lengthier, more slender fuselage

This version was called HS.1185, as shown below.

Source: harrier.org.uk

However, in March 1975, Hawker pulled out due to budget cuts and other reasons.

Later, Rolls Royce (ground) tested Plenum Chamber Burning in the Pegasus engine and made some progress in the 1980s. This was supposed to be used in yet another (supersonic) variant of Harrier. One of the versions was the HS. 1205, which had redsigned F-16 type inlets.

Source: harrier.org.uk

However, this also came to nothing and the project was shelved. For a full list, see Harrier site

To answer your question, it would be possible to make the Harrier go supersonic using more powerful engines and modifications to airframe (listed above).


Instrument Currency Scenarios I am currently trying to understand 61.57 and it’s instrument currency requirements for pilots. I have made some 2 scenarios here, and it would be great if I could get some feedback to my questions.

Scenario #1
In this one, I have completed 6 instrument approaches and one hold in March. By counting back, I am current until the end of August 31. My question is how many approaches do I have to do in September to regain my currency if I am trying to act as PIC during IFR in that month?

Scenario #2
This is where I get a little confused. Say I did 5 approaches and one hold. Because I didn’t do a 6th approach, when am I not current? Also what is the staring date of my “6 month grace period?” And alternately, if I did not do anything during the grace period, what is the date when I must do and IPC check?

To answer question #1 (assuming you are flying an airplane):
§61.57 (c)(1) states that:

  Use of an airplane, powered-lift, helicopter, or airship for
  maintaining instrument experience. Within the 6 calendar months
  preceding the month of the flight, that person performed and logged at
  least the following tasks and iterations in an airplane, powered-lift,
  helicopter, or airship, as appropriate, for the instrument rating
  privileges to be maintained in actual weather conditions, or under
  simulated conditions using a view-limiting device that involves having
  performed the following—

  (i) Six instrument approaches.

  (ii) Holding procedures and tasks.

  (iii) Intercepting and tracking courses through the use of
  navigational electronic systems.

As of Sept 1, to maintain instrument proficiency, you will be required to accomplish items i-iii in either an airplane under actual or simulated IMC (you must have at least passed the FAF and begun the final segment of the approach under these conditions for it to count) or have accomplished this in an approved simulator or FTD appropriate to the category and class of aircraft you seek currency in.  See §61.57 (c)(2).  Therefore in Sept, you would be required to accomplish at least 1 IAP in order to remain instrument proficient for that month.  It is suggested at this point to accomplish the required 6 approaches, hold procedures and course tracking in Sept to avoid repeating this process often while still maintaining instrument currency, though this is not required to meed minimum instrument proficiency standards.

Question #2:  see first paragraph of §61.57 (c)(1) - you must have accomplished tasks i-iii within 6 months of any IFR flight where you are acting as PIC.  In scenario #2 you are never current as you have not accomplished tasks i-iii within 6 calendar months for any month shown.  Assuming that you were instrument current in December of the previous year (items i-iii accomplished within that month), you still have a "grace period" to fly the approaches with a safety pilot starting in July of this year.  If items i-iii of §61.57 (c)(1) are not accomplished by December of this year, an IPC will be required as of January 1 of next year to maintain instrument proficiency.

Item iii of §61.57 (c)(1) is a little bit of a contention, but if you are not flying contact or precision radar approaches only to maintain instrument proficiency, you will have used navigational electronic systems to fly these instrument approaches or fly the holding patterns with so item iii can be counted towards with the accomplishment of items i and ii.


How do airports detect drones? Recently, there's been a lot of coverage in the news of drones disturbing operations at airports.
Do airports have a method to detect drones? If yes, how does it work? If it's still an emerging technology, is it something being looked into?

The number of drone sightings near airports is increasing (apart from last year), and frequently this results in temporary interruption of operations at airports, with enormous economic consequences. Most famous example is the December 2018 drone incident at Gatwick  which affected the airport operations for more than one day.

source: EASA Drone Incident Management at Aerodromes(pdf)
I live in Germany, where there have been over 500 sightings near airports since 2015, including several high-impact cases that resulted in the interruption of air traffic at major airports such as Frankfurt airport. Last year, the German Air Traffic Control (DFS) tested a number of drone detection systems  (press release, video link, article in German)  at Frankfurt and Munich airport as part of a market evaluation. Over 600 drone flights were performed, at distances up to 10 NM from the airport. I was involved in the design and execution of those tests and I can share a few observations.
At most airports in the world, drone detection still primarily relies on humans that happen to see a drone fly and inform the police, airport security or control tower by phone or radio.  There are a number of severe limitations to that approach, for example:

Drones are typically small and difficult to see at distances of more than 100 meters. At the scale of a major airport, it is plausible that drones can fly in safety critical areas for a prolonged time without being seen.
Position reporting to those that need to act upon the information (air traffic control, police, security staff) is delayed (phone call, relay of info) and inaccurate ("near the terminal", while the terminal building stretches over more than a kilometre).
Independent verification of the reports is difficult. This opens up the possibility to falsely report drones in an attempt to shutdown the airport.

To address these limitations, drone detection systems are under development, and some airports have deployed them with varying levels of success.
Ideally, a Drone Detection System (DDS) meets these high level requirements:

it detects all drones in the vicinity of the airport
it locates these drone(s) accurately
it does so without delay
it has a very low false alert rate (preferably no false alarms are raised at all)
it locates the position of the remote control (not needed for safety of air traffic, but helps the police in finding the drone pilot)
it does not interfere with aviation communication, navigation and surveillance systems.
preferably the system does not need any supervision; having staff 24/7 dedicated to drone detection is very expensive.

There are many drone detection systems available in the market, at various levels of maturity and some more suitable for airport use than others.  Most, if not all drone detection systems, make use of various sensor types.
There are four categories of drone detection sensors:

Optical / infrared cameras
Radars
Radio detection systems
Acoustic detection systems

