How to fly an RNAV (RNP) approach with VNAV in a Boeing 737? Let us assume you are flying a B737NG and you are cleared for an RNAV (RNP) approach which has several legs with different altitude crossing restrictions.

What do you enter in the MCP altitude window? The first, highest crossing restriction, and then keep repeating by entering the next restriction or does one enter the last and lowest one and let VNAV manage the step descents automatically?

If during this gradual VNAV descent procedure you have to use SPD INTV to alter an FMS commanded airspeed, does this "cancel" the VNAV profile, or will it adjust automatically to suit the new manually selected airspeed?

This usually depends on the airline SOPs. The safest thing to do when cleared for the approach is to set the MCP altitude window to the lowest altitude restriction because that way you won't forget to lower it again. As long as the aircraft is descending in VNAV PTH mode, it will follow all altitude crossing restrictions.
When cleared for final approach, one can also set the DA (Decision Altitude) for the approach instead of a crossing restriction into the window. This will allow VNAV to descend further and is required unless IAN  with G/P is used:

The Integrated Approach Navigation (IAN) modes are armed/engaged by
selecting the APP button on the AFDS Mode Control Panel (MCP). Once armed,
the autopilot/flight director will capture and track the localizer/final approach
course and glide slope/glide path.

(Boeing 737 NG FCOMv2 - 4.20.20 - Automatic Flight - System Description)
Once the aircraft is descending on final approach, one can set go-around altitude into the window:

When the FMC is “on approach”, the following features are available:

when preparing for a missed approach and the MCP altitude is set at least
300 feet above the current airplane altitude, VNAV will continue to
command a descent

(Boeing 737 NG FCOMv2 - 11.31.37 - Flight Management, Navigation - Flight Management System Operation)

Using SPD INTV will usually cause the vertical mode to switch from VNAV PTH mode to VNAV SPD mode, which does not care guarantee to comply with crossing restrictions:

If speed intervention is engaged:

during a path descent with flaps up on an idle leg, VNAV switches to VNAV SPD
with flaps down, VNAV remains in VNAV PTH

(Boeing 737 NG FCOMv2 - 11.31.38 - Flight Management, Navigation - Flight Management System Operation)
So if you already have flaps selected, VNAV PTH will remain active. During VNAV SPD operation, it is the pilots duty to ensure all crossing restrictions are met. Once the aircraft goes back on the path, VNAV PTH mode will engage automatically again (assuming you are on a STAR or approach):

A speed descent cannot automatically revert to a path descent, except during
STAR, approach transition, or approach leg with a vertical angle.

(Boeing 737 NG FCOMv2 - 11.31.33 - Flight Management, Navigation - Flight Management System Operation)
Changing the target speed in the FMS will compute a new descent profile. This usually results in the aircraft no longer being on profile and therefore the vertical mode will switch to VNAV SPD as well.
Note that VNAV SPD will not violate altitude restrictions, if possible:

The descent attempts to comply with waypoint altitude restrictions, and will not
violate these restrictions. The VNAV speed descent will not, however, guarantee
the airplane reaches an altitude restriction at the required point.

(Boeing 737 NG FCOMv2 - 11.31.33 - Flight Management, Navigation - Flight Management System Operation)


What are the disadvantages of a twin boom aircraft? Not to be confused with twin fuselage design.
Especially for high wing span UAVs, twin boom design seems to be a better option as it will reduce wing bending in flight. But it is not normally used, are there any aerodynamic(or otherwise) disadvantage of this design?

The main disadvantage is flutter. The booms add a mass behind the axis of elasticity which will twist the wing when it bends. This twisting will increase the local angle of attack when the wing bends upwards and increases the bending moment, and vice versa on downward deflection. Once the plane flies fast enough, flutter is hard to avoid. The main remedy is to make the inner wing stiffer in bending and torsion, so the flutter frequency is shifted outside of the flight envelope. This costs weight, though.


What are the wings and fuselage of an airliner made of? Which materials are used to make the wings and the fuselage of an airliner, for example the Boeing 767 or the Airbus A320?

Most modern planes are made mostly of aircraft aluminum. Or at least are skinned in aluminum. The spars and cross members may be made of other materials but I am under the impression they are mainly aluminum as well. Generally high stress areas like wing/fuselage joints may be made of more rigid materials like steel. Aircraft Spruce provides a nice break down of the various types of aluminum here. This answer to a similar question provides a nice break down.

As the technology gets better and the facilities grow in both scale and ability, composites like carbon fiber have found their place in planes like the 787. Here is a break down for the 787

(source)


Are the pitching moment and pitching moment coefficient of an airfoil given w.r.t. the aerodynamic center? When the magnitude and sign of the pitching moment and the value of the pitching moment coefficient are communicated, are these values given with respect to the aerodynamic center of the airfoil since the magnitude of the moment of a force is always dependent on the chosen pole O about which the moment is calculated (unless we are discussing pure moments which are independent on the pole O location).
Thanks!

A moment is a free vector. There is no need to add a reference point - it can be shifted fore and aft or up and down without changing its effect. This effect is a rotation without translation.

So yes, as you suspect already, the pitching moment is a pure moment.

If there is also a force around (like on a wing), you have two vectors: One for the force and one for its moment. Put the reference point along the vector of the total force, and that moment disappears. Shift your angle of attack to the zero-lift point, and the force will disappear, leaving you with a pure moment (which is the sum of all the remaining forces along the chord which balance at this angle of attack).

In two-dimensional potential flow theory, all angle-of-attack-dependent forces act on the quarter point. If you now define your reference point at this quarter point, all variations in lift with angle of attack have no moment around this point! What is left is the pure pitching moment which does not depend on angle of attack.

If, however, you choose some other point as your reference point, the change in the lift force with angle of attack will add a moment, so your pitching moment is some constant value plus an angle-of-attack-dependent contribution. If you pick your reference point behind the quarter point on an airfoil with positive camber, there will be one angle of attack where those two moment components balance and the pitching moment will disappear. Then your reference point is the center of pressure at that angle of attack.

So the split of the aerodynamic forces into a force and a moment is purely arbitrary. Technically, we could always sum up all forces in the center of pressure, but then we need to keep track where it is, because it moves with angle of attack. Therefore, we decided to make things a bit more complicated and express the aerodynamic forces as one force at the quarter point and a separate moment. Why the quarter point? Because then this moment does not change with angle of attack. That is the advantage of defining all forces in the neutral point. We always know where the force acts at the price of carrying an extra moment with us. At least, this moment is constant over angle of attack - that is the big advantage of choosing the neutral point as the reference point.

EDIT

When looking for older answers to link to, I found that the two of us are discussing pitching moments since years without getting anywhere. Like here, here or here. I'm afraid I will repeat the same stuff like a broken record, and you keep asking the same question over and over again. Maybe it is better that you explain in a new question what your current understanding is so I can work out where exactly we two talk past each other.


What is the effect of aileron deflection on the centre of pressure? Deflecting an aileron will increase or decrease the camber of the outer wing where the aileron is attached. I learnt on this platform that centre of pressure (c.p.) moves fore and aft with aileron deflection.

Is the c.p. movement as a result of aileron deflection significant?
If it is significant, how does one calculate the difference in c.p. with respect to aileron upward or downward deflection?