Optical / Infrared cameras
One way to detect drones is by cameras. But at the size of a major airport, you are almost always far away from the drone. This means that the camera must be able to zoom, zoom a lot. Some of the best cameras are able to show a clear picture of a 40 cm size drone at 2 kilometres, and a recognisable dot (distinguishable from birds) at several kilometres more. The problem is that with such zoom levels you have a very narrow field of view and so you are never going to detect a drone that you didn't know about before. Therefore cameras are not a good choice as primary sensor, they need to be pointed in the right direction by another sensor.
Once they are pointed at the object of interest, image recognition is often able to classify the object as drone or something else. Some camera systems do a great job optically tracking moving drones, others need manual input.
There are systems that use the camera subsystem for locating the drone, by combining the view angle of several cameras to calculate where the lines-of-sight cross. Laser range finders can also be used in combination with cameras to determine the distance of objects from the camera. Other systems just use the camera as means of classification of an unknown object, not for determining the position.
Having a combination of Infrared (IR) and visible light is beneficial. For one, because at night there is not enough visible light to see the drone, but also the contrast of the drone against the background is often much better in IR than in visible light.
Radars
Radars are an obvious choice of detecting flying objects near the airport. Unfortunately, air traffic control radars and surface movement radars are not able to detect objects of the size of a drone.
There are dedicated drone detection radars on the market, some with a background in military, border control or bird tracking, that are specifically tuned for drones. The maximum distance at which these radars detect a drone of course depends on the size of the drone, but also its shape and the manoeuvring it is doing. There are significant performance differences between radars, and even a good radar may perform badly if it is not properly tuned or sited.
Since these radars have to be very sensitive to pick up a radio reflection from a drone, they see almost everything: planes, cars, cyclists, birds, wildlife, trees moving in the wind etc. One of the biggest challenges for drone detection radars is to distinguish drones from other objects.
False alarms need to be balance with missed detections, which is a delicate process. In the end, a radar will occasionally classify something as drone while it is actually something else.
Radars transmit radio waves that potentially interfere with CNS systems at the airport. Most systems I have seen were operating at frequencies far away from aviation systems, and those that were operating at similar frequencies did not cause interference. (After doing a paper analysis first, the systems were switched on to see if there was interference. This was done at night when air traffic levels were low, and with all relevant people made aware before the system was powered up)
Radio detection
Radio detection systems work by detecting the radio signals transmitted by the drone or their remote control. These systems contain a library of signal properties and they recognise which signals belong to drones. Most systems I have seen cover vast frequency ranges.
The location of the drone (or its remote control) can be found by triangulation or multilateration. Some systems can decode data packages from specific drone models and read the GPS location directly from the drone and the remote control.
Since there are many radio signals transmitted at the airport, the background radio noise is quite high. Also radio signals reflect of large buildings such as hangars and terminals. This has an effect on the performance of radio detection systems. Some systems are severely impacted by the airport environment while other systems are less affected.
Of course these sensors only work if the drone transmits a signal, does so on a supported frequency, and with a protocol known in to the signal library. None of the systems I've seen was able to detect all drones in the test.
Acoustic
Finally there are acoustic detection and location systems, operating in the acoustic frequencies band.
These are passive systems that rely on detecting the noise characteristics of drones (specifically their propellers). They have a very limited range (up to ± 300 m) and are  susceptible to background noise, which degrades their performance.
Due to these characteristics the use of acoustic detection systems in airport environments is very limited. If there is any use at all, they could perhaps be used as a coverage gap-filler, for example between hangars.

A good drone detection system will make use of various sensors types and integrate their outputs using a multi-sensor data fusion and tracking system to a single, reliable situation picture. In every class of sensors there are some really promising products, but a  strong integration of a range of good sensors is hard to find. This market is developing quickly and I expect that the situation will be very different in a few years.


Is roll control achieved exclusively with spoilers on an A320 on final approach? As a private pilot I have zero knowledge about how big jets are being flown so I was a little surprised to see that smaller Airbuses (A32x) only use spoilers for roll control on final approach.

During the initial phase of the approach ailerons are being used but at a certain point they stay in the neutral position and roll is controlled by the spoilers.

When I mentioned this to the pilot when exiting the aircraft he was a little annoyed by the question and he said I must have seen it wrong. I've seen this numerous times since then so I'm pretty sure I wasn't mistaken.

Is this actually how the spoilers are used on approach, and if so, why?

Downward aileron travel must be restricted when flaps are deflected. Since flaps change the local incidence on the flapped part of the wing, the outer wing will experience an increase in its local angle of attack. The increased suction over the inner wing will accelerate not only the air flowing over the inner wing, but also that air which will flow over the outer wing as well. Left to itself, the outer wing would stall if powerful Fowler flaps are deployed on the inner wing.

Adding slats will push the stall angle of attack up, and now the outer wing is back in business. However, if the aileron is deflected downwards, the stall angle of attack is reduced again. The outer wing could stall simply due to the aileron deflection!

I do not need to point out that an asymmetric stall on approach is extremely undesirable.

Now roll control could still be achieved by only moving one aileron upwards. But this would decrease the induced drag at the wingtip at low speed. Remember, at low speed the induced drag is dominant (it is proportional to the inverse of the flight speed squared), so the drag change due to an aileron deflection will cause an inverse yawing moment at a time when the pilot wants to keep the aircraft lined up with the runway.

Using the spoilers instead will incur a higher lift loss for the same rolling moment, but will also increase local drag, which creates a helpful yawing moment into the opposite direction of the one created by upward aileron deflection. Also, the change in local lift from spoiler deflection is much greater once the flaps are fully deployed. This means that spoilers are much more effective for roll control in the landing configuration than in the take-off or cruise configurations.

THAT's why spoilers are preferred for roll control on approach. I am surprised, however, that you observed no aileron motion at all. Normally, the ailerons are still used, but with a much restricted travel range, as @DeltaLima mentions in his answer.


Who is responsible to check for radar-altimeter disagreement? I have searched the site but failed to find a similar question. I know that modern civil aircraft are equipped with multiple radar altimeters. Question is: In the unlikely event of those outputting different values, whose responsibility is it to determine that the values are different? Is it the pilots, or the aircraft is required to warn? Does it depend on the aircraft type?

The question is mostly concerned about this accident: https://en.wikipedia.org/wiki/Turkish_Airlines_Flight_1951

That depends on regulations but broadly speaking it is ultimately always the Pilot In Commands responsibility to make sure all instruments are functioning properly.

  § 91.3 Responsibility and authority of the pilot in command.

  (a) The
  pilot in command of an aircraft is directly responsible for, and is
  the final authority as to, the operation of that aircraft.

Now your probobly thinking "But if the autopilot is flying then who's really in command" well its the opinion of the FAA that even if the autopilot is taking care of everything you are still responsible for the aircraft and for making sure everything is working correctly. This question covers that more in depth.

As for things like warnings, alerts and auto-disconnects that depends on the specific system in question and the desired failure mode. For example if you have 3 radar altimeter inputs to an autopilot/auto-throttle the system can reasonably detect if one is erroneous and use the other two. There is lots out there on triple redundancy which is the basis of this.

What if the aircraft starts barking alarms at you saying "TOO LOW!, GEAR!" how can you even check that aside from cross checking your altimeters?

You can check your GPS altitude if available, while not a legal source for navigation you bet its a good backup.
If you are flying a stabilized approach (and in a plane like that you really should be) you should, at any given time know generally where you are on the approach and if you are on the glide slope (when available) you can gain a pretty good idea of your altitude.
In VMC, just look out the window!


RATO used to get civilian aircraft out of airports with inadequate runway length? There are plenty of stories of large aircraft accidentally, or emergently, landing at airports that don't meet the plane's runway length requirements.  The only solution to getting them out that I've read is stripping them to the point that some authority deems them light enough for takeoff on the short runway.

What I haven't read of are the use of any measures to increase the thrust available for the takeoff run.  The first idea that came to mind was the use of RATO.  Is this possible with civilian aircraft?  Is it done?  If not, why not?

(My second thought was to temporarily install water injection.  But then I figured that by the time a system like that could be tested and approved the plane could have been disassembled for overland shipping.)

The DH Comet was equipped with RATO.

The Fairchild SA226 had a RATO bottle in the tail to improve single engine performance.  As I recall, removing the spades on the main landing gear eliminated enough drag to render the RATO unnecessary.

Boeing built twelve "727-200/JATO" variants for use by Mexicana Airlines.

  The rocket installation was intended for emergency use only when
  flying out of a hot and high airport at maximum gross weight. Without
  the JATO, the aircraft would have to be payload restricted to account
  for the need to reach a safe altitude in the event of an engine loss
  after committing to takeoff. By having the JATO provision, Mexicana
  could operate its 727-200s at full payload. In the event of a loss of
  engine at past V1, the JATO unit would fire and allow the
  heavily-laden jet to reach a safe altitude and get aerodynamically
  cleaned up.

This old video shows a demonstration of that scenario with one of those special 727-200s:

  The JATO provision was eventually made obsolete by later developments
  in the JT8D engine that featured "APR" automatic power reserve. It
  sensed a power decrease from one of the engines failing on takeoff and
  automatically boosted the power to the remaining two engines by a
  significant margin.


How much is the minimum safe distance between two planes in-flight? According to FAA regulations, how far should a commercial jet be away from another plane in-flight?

How about military planes? They get really close when in formations... Is there a minimum distance?

The question specifically asks about FAA regulations, so I will try to provide specific citations.

Summary

For a commercial airliner (as the question asked), separation will usually be at least 3 miles laterally, or 1,000 feet vertically. In the enroute environment -- at higher operating speeds above 10,000 feet and based on the type of Radar and distance from the antennae -- a 5 mile rule is applied laterally. This is true in most but not all situations. There are exceptions: see below.

Note also the "or": it is allowed (and in fact rather common) for two jets to cross paths at the same moment, with one 1,000 feet directly above the other. The 3 miles is only required if the jets do not have at least 1,000 feet of vertical separation.

Pilots

The regulations do not set a specific distance that pilots flying under VFR must maintain from each other. There's just the following blanket statement:

§91.111, Operating near other aircraft: (a) No person may operate an aircraft so close to another aircraft as to create a collision hazard. (b) No person may operate an aircraft in formation flight except by arrangement with the pilot in command of each aircraft in the formation.

In addition, there is the catch-all "careless or reckless operation" clause:

§91.13, Careless or reckless operation: No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.

Pilots flying under IFR are subject to the same kinds of rules, and are still responsible for maintaining separation from other traffic when in visual conditions. The rules are the same: the pilot is given the discretion of deciding what a "safe distance" is.

Actual distance limits only come into play for the air traffic controllers, who are responsible for providing minimum separation distances between IFR traffic.

Air Traffic Controllers

The set of FAA regulations for air traffic controllers is FAA order 7110.65. These regulations are quite complicated and there are lots of exceptions and corner cases for various scenario. I'll try to give only the most basic rules here, but note that a full reading of 7110.65 would be necessary to understand all the minutiae.

Vertical separation:

4-5-1 Vertical Separation Minima. In general, 1,000 feet for IFR traffic (with exceptions).
7-7-3 Separation, 7-8-3 Separation, 7-9-4 Separation. 500 feet for VFR aircraft (with exceptions).

Lateral separation:

5-5-4 Minima. In general, 3 miles in the terminal environment and 5 miles in the enroute environment (with exceptions).

Things get more complicated in the following scenarios, and exceptions apply to them:

Oceanic environment (7110.65 Chapter 8).
Approach environment, especially at large/busy airports with parallel runways and special approach monitoring radar. (7110.65 Chapter 3, Sections 9 & 10).
Nonradar environment (anytime the controller is not able to "apply radar separation") (7110.65 Chapter 6).

For these scenarios the regulations are rather lengthy and complicated, so I would simply refer you to Order 7110.65 in general.

Military

When flying in airspace controlled by ATC, military pilots must follow the same rules as civilian aircraft. ATC will keep them separated according to the rules above. However, there are two main exceptions to this.

When flying in formation, such as for training, aerial refueling, or intercepting aircraft, they are much too close for ATC to provide safety. In these cases, ATC will allow the pilots to fly under MARSA (Military Assumes Responsibility for Separation of Aircraft). In the case of interception, if the fighter pilot is not able to see the aircraft they are intercepting before the minimum separation, they must break off. They can only fly under MARSA if they can actually ensure visual separation.

The FAA can also define an MOA (Military Operations Area). Once military aircraft enter this area, ATC is no longer responsible for them and they are under MARSA. When the aircraft leave the MOA, ATC resumes control. They can still be under MARSA for a formation, but ATC will keep that formation separated from other traffic.

In tactical formation flying, there is no limit. The Blue Angels routinely fly with only 18" separation between the wing tip and the cockpit canopy. Combat aircraft fly a variety of different formation patterns at varying distances, and pilots are extensively trained on each pattern and what the appropriate/safe distances are. (For example, a fingertip formation allows for much closer flying than a line-abreast formation.)


What is the reason for the sudden increase of French published accident and incident investigation reports in 1997? The number of accidents and incident reports available online at BEA has changed dramatically between 1996 and 1997:

There was an average of 2 reports per year until 1996, and an average of about 125 reports per year then.

As it is unlikely that there was a sudden increase in the number of events, something in the French legislation and/or the policy of BEA must have changed.

What was the actual reason for this change and sudden increase?

A related event could be the adoption by The European Community Council of:

Council Directive 94/56/EC of 21 November 1994 establishing the fundamental principles governing the investigation of civil aviation accidents and incidents [Official Journal L 319 of 12.12.1994].

European directives must be transposed in a country law and adopted by the country Parliament. This has been done for France in 1999, with the law "Loi n°99-243 du 29 mars 1999". This law actually modified the regulations related to civil aviation (Code de l'Aviation Civile, Livre VII).

Most of the articles added new obligations to the State and, from what I understand, no obligation for investigation or for releasing a public report existed before, so it may have been at the discretion of the Transport Ministry.

Two articles are of interest:

  Obligation to investigate every accident or serious incident with the
  aim of preventing any reoccurrence thereof.

and

  Obligation to publish an accident (or incident) report, containing
  safety recommendations where appropriate.

Regarding your count on the BEA site, this info is lacking some important details, for instance:

The hang-glider or ultralight aircraft are relatively new, and we don't know how it has changed the number of incidents/accidents. It's well known that in France hang-glider accidents where not investigated by the BEA because hang-gliders are not subject to an airworthiness certificate. It seems this has changed, but it's not clear to me.
You don't provide a list of events, only a list of reports, which as demonstrated in the Rio-Paris accident can multiply by 4 the count because of the 3 interim reports (this is now mandatory to publish a preliminary report each 12 months if the final report cannot be completed within the first year).


How does a nose down stall occur? Imagine I am in a straight down dive, at terrific speed, and if I wanted upward sharp turn and got the stick back far enough, would I stall (or spin)?
Does normal laws apply when the aircraft nose is vertically down?

Does normal laws apply when the aircraft nose is vertically down?

Yes, always. Physics doesn't care much about the aircraft's attitude and cannot be switched into different modes.

While the nose points straight down, a stall is hard to achieve since no lift is needed - the weight is balanced by drag and inertia. However, when you pull abruptly and start to pitch up, the aircraft can stall while the overall attitude is still partially nose-down. If the pitch inertia is small, the rotation about the lateral axis will increase the angle of attack quickly while the flight path is still more or less straight down. At this attitude the stall angle of attack can be exceeded and the aircraft will stall. This is not unusual - pilots are routinely warned not to pull too hard after ending a spin. And yes, entering the next spin is entirely possible. This is especially likely when the sideslip angle has not been set to zero after ending the first spin. In most cases, this second, unintended, spin is in the opposite direction from the first spin because the pilot maintained his anti-spin rudder deflection for too long.

There have even been cases of supersonic spins.

A few details will help, though. A high pitch rate will delay the stall to an angle of attack up to 50% higher than the stall angle at zero pitch rate. Flying faster will increase the Reynolds number, so stall is slightly delayed when compared to the stall conditions at 1g. Both effects, however, will increase the wing root bending moment, and pulling out of a dive will run a real risk of breaking off the wings when you pull too hard.