The CP of local sections where the ailerons are deflected should shift aft. In an asymmetric deflection case, very little pitch moment would be generated. For symmetric deflection (e.g. maneuver load alleviation, flaperon), there will be a net pitch moment, but should be small due to the outboard location of the panels, unless the wing is highly swept.

To estimate the effect, you can view ailerons as part-span plain flaps. You can use any of the semi-analytical methods from ESDU, DATCOM or Roskam. You can also use any vortex lattice method to get the answer. The estimates should agree fairly well for small deflections and low Mach.


Are crosswind landings easier with larger airplanes? I don't spot an answer to this question, so thought to re-post it here. Larger airplanes are heavier and would ostensibly be pushed less by the wind compared to a small airplane? Do larger airplanes have higher crosswind limits then?

Generally yes, since a 20kt crosswind requires considerably less crab angle on final at 135kt compared to 70kt.  The CRJ900 has a demonstrated max crosswind component of 32 kt (If I had those conditions in a 172 and had to put it down, I think I'd just land across the runway).
There is also a significant technique difference once you get above, say, 100,000 lbs, or depending on the engine location.  On heavies where you only have a few degrees of banking margin when the gears touch before the nacelles hit, the technique is to fly wings level crabbing into wind into the flare, and give a large boot of rudder just prior to touchdown, and get it down before you start drifting off to the side (you are effectively in a skidding turn, but there is a delay before you start to actually change direction and you have to achieve ground contact in that delay period, so you make it snappy and don't work on teasing out a smooth touchdown).
On smaller aircraft it's more the traditional sideslip technique, wing low with opposite rudder maintained to keep runway alignment through the flare and touchdown. My jet time was on CRJs and I would land in crosswinds slightly wing low with rudder to keep aligned, similar to landing a glider in a crosswind (it took about 7-9 deg of bank to risk tip contact and a crosswind would never need much more than 3-5, although tip strikes do happen in the CRJ fleet from time to time when pilots have an oops on landing).
Transport airplanes generally don't have a maximum crosswind component as a published limitation, but will list a "demonstrated" crosswind component with the statement "not considered limiting".  It means that it's only the maximum component demonstrated in testing, and you are free to land in stronger ones, but the manufacturer doesn't vouch for the ability of the plane to handle it.


What prevents DC-9 series aircraft from being reengined with more-efficient engines? All DC-9s except for the MD-90\B717 series use old, inefficient Pratt & Whitney JT8D series low-bypass turbofans, even the best of which have a paltry 1.74:1 bypass ratio.  We know that a DC-9 can accommodate a high-bypass turbofan; the final iteration of the DC-9 (the MD-90 series, including the Boeing 717) uses much more efficient engines with bypass ratios in the 4-5:1 range (which is the main thing distinguishing the 90-series aircraft from their predecessors).

Given that we know that a DC-9 is perfectly capable of using a high-bypass engine, and that the kerosene-guzzling nature of the JT8D is the main reason why DC-9s are being steadily phased out of service (as their low fuel efficiency makes it hard for airlines to make a profit from a pre-90 DC-9), why haven't the remaining older-series DC-9s been re-engined with high-bypass turbofans?

A re-engine program can be pretty expensive. This can make sense on military aircraft that take much longer to accumulate hours and cycles, or even cargo aircraft like the DC-8 in a similar position.

But the newest aircraft in the MD-80 series are going on 20 years at this point. They just don't have enough life left to justify that kind of expense. Most of them are not equipped with glass cockpits. When you add on the avionics upgrades needed, it becomes more cost effective to just buy a 737 or A320 which comes with the latest engines, avionics, and other features.

You're going to need a major manufacturer willing to take on the effort of an engine replacement program. That would point to Boeing or Airbus, and that would only cut into their existing narrow body sales.


For what is VMCL the minimum control speed? I would like to ask which of these options is correct:

  VMCL is the minimum control speed for:

  A) approach in go-around configuration
  B) approach in landing configuration
  C) go-around in approach configuration
  D) go-around in landing configuration

I would choose B but my friends disagree due to Certification Standards CS25.19 Minimum Control Speed stating that:

  VMCL must be established with -
  (6) Go-around power or thrust setting on the operating engine(s).

However that doesn’t mean that my VMCL is the speed that I use for go-around, right? It just says that to establish my VMCL, I need to use go-around power as a reference.

Therefore I think that VMCL is basically just the minimum control speed during an approach in landing configuration.

I would go with B as well.  Minimum Control Speed - Landing is for the critical case where you are in the least favourable possible configuration on approach, gear down, full flap, trimmed at approach speed, C.G. at least favourable location, etc etc, then have one engine fail and the other is put to full power. VMCL is the speed at or above which control can be maintained at that instant maximum asymmetric trust is applied, before the airplane can accelerate or the configuration can be changed.  Go around speed in the approach configuration will be faster than VMCL.  VMCL is the edge of the cliff.


Are there helicopters with vertically mounted turbines? On the helicopter representation in this answer, the turbine is horizontal, with gear required to drive the vertical-axis propeller.

Are there helicopters where the turbine is vertical and directly driving the propeller, without gear? What are the advantages and drawbacks, or why isn't it possible if so?

Discounting the V-22 Osprey and the other tiltrotors, the drawbacks are:

Wasted cabin space
L-duct inlet(s) needed to guide the air in
With that potential problems with getting the air in as the air prefers fewer turns
A reduction gearbox is still needed
Gearbox still needed for the tail rotor
Harder maintenance access as it will be buried into the fuselage
If the exhaust is channeled downwards: potential tarmac melting, fuselage weakening, and handling issues in low hover
If channeled upwards, well, that's where the rotor is
Otherwise, extra heat ducting needed, i.e., added complexity and weight and expensive materials
Any fire or uncontained failure will cause grave danger.

Below is the best illustration I could find, and it gives you an idea about the previous points:

http://www.melm-md.com/images/N600_image_02.jpg


How is the transfer of an aircraft managed when it landed at a different airport than the scheduled one? The question came to me when my flight could not land at the scheduled airport because of fog.

We landed in a nearby city (Turin instead of Milan). Is the aircraft transfered without passengers later on? Bear in mind that there are no flights scheduled between the 2 cities (120 Kms distance only).

The aircraft will most probably be re-positioned to wherever it is needed. Often this will mean sending a crew to wherever the aircraft is and having them ferry it empty. If it is some far-flung place, the crew which landed it there may wait until it is possible to re-position it themselves.


What is the diameter of the nozzle of Trent 1000? I am researching how much force is needed to divert the thrust of a Boeing 787 on its service ceiling of 13,100 meters and at a cruising speed of 913 km/h (data found on Wikipedia).

As I would like to fit a diverter to it, what is the diameter of the nozzle of Trent 1000 engine?

It is mentioned here that the fan diameter is about 111-112 inches. So the nozzle must be smaller than that as the engine assembly narrows at its rear end.

This PDF (p.5) shows a diagram of the exhaust nozzle of RR Trent 1000 measuring the diameter as 63 inches. This is the only reference I could find.


Is there any other category of aircraft in the FAA regulations besides 'Ultralight' that does not require pilot licensure? I am asking to clear up contention on my related question that was put 'on hold' by the moderators.  To my knowledge, there is no other category of aircraft besides 'Ultralight' as defined in CFR 14 part 103, that doesn't require any kind of pilot's license.  The 'experimental' category requires a recreational pilot license.

The problem with the 'Ultralight' category is that in practice it ends up being discriminatory against heavier potential users.

Yes, there is at least one. This probably was not what you had in mind, but the FAA has been very explicit about the following, for a variety of reasons.

Unmanned aircraft are explicitly regulated as aircraft. However, certain model aircraft require no certificate if operated under the provisions of §101. This is in contrast to all other unmanned aircraft which do require some sort of certificate or exemption, Small Unmanned Aircraft Systems regulated under §107 for example.

The following definition is from 14 CFR 1.1:

  Aircraft means a device that is used or intended to be used for flight in the air.

The following is excerpted from the FAA's FAQ page on Unmanned Aircraft Systems:

  What is an unmanned aircraft system (UAS)?
  An unmanned aircraft system is an unmanned aircraft and the equipment necessary for the safe and efficient operation of that aircraft. An unmanned aircraft is a component of a UAS. It is defined by statute as an aircraft that is operated without the possibility of direct human intervention from within or on the aircraft (Public Law 112-95, Section 331(8)).


What is the difference between inboard and outboard engines on a Russian IL-18 turboprop? I was looking at this picture of an Ilyushin IL-18 turboprop aircraft
Image hosted at https://i2.cdn.turner.com/cnnnext/dam/assets/160630220221-north-korea-air-koryo-il-18-nose-super-169.jpg
 and noticed that the outboard engines look significantly smaller than the inboard engines. Wikipedia lists specifications for four identical engines but the outboard cowlings don't seem to have air intakes.

Can anyone explain why?

The inboard cowlings seem to follow a different design structure in order to make room for the landing gear compartment. The air intakes are moved aft on the outboard cowlings.


Can a pilot change out of uniform during breaks on a long flight? How long is a pilot-in-command's break on ultra-, or long haul flights? Would they be required to keep the uniform on, or would they have the freedom to change out of uniform?

Related: How do pilots rest on long-haul flights?

The answer depends a lot on the jurisdiction and airline, but Yes, pilots can change out of uniform during breaks on long flights.

A 15 hour flight would have 4 pilots who are all in the flightdeck for takeoff and landing but then divide roughly 14 hours of cruising time into equal rest breaks.

Typically 2 pilots will be paired together and take breaks at the same time. At some airlines they would have 2 pilots take a 7 hour break and then switch off. At other airlines they  take two 3 1/2 hour breaks.

Each 2 person crew would be off the flight deck for half of the 14 hours spent at cruising altitude (15 hour flight) I find most crews prefer two shorter breaks as opposed to one long break. With a 3 person crew they would have each pilot take 1/3 of the cruising time as a break.

Most airlines operate with one main crew who does both the takeoff and Landing. This is because the second crew is generally less qualified. In some cases the "augment" pilots are only licensed to operate during the cruise portion of flight. Some airlines will operate with two fully qualified Captains and two fully qualifed First Officers and they can divide the takeoff and Landing between the two Crews.

Most pilots change out of uniform during their breaks. Some airlines require the pilots to wear their hat and tie when they leave the flight deck.  Others are much more relaxed.

I have seen pilots in T-shirts enter and exit for breaks.


What was the first supersonic wind tunnel? What was the world's first supersonic wind tunnel? Who built it and how did it work?

All I've found so far is this Wikipedia article that says:

  By the end of the war, Germany had at least three different supersonic wind tunnels, with one capable of Mach 4.4 (heated) airflows.

The citation in the article is a dead link.

The world's first supersonic wind tunnel started operating in National Physical Laboratory in Teddington, England in 1922.

It basically used a pressure tank upstream, which was used to generate supersonic flow in a cross section of 20.3 mm, which was used to study projectiles.

From the book Aeronautical Research in Germany: From Lilienthal until Today:

  During the same year, the National Physical Laboratory (NPL) in Teddington/England started operating, for the first time ever, a supersonic wind tunnel based on the pressure storage principle ...


What may be the cause of hot start problems in a Lycoming fuel injected engine? Cherokee Six, 32-300 1969. Lycoming engine is fuel injected with 450 SMOH. Cold starts are easy: 1/2" throttle, full rich, fuel pump on and hit starter after fuel flow starts. Runs great once running.
Hot starts are hit or miss. If it sits over an hour it may start but if I miss it it has to sit for at least another hour till I try again or forget it. I've tried 1/2" throttle, full lean with and without fuel pump; full throttle, full lean with and without fuel pump and all combinations I can think of. Most of the time it spins without any cough or catch. Due for 50 hour oil change and will ask that plugs and magnetos are checked. I'm afraid to go anywhere for fear of being stranded for hours.

The issue is undoubtedly vapor lock. In the fuel injected Lyc 540, the fuel injection plumbing is on the top of the engine which is the cold side of the engine when you are flying but when parked, heat rises! The following procedure runs cooler fuel through the plumbing while flooding the engine, then the flooded start procedure is necessary.

First, check your POH for the hot start procedure for your plane.  If not there, try this as it is for a very similar airplane:

Hot Starts on the Saratoga PA-32-300:

Intentionally flood the engine.  This runs cool fuel through the plumbing and "breaks" the vapor lock.

a) Full rich
b) Wide open throttle
c) Fuel pump on, watch fuel flow gauge
d) When gauge registers flow, wait 3-5 seconds then red knob to ICO and pump off.

Crank engine with red knob still at ICO and with throttle wide open at first. Gradually close throttle (at a rate that will take you about 10 seconds to close the throttle) and it should start up at some point.
When engine catches, do several things more or less simultaneously:

a) release ignition key
b) throttle to idle to avoid over-revving
c) mixture full rich.


Where can I find details about a Cambridge Scientific Instruments Fuel/Air ratio Indicator? In our restored Harvard Texan T-6 WW2 trainer aircraft, a Fuel/Air Ratio Indicator was installed. We cannot locate the wiring diagram of this system or information for the Cambridge Scientific Instrument Co. The instrument is P/N. 11622-1 Spec 94-27992. Please help us with the required information, or the indicator's pinout and functions.

The 11622-1 is a type A-7 single-engine exhaust gas analyzer. You can see it here on page 8. The instrument just tells you if the fuel/air mixture is too rich or too lean. Electrical inputs are simple; 12vdc and ground. Any competent instrument technician can open it up and see which pin is what. But that's only the beginning... the system includes tubes and an analysis cell, and the cell is probably pretty much useless by now, if it is still installed, but you can always try your luck.


How do you apply to offer WINGS credit for an event you are sponsoring? Douglas Rotary is sponsoring a fly-in October 3, 2015 in Douglas, AZ at the Douglas Muni. Airport.  How do we apply to get pilots WINGS credit for their training?

A fly-in by itself doesn't usually qualify for WINGS credit, but if you're also running educational programs or seminars there's a good chance you've got something that will qualify.

Normally you'd work with a FAA Safety Team representative to help set up the program so it meets the WINGS credit requirements (they have access to premade presentations, slides/videos, and with a little notice they can usually help find presenters/speakers), and the representative will then coordinate with your regional FAA Safety Team manager to get the programs listed in SPANS so people can sign up and receive credit.

If you have an account on faasafety.gov you can search the FAASTeam directory for representatives near you:

(I did a cursory search and I didn't see anyone in Douglas, but there are a few folks in Tuscon.)

If you can't get in touch with any of the representatives your best bet is to contact your local FSDO (Scottsdale in your case) directly and ask for the "FAA Safety Team Program Manager" - they'll be able to work with you to get the program coordinated and set up in SPANS.