How much the Concorde secondary nozzle contribute to thrust? Does anybody knows how much did secondary nozzles bucket type on the Concorde's engines contributed to the increase of thrust during take of and sub-sonic flight? This is picture of the nozzles  and here is air flow during take of  all images from this link https://www.heritageconcorde.com/variable-exhaust-nozzles  source states that secondary nozzles reduces noise, as well as improved engine performance. I suppose performance means thrust?

This source suggests at subsonic speeds, the secondary reduces drag but doesn't contribute significantly to thrust.

  At all times below Mach 0.55 – on the ground, take-off and initial
  climb – the buckets are set to 21 deg., forming a ‘jet-pump’ gap at
  the nacelle trailing-edge. The high speed jet exiting from the primary
  nozzle draws-in atmosphere air from above and below the nacelle to
  eradicate a source of base drag that would otherwise exit between the
  nozzles.


How do you check for soft or muddy ground on a backcountry landing? I've been curious about backcountry landings, but one part of the practice still seems to based more on instinct and experience than on well-explained processes.  I've read about making sure the strip is long enough, looking for power lines, avoiding campers and wildlife, checking your GA area, and other stuff, but what about mud?

Everyone seems to say that you do a pass close to the ground or a touch and go, but after that they seem to just make an educated guess based on how the ground looks and feels.  Given that a misjudgement here could be disastrous and prevent you from being able to accelerate to a take off, what's the actual process and list of things to consider to make sure you're landing in a good airstrip and not a foot of deceptive-looking mud?  Does a touch-and-go give you enough information or do you need to carefully consider things like recent rainfall, watershed shape, etc.?

Image from this video.

Usually, it's a combination of making a couple of low altitude passes over the landing site and visually observing the conditions of the terrain in the area and personal experience selecting a site based upon the knowledge of the local terrain and soil, recent weather conditions, local flooding, ground vegetation, etc.  It's a bush pilot skill based partly on intuition as well as instruction from senior airmen with experience flying in the area.


Which type of chevrons is applied on the B787? Which type of chevron nozzle was applied on the B787? I'm really confused about whether if it's sawtooth or sine wave.

Strictly speaking, its neither. Chevrons don't resemble either a sawtooth wave or a sine wave in mathematical terms. The closest is that it is a smoothed (modified) saw tooth form.

Image from compositesworld.com
Though the initial research used a serrated nozzle in saw tooth form, the final commercial application differs from it. However, the chevrons are repeatedly noted as having a saw tooth form, by NASA among others:

One of the most recent noise-reducing technologies shepherded through the research process by NASA and now making a difference on commercial jet engines is chevrons.
Chevrons are the sawtooth pattern seen on the trailing edges of some jet engine nozzles.

So, it's a choice between saw tooth and sine wave, its better to go with the former.
GE's patent for the Chevron describes the shape as triangular with qualifications:

... each of said chevrons being triangular in configuration
... said chevrons having a compound concave contour both axially between said bases and apexes and laterally between said trailing edges..

Image from Chevron exhaust nozzle for a gas turbine engine


What do the remarks of this metar mean? "RMK SC1SC5AC1 AC TR SLP116" Here's the current METAR from CYYZ:

CYYZ 140700Z 26011KT 15SM FEW025 BKN045 BKN080 M05/M08 A2984 RMK SC1SC5AC1 AC TR SLP116

Does anyone know what each section of text of the remarks section means?

So the posted parts means the following:

  RMK    = REMARKS
  SC     = Stratocumulus Clouds
  1      = 1/8 (0/8 = no clouds / 8/8 = overcast clouds)`
  SC     = Again Stratocumulus Clouds
  5      = 5/8
  AC     = Altocumulus Clouds
  1      = 1/8
  AC TR  = Traces of Altocumulus
  SLP116 = Sea Level Pressure of 1011.6 mBar (just add a 10 before the given number and divide by 10)

Cloud Types:

Ac-Altocumulus
As-Altostratus
Cb-Cumulonimbus
Cc-Cirrocumulus
Ci-Cirrus
Cs-Cirrostratus
Cu-Cumulus
Fc-Fractocumulus
Fs-Fractostratus
Ns-Nimbostratus
Sc-Stratocumulus
St-Stratus

(Source: Wikipedia)

Octas:

(Source: Navcanada)


How much horsepower to weight is required for a 1:1 thrust ratio? I did a math experiment today, but wanted to verify the results.

I wanted to figure out how much horsepower is needed for an aircraft to hover in midair with no wings/lifting body (1:1 thrust/weight).

So to start I found that a unit of horsepower is the amount of work required to move 550 pounds 1 foot every second.

Then I wanted to find out how many foot/pounds of force gravity places on 1 pound of material. With a static acceleration of 9.8m/s (32f/s), it seems that 1 pound has 32 foot pounds of gravity acting on it.

Now for my equation:

Hp = 550 (f/p)
Gravity = 32 (f/p)
Hp / Gravity = 17.1875

So it seems 1 hp can hold 17 pounds in the air, but to incorporate propeller inefficiency, I just used 70% of that. This leaves me with 1 hp being able to vertically lift 12 pounds when accounting for the inefficiency of a propeller.

Is this a relatively correct assumption? I know thrust changes as the aircraft accelerates, but in a hovering state the acceleration is 0 f/s as a whole. So will these numbers apply?

Edit: I initially used a specific ultralight to prove how high the numbers were, but I used wrong data from the spec sheet. So I removed that reference from my question.

Horsepower cannot be converted into thrust without knowing the speed at which this is done. In your case this is zero, so the special equation for static thrust applies. With $T$ for thrust, $P$ for power, $\rho$ for air density and $d_P$ and $\eta_P$ the propeller diameter and efficiency, respectively, this is $$T_0 = \sqrt[\LARGE{3\:}]{P^2\cdot\eta_{P}^2\cdot\pi\cdot d_P^2\cdot\frac{\rho}{2}}$$
With this equation you cannot simply say how many HP are required to lift that many pounds; instead, you need to add the propeller geometry. If we assume a very efficient propeller with a large diameter, we can combine blade size and mass into disk loading – here 8.5 kg/m² or 41.5 lbs/ft² is a good value. Now we can write, using standard atmospheric density at sea level:
$$\frac{T_0^2}{P^2\cdot\eta_{P}^2} = \frac{2\cdot1.225\,\frac{\text{kg}}{\text{m}^3}}{8.5\,\frac{\text{kg}}{\text{m}^2}\cdot g} = 0.0294\,\frac{\text{s}^2}{\text{m}^2}$$
assuming for simplicity a 1 m propeller diameter. If we now use 70% for propeller efficiency, as you did in the question, the power to mass ratio becomes 8.33 Watt power per N of weight force or 81.7 Watt per kg. Converted into funny units this is 0.05 HP per pound of mass. 1 HP can hold 20 lbs in the air, but that will not be enough to even spin up the propeller unless it can adjust blade pitch. For practical usability I recommend to double that number. If the propeller diameter shrinks and disk loading increases, efficiency will drop and the power required will rise.


What is the origin of the waypoint WORUM (neayby Houston)? I'm reading the book "Skyfaring".
There is a sentence "TQELA, WORUM, CRVZA (beer), CARNE (meat),
QUESO (cheese) for the city’s cross-border
culinary traditions"

What is the origin of WORUM?
Is it related to some food?