If you intend to do regular events with safety programs it would probably be worth getting yourself or someone else in your group set up as a FAASTeam representative. There's some (really simple/basic) training involved, and usually the regional program manager or one of the lead representatives in your area will want to meet with you just so they know what you look like, and once you're set up you'll be able to submit requests to have your programs listed directly.


Is the FlightGear sim accurate in 2018, specifically Cessna 172 aircraft? So, I fly on FlightGear as I like the fact its free. But is it accurate enough to train to fly the real Cessna 172? Are there any better free flight sims? Please give your honest opinion and please try FlightGear before answering my question, I can't deal with people telling me their opinions when they have not tried FlightGear yet.

It completely comes down to what you want to train in the sim. I have used FlightGear in my beginnings of when I started my interest into aviation. I guess you are using the default C172. What you can do in this sim is learning to navigate, use the instruments (at least the ones that are modelled to work correctly) and maybe learn checklists. But that's it actually.

If you really want to train for a real Cessna 172 aircraft, I can highly recommend getting Prepar3D and the C172 by A2ASimulations. It's one of the best flight simulation addons in the flight sim industry at this time. A full simulation of everything from oil viscosity to water contamination of fuel. Even a real C172 walk-around is implemented (with fuel inspection, tire inspection, everything) and a full damage model. You have to treat it like in real life to not damage parts. And you can also install a GTN750, GTN650 or GNS systems in her, if you own one of that beauties in real life. Also study level of course.

What you can't train in flight sims, however, is the hand flying.

I own all those addons and can't stop recommending it.


Why the pitot tube is heated but the static port isn't? In some light aircraft, like a Cessna 172, the pitot tube is heated but the exterior static port isn't. The usual reason I've heard is that static ports are much less susceptible to icing, but why is that the case?

Some aircraft DO heat the static port, if only by side effect: Piper PA28 aircraft (Cherokee/Warrior/Archer/Arrow ) for example have the static port located on the pitot mast.  When pitot heat is turned on in these aircraft the entire mast heats up, including the static port.
This characteristic would be the same on most aircraft with a combined pitot/static mast.

As for why most light aircraft don't heat the static port, there are two big reasons.

First, as you've heard, static ports are normally not located in areas susceptible to icing -- they're on the side of the fuselage in areas of relatively undisturbed air (on most of the Cessnas I'm familiar with they're forward of the doors, on other aircraft they're right around where the registration numbers get painted). As Falk pointed out this means ice normally won't accumulate there.
As an additional protection some aircraft have multiple external static ports (connected to a shared static line) so that if one port does get iced over you still have a source of reference pressure.

Second, a static failure is relatively easy to deal with - particularly in unpressurized light aircraft.  Many unpressurized aircraft that are regularly flown in instrument conditions will have an alternate static source in the cabin (which is simply a valve that opens the static line to cabin air), or the traditional "break the VSI glass" solution can be used to get a mostly-working altimeter.


How do you program circular flight plans in GPS? In a recent Buzzfeed article about aerial surveillance, I have seen a map showing perfectly circular flight patterns. Given how precise the patterns are, I highly doubt that the aircraft were being hand-flown.

According to the article, the planes used for this kind of survey work are GA (among others, Cessna and Pilatus PC-12 are mentioned in the article).

I was wondering how would you get to fly such a pattern using a GA-grade autopilot?

I did some research and so far the only way I could find was selecting a circular pattern in a G1000 equipped with the "Search and Rescue" additional SD card. However in this case it seems you can only have a circle where the centre of the circumference corresponds to a known waypoint (See third last page in this PDF manual), so probably that method doesn't offer enough flexibility for the purpose of aerial surveillance (e.g. what if there are no waypoints over the area you need to survey?).

Another way I imagine you could achieve something similar to a perfect circle could be by carefully setting aileron/rudder trim I guess, but I doubt you could hold a stable pattern over time just by using trim controls, due to wind, turbulence etc.

Does anyone have more insight on how you could achieve a perfectly circular flight pattern while being able to arbitrarily place the centre of the circle in any geographical location?

Custom waypoints would work, but it would not be a smooth circle. And as far as I know looping the set of waypoints is not straight forward. Another way is the CWS (control wheel steering) function of some autopilots such as the King KFC 225 to hold a bank angle and to control the radius with the speed, but as you pointed out in a comment, it would not be perfect due to the wind.

Also a unit such as the Garmin 650/750 available for FAR 23 (small aircraft) has the new radius-to-fix and orbit functions:


How do insects decrease aircraft performance? Is it true that insects hitting the fuselage could decrease aircraft performance and increase fuel consumption? Insects are small in mass. I think even a large amount of them accumulate on the aircraft's outer skin, it wouldn't be significantly enough to affect fuel consumption.

You're right, insects are very small, so they influence things which happen on their scale. The one important phenomenon on an airliner which has the scale of insects is the boundary layer, the sheet of air around all wetted surfaces where air speed changes from zero (relative to the plane) to the speed it has at some distance. This is called the boundary layer. Its thickness changes from zero at the stagnation point to several centimeters at the end of a long fuselage.

What does the boundary layer look like?

At the leading edge, the boundary layer starts with a thickness of zero. Now friction with the wing will cause some air molecules to be slowed down, and soon you get a sheet of air in which the molecules closest to the aircraft's skin will move with the skin, and the more you move away from the skin, the less they are slowed down. Initially, the layers of air within the boundary layer show no cross movement of molecules. Compare it with a multi-lane road with bumper-to-bumper traffic where no car changes lanes. Since all molecules move along their layer of air, this is called laminar flow (lat. lamina = layer).

At some point downstream oscillations will develop, and once they become unstable, molecules will move between the layers of air. Now you have faster ones from more distant layers moving closer to the skin and kicking the slow ones there ahead, and slower ones from close to the skin moving away, slowing down the more distant layers. Now the cars in your multi-lane road cross lanes, and the result is that all lanes but the rightmost one will move at similar speed. Since the cross-flow is the result of turbulence, this boundary layer is called turbulent.

Speed profiles of laminar (left) and turbulent (right) boundary layers. Image source.

Consequences for drag

The cross flow causes the turbulent boundary layer to have a much steeper speed gradient at the aircraft skin, causing much more friction drag. At the same time, more energy is taken from the flow due to friction, so the whole boundary layer becomes thicker. If you look at the local friction drag, the parameter plots made possible by XFOIL are quite illuminating.

Friction drag over chord for an E502mod airfoil at 3° AoA. Blue: Top surface, Red: Bottom surface.

The plot shows the friction over chord for an airfoil at 3° angle of attack. All flow is attached (save for one small separation bubble on the bottom near the transition point). Can you spot the transition points from laminar to turbulent flow? Yes, it's where the friction drag jumps up and stays annoyingly high downstream. Note that the flow around an airliner's wing happens at a much higher Reynolds Number, so the transition points are closer to the leading edge than in the plot above. I chose the low Reynolds Number in the plot above because it shows the phenomenon more clearly.

But you see also a friction spike at the nose! This is caused by the very small thickness of the young boundary layer. Even though it is laminar, it shows a high friction contribution simply because it is still very thin. Now imagine you have both effects, a thin boundary layer and the bigger friction of a turbulent boundary layer, added together. This is what bugs at the leading edge of the wing will give you! They make the wing's surface rough and drive up friction losses due to an early transition of the boundary layer into turbulent flow.

Consequences for maximum lift

But there is also a second effect: The longer the boundary layer develops, the more the flow loses the ability to slow down and increase pressure towards the trailing edge. A flow's energy is either speed or pressure, but if the energy of the flow is sapped by friction, none of both is left when needed to negotiate the last half of the wing's shape. The flow will separate earlier if it had been turbulent from the beginning, and the wing will stall at a lower angle of attack. This is the second negative consequence of bugs on a wing. It can be mitigated by careful airfoil design, but then this airfoil will show lower bug-free performance.

Glider pilots know this very well, especially those who flew planes which used the Wortmann FX 67-170 airfoil. It had excellent L/D without bugs, but both rain and bugs converted the plane to something resembling a brick. I once flew a Janus B into a shower, and minimum speed increased from 80 km/h to 110 km/h. A few seconds of flying at higher speed cleaned the wing, but then it was time to land, because I had lost so much altitude.


Which airlines allow you to gain hours with them by paying them As a fresh grduated pilot is there any airline that you can pay them to fly with them without a salary just to build hours?

There are pay to fly services, however regulations require a minimum number of hours before you can do any kind of part 121 or part 135 work. So while you can get some extra hours in Jets by paying them that way or turbo props or whatever else you’re looking for, it’s not recommended. Probably the best starting gate once you have a commercial pilot certificate is just doing flight instruction work until you gain the required 1500 hours.  After that you should have offers for most of the regional carriers to work for them as a first officer.


Why B-52 engines produce lots of smoke only during take-off? During the flight it seems there is no smoke, it's only during takeoff. Here is an example

This question explains that smoke is caused by unburnt fuel but why is there so much more during takeoff?

There are a few reasons.

The first is that the engines run more efficiently when the aircraft is moving. As the aircraft moves faster, it's easier for it to get more air into the engine, which helps it run more efficiently.

The second reason is even simpler: like most aircraft a B-52 only rarely runs at full throttle. Most of the time that it's cruising, the throttles are backed off to just overcome the drag and maintain the current speed and altitude (though flying at terrain avoidance level is a rather different story).

The last reason is that the larger amount of smoke is largely an illusion. When the aircraft is moving, it's actually still generating close to the same amount of smoke. We're seeing the smoke from running the engines at full throttle for roughly a minute. During one minute of normal flight, it would generate somewhere close to the same amount of smoke--but instead of being concentrated in the 1 mile or so that we see here, it would be spread across something like 10 miles of sky. In addition, at altitude, you're a lot more likely to have much stronger winds, so exhaust from a minute ago will be relatively widely dispersed, rendering it much less visible than we see here (with air that looks like it's almost perfectly still).


When to slow down to Va for turbulence? I'm trying to figure out when it is safe to continue at normal cruise speed and when I should slow down to avoid over-stressing the airframe due to turbulence. (of course I have asked instructors but I'd like to get some more opinions)

My aircraft has a maneuvering speed listed, Va, also listed as the turbulence penetration speed. When should I actually slow down to this speed (i.e. light, moderate, severe turbulence)? How long should such turbulence occur before I slow down (after a bump or two, when the turbulence is observed to be intermittent or continuous, should I slow down immediately)?

For the scope of this question I'm asking specifically about light single-engine general aviation aircraft that cruise around 140 kts. Va is 112 kias. I'd be interested in hearing about larger/smaller aircraft too though.

Answers from actual pilots with experience in this area, please (I would hope all pilots have some :-) ).

To help you understand this, I feel like we should discuss how Va is determined.

14 CFR 23.337 Limit maneuvering load factors.

  (a) The positive limit maneuvering load factor n may not be less than—

  (1) 2.1 + (24,000 ÷ (W + 10,000)) for normal and commuter category
  airplanes, where W = design maximum takeoff weight, except that n need
  not be more than 3.8;

  (2) 4.4 for utility category airplanes; or

  (3) 6.0 for acrobatic category airplanes.

  (b) The negative limit maneuvering load factor may not be less than—

  (1) 0.4 times the positive load factor for the normal utility and
  commuter categories; or

  (2) 0.5 times the positive load factor for the acrobatic category.

  (c) Maneuvering load factors lower than those specified in this
  section may be used if the airplane has design features that make it
  impossible to exceed these values in flight.

Based on that regulation, the light single-engine aircraft I've flown have a +3.8/-1.52 load factor limit in normal category loading.  The aircraft is designed to structurally handle those load factors up to max gross weight.

A pilot could exceed those limits by making abrupt control inputs or "pulling G's" because by pitching up abruptly, for example, the pilot is increasing the angle of attack and accelerating the aircraft.

At speeds above Va, the wings are capable of producing enough force to damage the structure with positive or negative loading.

This is a V-G diagram (Source)

The curved line represents the load factor at which the aircraft will stall at a given indicated airspeed.

Va is the speed at which the aircraft would reach its maximum positive load factor limit at the same time it would experience an aerodynamic stall.

An angle of attack less than or greater than the critical angle of attack will cause the wing to generate less lift than it would at the critical angle of attack.  Therefore, the stall point is used to determine where the maximum load factor would occur for a given speed.

Maneuvering speed, as you can see by the chart, has no safety buffer above it, which is why Dave's answer has some details about inadvertently increasing speed.

Flying below maneuvering speed would be a better guideline to follow if you feel that there is are hazardous levels of turbulence.

Let's say you are flying well below Va, and you are hit by a strong vertical gust.  The vertical, upward, gust will increase the angle of attack by momentarily changing the relative wind, and therefore increase lift.  If that change in angle of attack exceeds the critical angle of attack, then the aircraft will aerodynamically stall rather than continue to produce greater lift. In that case, the stalled condition would protect the structure of the aircraft by acting as a limit on structure loading.

Now to answer your specific question:

It is unlikely that severe turbulence will come as a surprise during flight in a small single-engine aircraft.  I have never personally experienced turbulence where I felt the need to decrease my speed.

It is a judgement call that needs to be made by the PIC, and I would slow down whenever I am experiencing consistent turbulence that is causing me to physically perceive unusually high G-forces.

If a pilot is in that scenario, there has likely been a poor go/no go decision regarding the flight.  If you need a quantifier, "severe turbulence" would be the classification for which this matters.

I have experienced moderate to severe turbulence in flight, but those were attitude altering gusts; the load factors imposed were not near limits.


Is a lower CHT on one cylinder cause for concern? I have some concerns about the level of maintenance and service on some rental airplanes on an out of town trip.
The first airplane I rented had some misgivings, so I went on a different one today. It was a Cessna 172 and had a Cylinder Head Temperature (CHT) monitor, which I hadn't seen before. So while on an extended leg I fiddled with it. Three of the cylinders where 340-350.  However, cylinder 2 was only 280. That seems like a significant outlier.
Since most rental airplanes don't have a CHT monitor, I don't have experience with what is the normal temperature. Should I be concerned that one cylinder isn't close to the other three?
Below is some information about the airplane and that flight:

Power: 2400 RPM
Altitude: 7500 feet
Oil Quantity: 7 quarts
Oil Pressure: Top of green arc
Oil Temperature: Normal

Airplane was fresh out of a 100-hr maintenance
Anomalous Cylinder: #2

There was some issue with the engine during the 100-hr maintenance which held it up, but was signed off by a mechanic with an IA.
I am unsure about the quality of maintenance service there. We had two flights cancelled in a row because the carburetor  was fine when it was warm, but this cold snap (38°F) is making it hard impossible to start. In addition, there are all sorts of small squawks such as trim that was out of whack etc.
The bottom line is that I am concerned about my safety as probably I will be renting one more time. Is the above mentioned scenario something which could indicate a problem, or more likely something harmless? I have doubts that the maintenance is top quality, but I'm unsure if it's bad enough that I should just opt out.