It's worm, as in tequila worm:

  Perhaps even more popular than the actual tequila is the worm crawling around the bottom of the bottle. But the worm, or gusano, actually originated with tequila's “lower-quality” cousin, mezcal, largely as a marketing ploy. The gusano is the larvae of a type of moth that lives on the agave plant.—huffingtonpost.com


Why does one of the Boeing 777's spoilers deploy differently from the rest? There is one spoiler that isn't totally deployed. Look:

Source: Airliners.net

And also on this other picture:

Source: Airliners.net

Why aren't they deployed at the same angle?

Roll control is provided by 2 flaperons, 2 ailerons, and 14 spoilers.

Spoiler panels 4 and 11 are controlled mechanically rather than electrically. They are driven directly from control wheel deflections via a control cable. In case of compete electrical loss, limited roll control is available until the electrical system is restored.  This probably explains the difference in rigging.

Spoiler panels 4 and 11 are also used as speedbrakes, both in the air and on the ground. The speedbrake function for this spoiler pair only has two positions: stowed and fully extended. The speedbrake commands for spoilers 4 and 11 are electrical in nature, with an ACE (actuator control electronics) giving an extend or retract command to a solenoid operated valve in each of the actuators.

Once that spoiler pair has been deployed by a speedbrake command, there is no control wheel speedbrake command mixing, as there is on all the other fly-by-wire spoiler surfaces.

Boeing B-777: Fly-ByWire
Flight Controls


What are the contributors of lift-dependent drag? If someone wants to plot the drag polar of an aircraft, we have

$$ C_D = C_{D,o} + C_{D,i} $$

form/skin drag plus the induced.

However, as I feel, form drag and probably skin friction drag (parasite drag) also lift-coefficient dependent. For example,
as the angle of attack increases, the shape will be different and so the parasite drag will be different  .

Does that increment in parasite drag considered in the induced drag term because it's lift-dependent?

Where should we add this contribution of drag in terms of
the drag polar formula? in the lift-independent part or in the lift-dependent part?

If someone wants to plot the drag polar of an aircraft, we have
$CD=CD_o+CD_i$
form/skin drag plus the induced.

Something to note here is that there are more than one mathematical "model" to describe the relationship between lift and drag. What you have described is known as "Parabolic Drag Model", where we consider a parabolic relationship between lift and drag.

Does that increment in parasite drag considered in the induced drag term because it's lift-dependent?

Imagine an airfoil drag polar (2D polar so there is no induced drag taken into account). If this polar can be closely modelled as a skewed parabola (skewed due to camber) then $e$ in the induced drag term $\frac{CL^2}{\pi*e*AR}$ takes care of this drag rise up to some extent. This kind of a 2D airfoil drag model is usually applicable for mostly turbulent airfoils such as NACA 4 series in high Reynolds number flows.

Where should we add this contribution of drag in terms of the drag polar formula? in the lift-independent part or in the lift-dependent part?

Above said, mostly laminar airfoils such as FX163 or NLF series airfoils which operate in relatively low Reynolds numbers does not fit the above model fairly well. These airfoils exhibit a linear relationship between lift and drag at-least for the operating range which is not affected by massive separation. So in this kind a scenario its best to modify the above equation with a linear term as well.

$CD=CD_o + k CL + \frac{CL^2}{\pi e AR}$


Would in flight engine failure of the passenger jet be reported? A passenger plane with two or more engines would probably land without big difficulties with one engine inoperative.

Would the passengers be notified, asking to take the brace position during landing?
Would such event have an accident investigation report?
Would this report later be available for general public, as are the reports of various significant air disasters?

To certify any Commercial Jet Plane worthy of flying,

  a 2 engine plane must be able fly with just 1 engine running

  a 3 engine jet like the Lockheed L1011 or DC10 with just 1

  a 4 engine jet with just 2. Although not a requirement, the 747, A380  could fly on one engine but are not be able to maintain the cruise altitude indefinitely and will eventually become  long-range gliders so they must land immediately.

  The landing approach and procedures vary on how much the plane weighs prior to landing.

Would the passengers be notified, asking to take the brace position during landing?

Brace Positions are only assumed on impact emergencies, like crash landings. Losing an engine doesn't mean you are going to crash so no brace will be assumed.

Would such event have an accident investigation report?

There will be an investigation but if the plane landed safely it wont be filed as an accident. The plane plus the engines passed its FAA, EASA, etc Certification so it may be a number of reasons why the engine malfunctioned, like bad maintenance, faulty parts etc.

This is an extract from wikepedia -- http://en.wikipedia.org/wiki/Turbine_engine_failure

  Shut downs that are not engine failures

  Most in-flight shutdowns are harmless and likely to go unnoticed by
  passengers. For example, it may be prudent for the flight crew to shut
  down an engine and perform a precautionary landing in the event of a
  low oil pressure or high oil temperature warning in the cockpit.

  However, passengers may become quite alarmed by other engine events
  such as a compressor surge — a malfunction that is typified by loud
  bangs and even flames from the engine’s inlet and tailpipe. A
  compressor surge is a disruption of the airflow through a gas turbine
  engine that can be caused by engine deterioration, a crosswind over
  the engine’s inlet, ingestion of foreign material, or an internal
  component failure such as a broken blade. While this situation can be
  alarming, the condition is momentary and not dangerous.

  Other events such as a fuel control fault can result in excess fuel in the engine’s
  combustor. This additional fuel can result in flames extending from
  the engine’s exhaust pipe. As alarming as this would appear, at no
  time is the engine itself actually on fire. Also, the failure of
  certain components in the engine may result in a release of oil into
  bleed air that can cause an odor or oily mist in the cabin. This is
  known as a fume event. The dangers of fume events are the subject of
  debate in both aviation and medicine.

Would this report later be available for general public, as are the reports of various significant air disasters?

It depends, as you read above the flight deck crew may shut down an engine because of low oil pressure. Would that have significance of becoming public knowledge?

However as an example, if the problem of an engine failure escalates to the manufacturer then the airline may ground the fleet and that then becomes public knowledge and the reason why because the airliner would give media statements.

It all depends on the severity of the problem. Like this one that got reported --- http://www.b737.org.uk/accident_reports.htm

  20 August 2007; B-18616, 737-800, 30175/1182, FF 11 Jul 02, Okinawa,
  Japan:

  Engine #1 caught fire shortly after docking destroying the aircraft.
  No fatalities. The cause was the detachment of a slat pylon bolt which
  fell off and pierced the fuel tank which leaked and ignited. According
  to the JTSB the washer that should have held the nut in place probably
  detached during maintenance 6 weeks before the incident. The design of
  the nut has since been changed to limit the likelihood of detachment.


What navigation publications are required when flying from KSNA to KAVX? If I'm flying to Catalina Island (KAVX) from John Wayne Airport (KSNA), which is 30 miles over water, what navigation publications are required to be on board for the route of flight?

You'll need the navigation publications appropriate to your flight.  If you're flying under VFR, then you'll need the appropriate sectional chart, and should have airport diagrams.  You'll need to be aware of the airspace and how to communicate and navigate in it.  You should have taxi diagrams.  For VFR flying, I've always made a practice of having the appropriate terminal charts, sectionals, WAC's, and airport publications.  I have also always carried IFR enroute charts, as these provide useful information about communications enroute, as well as airspeed and navigation information.