A 60 degree difference in CHT is something worth investigating: It may not be a safety-related issue, but you want to know if it's "normal" for the engine or an indication of a problem.

In order from roughly "Most Benign" to "Worst Case", the CHT difference could be the result of:

A faulty CHT probe
Either the probe is not seated in the cylinder head well properly, or the probe/wiring is defective and causing an inaccurate reading.
Different CHT Probe Types
Spark plug gasket probes can read higher than "normal" cylinder head probes, so if gasket probes were used on cylinders 1, 3, and 4 and a standard bayonet probe used on #2 that would account for the difference.
(I can't see why this would be the case though: usually you have one gasket probe - on the cylinder where the manufacturer's CHT gauge was connected because it's using the normal bayonet probe hole - and bayonet probes in the rest, not the other way around.)
A mixture imbalance
Cylinder #2 is running richer or leaner than the other cylinders.
Since you say the engine is not running rough Cylinder #2 would probably be running rich.  (This is normally something you'd notice with Exhaust Gas Temperature rather than Cylinder Head Temperature.)
You can check for this by watching the engine monitor as you lean the engine: Cylinder #2's EGT will peak at a different time than the rest of the cylinders (early = leaner, late = richer). On fuel-injected engines this can be adjusted for by using tuned injector nozzles. On carbureted engines you just have to live with it - but at least you'll know the cause and be able to determine what "normal" is for this engine.
Fouled/Failed Spark Plug
Easy to check in-flight: Cycle each magneto; If the cylinder dies on one you've found the culprit.
(Something you should probably catch on your preflight mag check too.)
A cooling problem
If the cooling baffles for your engine are worn/damaged/incorrectly positioned you might have anomalous airflow that's directing more cooling air over the #2 cylinder head than the rest of the cylinders.
(Normally you find baffle problems because one or more cylinders are running hot, but cold is possible too.)
A cylinder problem
An exhaust valve that's not fully closing can cause low CHTs - your mechanic can confirm this with a borescope (or a compression test -- they'll hear air escaping through the exhaust if the valve isn't closing).
If this is the case it's something to attend to, because if it's ignored it can lead to an exhaust valve failure, which can ruin your day.


Accelerated Stall, what is it, how do you prevent it? I've read a lot of NTSB crash reports regarding small, GA aircraft (just trying to figure out what went wrong and what to avoid.)  There seem to be a lot of reports that talk about "low altitude high speed stalls" (accelerated stall) happening on approach.

What is an accelerated stall, and how is it created?  What is the best way to avoid one?  As they seem to cause a lot of GA accidents...

Short Version

An accelerated stall is a stall that occurs at an airspeed higher than normal due to a higher load factor (g loading).

Longer Version

When an aircraft is in a bank or when pulling back on the yoke quickly, the wing has to create additional lift to support the aircraft since the load factor has increased.  This increases the angle of attack beyond the critical angle of attack (the point where the wings can no longer produce enough lift to support the airplane) and the airplane will stall even though it is above the normal stall speed for the airplane.

Example

Let's say that we are flying an airplane that has a "normal" stall speed of 60 KIAS.  If we start a 60 degree banked turn, this will increase the load factor on the airplane by two (2g) and will increase our stall speed by 41% to 85 KIAS.

A fairly common cause of this type of accident is when a pilot overshoots the runway centerline during the turn from base to final and steepens the bank angle in order to "turn faster" to get back on final.  Because the stall speed goes up, they can stall the airplane even though they normally never even come close at their current airspeed.

Instead, they should have just maintained the current bank angle that they had (instead of steepening it) and continued the turn back to re-intercept final using normal maneuvers.  If that couldn't be done in the amount of altitude/distance available, then simply go around and come back to try again (while making the proper wind correction in the turns this time).


Are there any airplanes with the Centre of Gravity behind the Centre of Pressure? We are used to seeing a forward CG in all airplanes, from a small Cessna to a large Boeing.

I wonder whether any airplane with CG behind wing CP exists in production? Is it even possible? If so, what about its stability?

EDIT: I mean wings CP, because it is the most common definition of the airplane CP. Usually for civil aircrafts the wings produce lift, and the tailplane produces downforce to counteract wings rotation moment about CG. These aerofoils (wings and a tailplane) have 2 separate CPs and two forces in the opposite directions. I ask about the wings CP in normal wing AoA ranges (excluding aerobatics, critical AoAs etc.)

In steady flight both points are exactly at the same lengthwise location. The pilot is able to shift the center of pressure somewhat by moving the elevator. Therefore, every aircraft that pitches up will become one where the center of gravity is behind the center of pressure.

But I guess you did not request this answer but a different one. That, however, would require a different question: Are there any airplanes with the Centre of Gravity behind the Aerodynamic Centre? Please read here about what those different centres are.

Most aircraft are indeed built such that the center of gravity is ahead of the aerodynamic center. This allows the pilot to let go of the stick momentarily and gives him valuable force feedback on the stick when the aircraft's speed or load factor changes. And it is required for civilian aircraft so they can be certified.

However, placing the center of gravity further aft has become a de facto standard for military combat aircraft with supersonic capabilities. Please read here why this helps. To summarize:

Agility is enhanced
Supersonic trim drag is greatly reduced
Smaller wing and tail surfaces are needed for the same take-off and landing performance.

The first aircraft which was designed with the center of gravity behind the aerodynamic center was the Wright Flyer I. When designers found out that aircraft could be made naturally stable, they placed the center of gravity more forward, and only with fly-by-wire controls did they return to the rear center of gravity location. The first design to do so on purpose was the F-16, and today every fly-by-wire combat aircraft with supersonic capabilities follows the same path.


Why are aircraft not equipped with nose cargo doors when being converted from passenger to cargo variant? I'm trying to find any substantiation / design justification why none of the OEM/STC holders are able to provision nose cargo doors on A321 / B757 / B767 / B777 fleet as part of P2F (passenger to freighter) conversions.
I'm sure there would have some discussions/presentations on this, appreciated to guide with reference.

The simple answer is there isn't the space. The problem with nose cargo doors is the cockpit. The cargo door must be high enough on the aircraft to line up with the main cargo hold to be useful. On larger aircraft designed with this in mind from the start, (747, C-5, An-124, An-225) the cockpit is placed in a location above the main deck so that the cockpit does not obstruct the path from the main cargo hold to the nose door.
This is specifically why the 747 was built as a two deck aircraft with the hump. They wanted to be able to fit a nose door, so they needed to put the cockpit on a raised deck, which left them with the hump deck for aerodynamic reasons.
However, on current generation narrow bodies, they simply aren't tall enough to do this. They are single deck aircraft, with the cockpit located directly in front of the main payload deck.


How to find runway Centerline spacing on airport diagrams? I know that airport diagrams show the exact position of airport runways in DMS (degrees, minutes, seconds) and therefore could use trigonometry to find how far apart runway centerlines are from adjacent parallels, but is there a more accurate way to find runway centerline spacing on these diagrams?