These days, a subscription to Foreflight usually covers it, and I've flown all over using little else.

If flying IFR, you'll need appropriate IFR publications.

You should have a copy of the airport information, airport facility directory, or equivalent, etc.  If you don't carry and reference the current data, and end up getting busted, you'll have little to fall back on as an excuse.


How did aircraft flying in formation use flares to communicate with each other? The image shown in the question Which aircraft had such a luxurious-looking navigator's station? (also below) shows a flare gun with several flares.

A comment below the question says:

  On the bonus question: flares were often used for communication between planes, sometime even when all planes in the formation had radio (to the point that new flare systems had to be designed for high altitude pressurized bombers at the end of WWII). Given the number of cartridges right under the flare gun, I think it is indeed for communication and not just for distress signaling, so you would probably want it readily available.

How common was it for this to be used, and how effective was it, especially during the day? When would it be used, and what would the flare indicate or communicate?

cropped and sharpened from https://i.stack.imgur.com/K0cO2.jpg used in here.

 Source.

This break down of a B24 Missions seems to outline the usage of flare on a mission well:

Assembly
After takeoff, a pilot usually kept on a straight course for about two
minutes, in part because the B-24 was not very maneuverable until it
had gained speed and altitude. The pilot then climbed at a
predetermined rate (about 300 feet per minute at 150 mph Indicated Air
Speed) to the assembly area assigned to the 392nd. A ship, usually
41-23689, Minerva, orbited a radio beacon at the prescribed altitude and fired designated colored flares to signal 392nd BG a/c. The
specified assembly altitude was based on cloud conditions. Clear skies
meant assembly could be at 5,000 to 10,000 feet. Often, though, pilots
had to climb to 20,000 feet or higher to get above the clouds, relying
on instruments to maintain headings, speeds and timed legs.

When on the actual mission...

Mission Routes
Routes were scheduled to avoid major flak areas as much as possible
and to mislead enemy fighters about the intended target. Each route,
then, included several course changes until the Initial Point (IP) was
reached. The IP was usually an unmistakable landmark, both visually
and on radar. At the IP, the lead plane alerted the formation to turn
toward the target by signal flares or by opening its bomb bay doors.
All other planes promptly opened their own bomb bays and made the turn
while simultaneously aligning in trail. On the bomb run, Groups were
usually two to five miles apart.

Then on the way home:

Wendling
By this time, the formation was down to a few hundred feet. While in the landing pattern circuit, the engineer confirmed the
wheels were down and locked. Planes with wounded aboard or severe
battle damage fired two red flares and landed first.

A few sources cite the M8 Flare gun being the one that was likely used during WWII. There is a discussion that covers some of the colors and cartridges here.

This book which talks about flare usage quite a bit, discusses a lot of the ground <-> air communication done with flares:

The increasing amount of airfield bombardment led to the establishment
of false aerodromes. These false fields gave the appearance of active
aerodromes by day and night in the hope that enemy bombers would be
fooled into releasing their bomb loads a mile or two from the real
airfields. At night, active fields remained concealed in darkness
until an airplane was properly identified through flare signals.

It goes on to say:

Signaling devices were necessary for communications with the ground.
The flare gun provided the most basic means of signaling. The Allies
used the Chobert, more commonly known as the Very pistol. British and
American night squadrons used three different colored Very cartridges,
red, white, and green. Each cartridge had a different milling along
its metal base flange so that aviators could quickly feel for the
correct shell in the dark.

Some flares were also used for ground illumination

Illuminating flares were developed for target acquisition and landing
assistance. One of the most important was the Holt wingtip landing
flare. Useful in an emergency or under inadequate field lighting
conditions, the Holt flare was ignited electrically (by dry cell
battery) from a push button in the cockpit. Typically the flare would
burn for a maximum of two minutes at up to 20,000 candlepower.


How can someone get clarification from the FAA on the intent of a regulation? How does someone request a letter of interpretation from the FAA if a particular situation surrounding a regulation or policy is not clear?

Generally there are two ways to get an interpretation of regulations from the FAA.

The first (less-formal) way would be to call or email your local FSDO with your question -- they will usually get back to you pretty quickly with a verbal opinion or an email (on the two occasions I've contacted the local FSDO I've gotten an answer within 1-2 days).

Interpretations from your local FSDO are essentially valid within their coverage area (if you get an opinion from Farmingdale in New York the folks in Los Angeles, California may not honor it).

The second (very formal) way is to write an actual on-paper letter to the FAA's office of the chief counsel requesting a legal interpretation of some specific regulation(s) and mail it to their office:

Office of the Chief Counsel
800 Independence Avenue SW
Washington, DC 20591

This request should be as specific as you can make it, for example:

  May the FAA hold a person who performs maintenance on an aircraft responsible for the proper performance of that maintenance under 43.13(a) and (b) when that person was not the person who made the maintenance record entry required by 43.9?

If your request is inspired by some ongoing action (say a warning letter) you should reference it and include a copy to help speed things along.

The FAA's legal staff will chew on the request for a while, and eventually produce a legal interpretation (the linked interpretation, which answers the example question above, took about 5 months, and it is a relatively simple interpretation - The more complex the regulation that needs to be interpreted the longer it will probably take).

Counsel interpretations are organization-wide precedent, and on occasion they can overrule opinions/decisions from a FSDO. They are effective "The Final Word" on how regulations are to be applied (unless overturned in court).


What aircraft maintenance logs should I have available for a private checkride? I’m about to do my Private Pilot Checkride in my own plane. I’ve been trying to find a list of things that will be needed for the Checkride in the logbook, as I had thought it was an updated annual, ELT battery is within time, and all ADs are complied with. My CFI said he thought it needed other things like VOR checks in the last 30 days and transponder checks and more.  Is there a specific part in the FARs or somewhere that lists out exactly what it needed for VFR day/night flight as far as maintenance logs to keep the plane airworthy? I’m not talking ATOMATOFLAMES, but maintenance logs (aircraft, engine, prop) specific?

The answer to your question is in the Private Pilot ‒ Airplane Airman Certification Standards

  Task   B. Airworthiness Requirements References   14 CFR parts 39, 43,
  91; FAA-H-8083-2, FAA-H-8083-25

You should read and understand CFR §91.203, §91.205, §91.207, §91.209,  §91.213, and §91.215.

The DPE will want to know that the aircraft is legally permitted to operate in the airspace that you will be taking your practical test in.

They will check that the AR(R)OW documents are in the aircraft—Airworthiness Certificate, Current Registration, probably not Radio License since it is not likely your test will take you to another country, Operating Limitations, and current Weight and Balance. Operating Limitations would include placards; any documents required by STCs for installed equipment e.g. GPS or autopilot; and for newer aircraft flight manual (AFM) for that aircraft.

They will check the logbooks to make sure that the aircraft is legal maintenance-wise. Annual Inspection and 100-hour AD items obviously. Some 100 hour items can be signed off by the pilot e.g. Bendix switch check while others need to be signed off by an A&P e.g. muffler shroud inspection. Most A&Ps use a computer system that prints out a list of ADs and when they were complied with. It is a good idea to put a sticky-note on the annual and recurring ADs pages so you don’t waste time looking for them.

ELTs are usually checked at annual time, but they can get out of sync with the annual and they will check that. If you are taking the test in an aircraft with an electrical system, the transponder must be working and checked in the last 24 months. You don’t need a pitot-static check for a private checkride, but if you are going to file and fly IFR flight plan—as you will for your instrument rating you will need one.

If there is any inoperative equipment e.g. LORAN then it must be logged and marked.

ADSB isn’t required yet, but after January 1, 2020 if you fly in the covered areas your aircraft must have it installed. CFR §91.225.

On an instrument check-ride the DPE will want to see that the database is current if you have a GPS and that the VOR check has been done.

They will also check to make sure that your medical is current and that you have all of the endorsements necessary for the aircraft e.g. complex, high performance, tailwheel.


The numerical method used by F.X Wortmann for inverse design Apologies in advance for a long-winded question.

Can someone who understands some airfoil theory and German language please help me understand the method outlined in this paper from Prof. FX Wortmann.

I have back-quoted what I've got so far with Google translate but it doesn't appear to be accurate especially with the given context.

  If you reduce the work of E. Truckenbrodt [8] to calculate the profile shape from the speed distribution to the mere calculation, you come to the following series of formulas: 1. At the n positions of the dimensionless profile depth

  Table 3 contains the constants bm for N24. The following applies to
  the pth approximation step

  Table 3 contains the constants is the slope of the tangent to the
  profile shape and results from

  The values amn are listed in Tables 4 and 5. The values amn for
  even-numbered m - n disappear. The table amn with m = 2, 4, ... is
  therefore used to calculate the values y, 'for n = 1, 3, ... A good
  estimate of the correction values x, which greatly abbreviates the
  approximation, can be obtained if one starts from the examples already
  calculated. Eq. (A2) includes (N-1) 2 232 529 operations that can be
  performed in about two hours with a simple electrical calculator. 2.
  The skeleton line of a curved profile, in which the speeds on the top
  of the profile (index o) and underside (index u) are different, is
  calculated

  Table 3 contains the constants dmn. The value for z is already known
  from the above calculation. This skeletal line still needs a minor
  correction, which is reflected in

  The constants pa, qn are given in Table 6. The values y, and are
  according to Eq. (A2) and (A5) known. The angle of attack a is
  calculated

  The values d are also shown in Table 3. The final coordinates of a
  curved profile then result from (A11) = y (s) 4ys) + y (). The
  positive sign applies to the top of the profile.

Also with my very limited understanding with possibly incorrect translation, I've got the following questions so far

What is the $(t)$ in $\bar{y}_n^{(t)}$ in (A-2)
what is the $p^{th}$ approximation in (A-3)
What is the $(s)$ in $\bar{y}_n^{(s)}$ in (A-2)
Does (A-6) refers to the camber line calculation?

What is the $(t)$ in $\bar{y}_n^{(t)}$ in (A-2)

The text doesn't say. t in German normally denotes chord (Tiefe), but since we later see $\bar{y}_n^{(s)}$ for the camber line (Skelettlinie), it here stands for Tropfen, denoting the thickness distribution of the uncambered airfoil.

I would also interpret the long dash between $\bar{y}_n^{(t)}$ and the sum in A2 as a misprinted equal sign. Otherways the whole sequence will not add up.

  what is the $p^{th}$ approximation in (A-3)

You mean $p$th, don't you? p is used as the index of a stepwise approximation. $f_m^{(p)}$ is the $p$th iteration of $f_m$.

  What is the $(s)$ in $\bar{y}_n^{(s)}$ in (A-2)

That is only in A 6 and indicates that $\bar{y}_n$ this time denotes the ordinate values for the camber line (see above).

  Does (A-6) refer to the camber line calculation?

Yes.


Can I fly IFR in VMC if my instrument rating is not current? If my IFR rating is not current, does this mean I can't fly by IFR, or does it just mean that I can't fly in IMC?

It seems that if it's just the latter, then it would make sense to fly IFR if the weather is still VMC. It would allow me to stay current on procedures while waiting for the opportunity to get my rating current.

No, you cannot. 14 CFR 61.57 (c):

  (c) Instrument experience. Except as provided in paragraph (e) of this section, a person may act as pilot in command under IFR or weather conditions less than the minimums prescribed for VFR only if: [instrument currency requirements]

To accept an IFR clearance as PIC, even in VMC, you must be instrument current.


Does lift-to-drag vary with anything beyond geometry? The basic equations for both lift and drag show dependency on $v^2$, air density and some constants. At a glance, Lift to Drag ratio should depend on nothing else than plane geometry. Although I realize in aerodynamics things are rarely that simple - and so, I wonder how constant these constants actually are?

In particular - let's say we use a very efficient air-augmented rocket engine (so thrust doesn't degrade with air pressure) of a moderate thrust, sufficient to achieve take-off and a modest climb rate. We also somehow manage to shield the airplane from overheating. As it climbs, air density drops, speed rises - could it reach orbital speed eventually? Or will it encounter conditions where it can neither accelerate to gain more lift, nor climb to reduce drag?

Yes, it is entirely possible that the airplane will hit a wall where no more acceleration is possible while climbing. This is called the sound barrier. Besides the geometry, both the Reynolds number and the Mach number influence L/D.

As a rule of thumb, the zero-lift drag coefficient at least doubles when accelerating to Mach 1. Slenderness and sweep help to keep this rise down, but some is inevitable. Below is a plot with test data from the linked NACA Research Memorandum A55F06 for wings of different aspect ratio.

Figure 9 from NACA Research Memorandum A55F06 (picture source)

Just so you don't think this can be overcome by clever design, below is a survey of the drag rise of supersonic aircraft, taken from a short course on fighter design by Ray Whitford. While engineers could lower the drag rise with increasing experience, the Sears-Haack body shows that there is a theoretical lower limit that cannot be avoided.

Supersonic drag rise survey by Ray Whitford. Note that here the absolute increase in overall drag is plotted. This graph includes all effects like trim drag which contribute to the supersonic drag rise.

However, this can be overcome by inserting a short dive which helps to gain enough kinetic energy to overcome the drag hump shortly past Mach 1. But you need to start with enough excess thrust so any net thrust is left in the supersonic region.

By the way, the thrust of your ideal rocket will increase on the way up because the atmospheric pressure drops. This will result in an increase in the pressure difference between the thrust chamber and outside pressure.


In normal condition, is the Airbus A320 APU fuel supplied by the main engine boost pump only? I have a question relating to APU fuel system.
As said in material, there are 2 system supplying fuel for APU to operate. 1st one is primary and 2nd one is secondary.

In the 1st system or low pressure system, there is an APU fuel boost pump locating in LH main fuel tank. Its said that this boost pump is used in starting process or in condition when fuel feed system is lower than required.

Does it mean that, in normal condition, APU fuel boost pump is OFF and fuel is supplied to APU by Main Engine Boost Pump (or Engine Driven Pump)?

The LH fuel line feeds the APU unless the pressure is too low. In this case the advisory "FUEL LO PR" will be displayed on the APU page of the lower ECAM and backup pump is started.

From the A320 family FCOM:


What are the differences between types I, II, and III emergency doors? What are the differences between types I, II, and III emergency doors?