The scale is usually depicted on the chart. Just take a ruler and measure.


Can the removal of a duty-free sales trolley result in a measurable reduction in emissions? Recently, SAS announced that:

  Withdrawing tax-free sales will reduce the overall weight of the aircraft, which in turn will reduce fuel consumption and emissions.

I could buy that this might be a polite way to say "we don't make any sales, consequently, the trolley with the items is just ballast; throw it away". It's not the first time that I hear it though: some time ago I remember an article in Aegean's Blue magazine about saving 15 tons of CO2 in one year just by replacing the pilots' flight bags with the electronic equivalent (EFBs). Unfortunately I don't remember in which issue was that.

Now assuming that a trolley weights approximately 25kg empty so let's say 30kg with the merchandise. The L/D ratio of a an Airbus A320 is 16,3 and thus a 30kg reduction in the weight will result in approximately 18,05N reduction in drag and and an equal reduction in the thrust required to maintain the same speed.

Compared to the ~150kN that 2 CFM56-5B4 produce in cruise, The 30 Kg reduction in weight will result in 0,00012% a 0,038% to 0,06% reduction in required thrust (depending on the aircraft gross weight), which seems like a drop in the ocean. Can this make a difference measurable by an airline company at the end of the month/semester/year/century?

Yes, the removal of payload will result in a measurable reduction in emissions.

First, I have to correct some math: the engines don't run at 150 kN in cruise - that would indicate a L/D ratio of only 4-5, since a typical A320 weighs around 60 tons mid-flight (give or take 10). Fuel consumption is roughly proportional to weight, so 30 kg out of roughly 60,000 kg is a 0.05% reduction, not a 0.00012% one.

As to whether one would be able to move the throttle down exactly 0.05% to benefit from the reduced weight and leave everything else equal, no, not even the autothrottle is so precise.

But that doesn't mean fuel consumption is unaffected: the balance of an aircraft's weight, lift, drag and thrust always affects its energy. At the exact same throttle setting, the aircraft will climb a fraction of a percent faster, and so finish its climb and go to lower fuel burn at cruise a few seconds sooner. In cruise, maintaining the same flight level, it can spend a minute or two at a bit lower throttle, or trimmed a bit down, which will make it arrive a few seconds sooner.

This is fundamental conservation of energy: if less of it is expended in drag, then either the engines have to work less, or the aircraft will climb or accelerate. This happens all the time - an airliner in level flight doesn't stay in perfect equilibrium throughout, but corrects for energy changes every once in a while. Even if it doesn't change how much fuel is pumped into the tanks that particular day, the effect is cumulative across all sources and persistent through random variations.

Overall the effect of weight on fuel burn is proportionate. Depending on the distance, everything aboard an aircraft can consume 10% to 50% of its weight in fuel each flight. A very rough ballpark is that, over the life of an A320, each pound of operating empty weight will cost an extra ton of fuel.

To be more specific, given the A320's fuel burn of roughly ~2,500 kg/hour, adding the trolley requires an extra 1.25 kg of fuel per hour it's in the air. For a 4-hour flight, that's an extra 5 kg of fuel. Over the 60,000 flight-hours an A320 will serve in total, that's 72,000 kg of fuel. So behind that trolley is 3 large tank trucks' worth of fuel to keep it in the air, and ~225,000 kg of CO2 emissions.

As an aside, in-flight sales of random junk really serve the airline, not the passengers. The profit margin on these items can be up to 100 times the tiny profit margin on economy tickets themselves. With airports packed full of duty-free shops, there's no shortage of shopping opportunities on an air trip. Cramped and burning fuel for every item on board, whether it sells or not, aircraft don't make for a very efficient or practical storefront.

Airlines only make up for this inefficiency by exploiting their cabin crews as free salesmen. Of course, airlines' environmental consciousness tends to correlate strongly with the price of jet fuel, so once sales profit no longer covers the extra burn, it's time to go green.


How do I fly to Borrego Valley (L08) if it's inside a CA state park? I'd like to fly to Borrego Valley Airport (L08) and it's inside California's Anza-Borrego Desert State Park.  How does this work, given that I'm supposed to say at least 2000 feet AGL when flying over a state park?  Am I allowed to descend below 2000 feet AGL only for takeoff and landing at the airport, or can I be below 2000 feet AGL at other times when flying over the state park?

Having never flown into this airport before, I would study the sectionals and charts very carefully. The state park is one concern. There are specific notes on the sectional requesting aircraft to stay above 2000 feet. There are also restricted areas, military training areas, laser illumination areas, etc. all around the airport. Not to mention the rising terrain and obstacles as high as 7100 feet around an airport with a 522-foot field elevation and a 10,000-foot Minimum Sector Altitude. Then, the airport having to have a specific obstacle departure procedure is a big red flag for me.

If it were me flying into this airport, I would request flight following to the RNAV (GPS) RWY 26 IAP all the way in to the traffic pattern. Then, I would land straight in or circle to runway 8. The minimums are probably a full 1200 feet above the field elevation for a reason. Remember to circle South of the airfield (right traffic pattern for runway 8). And, do not overfly the area Northwest of the field nor the elementary school 1 nautical mile West of the field. This is from a quick glance of the charts and chart supplement.


Why is white light more visible than red and green on this Cessna model on a sunny day? Source: https://i.ytimg.com/vi/hNgu1vypV3g/hqdefault.jpg

VS

Why is the white Landing light more visible on a fine sunny day and not the other two? I would have assumed the Red one to be most visible during flight, followed by Green but from a distance on a sunny day you can only see the white Landing LED very clearly but neither of the other two. Could it simply be because of the intensity of white light being more (having 2 bulbs) as compared to the other two having 1 bulb each?

This picture does illustrate the position of lights but since it was taken in a dark setting it doesn't do justice to how it appears during the day. Those red and green lights are not visible at all from any distance beyond a few meters.

When its a little dark (evening / cloudy) they are clearly visible.

Please Note: It is not really so that I expect to see all these 3 lights clearly on a bright day, obviously against sun light they stand no chance. I am just curious as to why the colored ones are far less visible than the white one.

The navigation lights are meant to give away the airplane's orientation (i.e. which direction it is travelling) at night. The red and green lights are small. Their purpose is to be observable from a long distance, which only requires a small light. For the same reason, the anti-collision red lights of tall structures are small as well.

The landing lights (white light) are meant to illuminate the runway, taxiway or any terrain in front of the aircraft. The light must travel to a certain distance in front of the aircraft then bounce back. They are very bright. To experience that, go to a busy airport at night and stand near the approach end of a runway. In fact, standing next to an airliner with its landing lights fully on would likely damage your eyes.

Landing lights must also be turned on when any aircraft enters an active runway. Here they serve the same purpose of turning on the headlights of your car at night: to make sure others can see you. This regulation came after an airliner landed on an occupied runway at night. It was determined that the plane on the ground blended in with the runway lights, and the pilots of the landing aircraft were unable to spot it. Had the plane turned on its landing lights, the very bright light source would have give away their position and alerted the crew to abort its landing.

The model is simply a replica of the real life scenario.


Could a knowledgeable passenger have saved Helios Flight 522? Helios Flight 522 crashed when the crew unknowingly suffered hypoxia. Although the masks in the passenger cabin deployed, the crew were unaware of this and didn't realise the extent of the problem.