The major difference is size.
FAR part 25.801 defines them as:

(1) Type I. This type is a floor-level exit with a rectangular opening
of not less than 24 inches wide by 48 inches high, with corner radii
not greater than eight inches.
(2) Type II. This type is a rectangular
opening of not less than 20 inches wide by 44 inches high, with corner
radii not greater than seven inches. Type II exits must be floor-level
exits unless located over the wing, in which case they must not have a
step-up inside the airplane of more than 10 inches nor a step-down
outside the airplane of more than 17 inches.
(3) Type III. This type
is a rectangular opening of not less than 20 inches wide by 36 inches
high with corner radii not greater than seven inches, and with a
step-up inside the airplane of not more than 20 inches. If the exit is
located over the wing, the step-down outside the airplane may not
exceed 27 inches.

There is also a type IV:

(4) Type IV. This type is a rectangular opening of not less than 19
inches wide by 26 inches high, with corner radii not greater than 6.3
inches, located over the wing, with a step-up inside the airplane of
not more than 29 inches and a step-down outside the airplane of not
more than 36 inches.

If you read the full text, you will also find definitions for ventral and tailcone exits. For pictures I refer you to this answer.


When does a Maximum Taxi Weight Restriction happen on a 737-800? This is the first time I experienced restriction in taxi weight on B737-800. Have you experienced that before? All I know that restrictions will be from MZFW/MTOW/MLW.

When does it happen?

The Maximum Taxi Weight (MTW) is one of the weight limitations in the FCOM:

(Boeing 737-800 FCOMv1 L.10.3 Limitations - Operating Limitations - Weight Limitations)

It can be the limiting factor when the actual takeoff weight including required fuel is very close to the MTOW and adding taxi fuel would get you above this limit. Note that the difference between MTW and MTOW is only 500 lbs (about 2 hours of APU use or 35 minutes of taxi). If the required taxi fuel is higher than that (which can happen at a busy airport), you are limited by MTW.


What is the requirement of a higher glidepath/approach angle and is this a flight safety issue? To reduce noise in the London urban area, Heathrow airport has announced plans to trial a steeper approach for aircraft landing - changing from 3 to 3.2 degrees. If this is successful, they will further steepen the landing approach to 3.5 degrees. Moreover, London City airport uses a 5.5 degree landing approach.

What are the requirements of higher angle landing approaches and how do they affect flight safety?

The most important sources of noise in an aircraft (especially during landing) are the engines, the slats/flaps and the gear. An aircraft’s angle of descent has an effect on the noise experienced by people below. Steeper the angle, the less time an aircraft spends at low altitudes, which means that fewer people should be affected by higher levels of noise.

The reduction in noise (as experienced in Frankfurt airport) due to steeper (3.2$^{\circ}$) landing approaches are due to the following:

The aircraft is higher (compared to the 3$^{\circ}$ landing angle), and as a result, less noise is heard by the people below.
The deployment of flaps before intercept may also be 0.7 nm closer to the airport as the ILS intercept occurs 0.7 nm closer to the airport.
Thrust increase for a/c stabilization 0.3 nm closer to the airport; this results in lower noise. The thrust required was also reportedly lower.
Landing gear deployment is 0.4 nm closer to the airport (at the same height of 2000 ft)

The differences due to changing the glideslope to 3.2$^{\circ}$from the present 3$^{\circ}$ is shown in the following image:

Image from Experience with the steeper approach angle of 3.2 degrees - Presentation by DLR

As for safety, this system is expected to pose no problems as most aircraft and pilots are easily capable of doing this. As an added measure, the trial is only being carried out when visibility is good (Cat I) and the trial is optional (for aircraft using ILS anyway).

Some airports (like London City) have very steep landing approaches for clearing obstructions and may require special certifications and aircraft modifications.


What is the difference between IMU (Inertial Measurement Unit) and AHRS (Attitude and Heading Reference System)? According to online sources like wikipedia the difference belies on the addition of an on-board processing system (I assume it means a microcontroller) which outputs already processed data and not simply the raw measures of the different sensors.

However I would appreciate the take on this of someone who has experience on the subject, a more practical standpoint.

An INS calculates and updates the vehicle's position (latitude and longitude), alongside the orientation. It needs to be initialized on ground, with the aircraft completely still.

An AHRS does not record/update the position. It outputs real time orientation (attitude and heading) only. It thus can be initialized in flight.

(Image source)

Image shows the INS panel of a Boeing 747-400, notice the ATT (Attitude) option, that's an AHRS-only mode. NAV includes both AHRS and navigation.

ATT can be initiated in-flight, it requires a manual entry for the heading while flying straight and level. So in other words, an INS includes AHRS-functionality.

  Sidenote on terminology: INS nowadays is predominantly used to describe an Inertial Platform, Inertial Reference System (IRS), and Inertial Reference Unit (IRU/IMU); and often used to describe all three of these units (as one system) typically installed on a single aircraft.


What is the fastest unpressurized aircraft? What is the fastest unpressurized aircraft with oxygen  masks that can carry people?

Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor.

  The operational ceiling was limited by what the pilot could endure for
  several minutes while breathing oxygen from a mask, without losing
  consciousness. Pilots underwent altitude chamber training to harden
  them.

Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.

The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h.

About 370 were built.


Are there any airliners and or aircraft that use bleed air and excess heat from the engine to prevent icing are there any airliners and or aircraft that use bleed air and excess heat from the engine to prevent icing.  Rather than use battery power or alternators, this would make sense from an energy efficiency point of view because the use of alternators and batteries would use more fuels if providing electricity for heaters and recharging the batteries

Quite a few actually, and this has been dealt with here before. Most systems use the bleed air from the engine, which is directed through pipes to wing leading edges, engine cowlings etc.

For example, the Boeing 767 anti-icing system, from this question:

Image from Aircraft Anti-Icing Systems by Leslie Mehl and Annie Parsons

This question has answers which explain about the anti-icing systems in large aircraft.

Boeing 727 anti-icing system, image from this answer


What is the minimum altitude over Egmond aan Zee when landing at Schiphol? I live in Egmond aan Zee which is about 36 km flight distance from Schiphol.
It seems that aircraft fly over Egmond when there is a southerly wind so they can make a landing approach to Polderbaan runway.

Where can I find the minimum allowed altitude when flying over Egmond?

Approaches into Schiphol are usually vectored by ATC during the day (see below for night operations). This means the controllers are giving instructions to pilots depending on the current traffic, which makes it hard to say when exactly planes will overfly Egmond aan Zee.

If the Polderbaan (runway 18R/36L) is used for landing from the North (18R), planes will be vectored to the PEVOS waypoint shown on the following chart (source: Jeppesen):

At this waypoint, the aircraft should be at an altitude of 2000 ft (about 600 m):

I marked the approximate position of Egmond aan Zee on the map. There is no given minimum altitude there, so planes could already be at 2000ft, but they are likely still higher (3000 to 4000 ft) see below for update.

During night operations, Schiphol is a lot stricter about where planes can fly and how they can descent. The following night approach chart shows how planes will fly if they land on 18R:

Egmond aan Zee is between EH602 (where planes will be at FL70 or about 2100 m) and EH607, which is on a dashed line indicating a continuous descent path, where planes must fly at idle thrust from FL55 at NIRSI down to the ILS intercept altitude (2000 ft).

I just had a look at flightradar24.com and saw a KLM Boeing 737 flying quite close to Egmond:

According to the ADS-B data, they were between 1800 and 2000 ft when flying just south of Egmond an Zee (I marked the point where they leveled off at 1800 ft). I assume this data is somewhat inaccurate since they should not descent below 2000 ft before intercepting the ILS. Other aircraft were also at similar altitudes around Egmond, so I assume overflights at 2000 ft are quite common.