If I was a passenger on a plane at cruise altitude and the oxygen masks deployed, I know that there is only about 10 minutes oxygen available. I would expect the plane to immediately descend to a lower altitude.

If I don't notice any change in attitude and there is no passenger announcement after a few minutes, would it help to alert a flight attendant to check that the pilots are aware of the issue? Or would it be best to assume that 'they know what they are doing' and just keep quiet?

It's never bad to inform a crew member about anything you think may be abnormal.

In this special situation, as you know that the oxygen masks deploy automatically if the cabin altitude raises to high, you expect the aircraft to descend in order to allow the pax to breathe normally again. If this doesn't happen it would be possible that the crew deployed the masks manually for no reason or the automatism faulty detected a low cabin pressure. In both cases I would expect an announcement that the oxygen masks are not needed and can be put down safely.

If neither of those options happens something is definitely wrong. Either the pilots already suffered hypoxia or they didn't notice the passenger masks drop at all.
In my opinion the cabin crew should be trained to know what's normal procedure if masks drop, but I would definitely tell a flight attendant about my concerns if nothing happens after two minutes or so.

You always have to remember: In high altitudes you become unconscious after less than 30 secs without the mask, and as the masks can only produce a limited amount of oxygen it's a really time critical issue to reach the flight-deck and initiate a descend if the pilots are already out of service!


Why have none of the US /UK/Japanese airlines ordered the Irkut MC-21 yet? A new airliner was launched on June 8, 2016 by the Russian company Irkut Corporation.

Source: https://www.flightglobal.com/

The airliner looks good on paper:

Empty plane weight is reduced due to composite and prospective alloy materials and lighter new generation onboard systems
Wide cabin (3.81m), MC-21's cabin is wider than the cabin of an
Airbus A320 by 12 cm and a Boeing 737 by 28 cm. As a result, two
passengers can walk through the aisle without blocking each other.
The wide cabin also supports the biggest luggage racks in its class
in order to accomplish the goals of decreasing MC-21's turn-around
time at the airport (important for charter and low-cost carriers),
providing better comfort for passengers, and making the cabin more
suitable for carriers' purposes
Relatively to the current models MC-21’s engines will emit 20% less
СО2 per passenger seat
High cost operational effectiveness, international certification
under AR IAC, EASA, FAA requirements, individual logistic and
maintenance support during all life cycle will make the aircraft
attractive both for Russian and foreign carriers

Source: Wikipedia

Almost all of the orders look like they come from Russian airlines. But why haven't any US/UK/Japanese etc. airlines ordered any planes?

Ordering an aircraft is a very complex business, and implicates a number of matters that don't actually (on the surface, anyway) have anything to do with the aircraft itself. Consider this from the point of view of a western European or north American airline.

Broadly political obstacles

These could include:

international trade agreements and disputes (Russia's relations with much of the world are not very friendly at present)
existing suppliers ("Lovely relationship we've enjoyed since the 1970s; it would be a shame if someone carelessly trampled it underfoot and soured it.")
governments and politicians ("Oh really? Well, it certainly has been nice supporting you fearlessly in Parliament/the Ministry/Congress/against my opponents who hate your guts. Bye!")
national expectation ("Traitors!")

History and reputation

Russian (like Chinese) design, technology, engineering and manufacturing have a reputation in the west that works against them. These things take decades, not years, to be overcome by stellar achievements in those fields (consider how long it took "Japanese" to become regarded as a positive rather than a negative adjective when applied to products).

However unfairly, this feeds in to other obstacles:

customers ("Get on a Russian airliner? No way!")
shareholders ("You're proposing to buy a what?!")

Risks

Then, it doesn't make sense to buy one plane to "try it out". Even with a single aircraft of the type, you'd need the same training, certification, engineering support, etc etc as if you had a fleet of them.

So, you'd be looking at a fleet of them, and now you're facing the purchase of a fleet of aircraft that very much represent an unknown quantity. It's a huge, huge risk; no wonder airlines cleave so strongly to the Airbus/Boeing duopoly - maybe it's not entirely to their economic advantage, but they don't expect too many surprises, which is arguably more important.

Some of these risks:

are simply technical ("How good will the product turn out to be in the long run?")
are economic ("Could the trade climate make spare parts and maintenance prohibitively expensive in the future?")
hinge on international relations ("Could our government ban us from working with this supplier in future? Could a meddling Kremlin disadvantage us?").

How could it work? Will it ever work?

Give it a couple of decades.

If airlines in places like India, Indonesia, Brazil, with fewer historical attachments and political obstacles than in the west, find that the aircraft and manufacturer prove to be good for them, bit by bit we may see that influence trickle into western airlines, especially as those non-western airlines and the economies they belong to expand and become more influential themselves.


Why are follow-me cars used in Gran Canaria Airport in good visibility for incoming aircraft? I've been at GCLP (Las Palmas de Gran Canaria, Spain) a few weeks ago.

I've observed that all incoming traffic was guided by a Follow Me while outgoing traffic went without one.

I was curious about that and looked for an airport chart to find more about it. The latest I've found is a Jeppesen chart from DEC 2004... so a bit outdated.

There it states in the LVP (Low Visibility Procedures) section only, that pilots have to wait at apron entry for the Follow Me while outgoing traffic should avoid requesting clearance in such a situation.

There is no notice, about what applies when LVP is not active.

What could be the reason for doing this even in good weather situations (visibility was at least 10km, if not even more)?

The unspoken truth (no references, sorry) for this also applies to many airports around the world (not just Gran Canaria). The follow-me service is not free, and it's one of the ways airport operators make money and create jobs for the community.

The Spanish Aeronautical Information Publication (.pdf) for GCLP does not state a reason, merely confirms what you saw, a follow me for all arrivals regardless of LVP. Here's an example of such paragraph:

  ATC clearance to land at RWY 03L, leave runway via TWY R1R or R1L, following TWY R2 and wait for ATC clearance to enter apron via Gate J, following the instructions of "FOLLOW ME" vehicle, via TWY R10 to the stand (...)

Also see a related discussion on pprune.org.


Would a slotted flap still work if the prop is located mid wing between the wing and flap? Would a slotted flap still work if the prop is located in a slot mid wing between the wing and flap?

This would be for a distributed propulsion application for an multi prop ultralight triplane, where the each prop would need to provide distributed propulsion for multiple wings vertically.

Yes.  Inserting a prop aft of the wing and forward of the flap would not reduce the flap's effectiveness.  If anything, the stronger, more concentrated propwash would make the flap more effective.
The answer doesn't change for a triplane.  But off topic, I wonder about how to design a triplane that's light enough to be in the ultralight category, what advantages it would have over a biplane or monoplane.  Commercial interest in triplanes petered out eighty years ago.


How does the Piper Archer III electrical system work? I can't seem to understand how the electrical system works:

What does the ammeter display?

What is said on my POH is that the ammeter displays the alternator output. My question is what output is that? In amps?

Does the battery power the DC bus?

It seems the battery takes AC from the alternator, convert it to direct current, and then feed the entire electrical bus system while still being charged. Is that correct?

I think you misunderstand how the alternator, battery, and electrical system are connected. An alternator puts out DC voltage into the electrical bus. It is used to provide electrical power to aircraft systems, as well as to recharge the battery from cranking and pre-start light and radio usage.

You can see this in the wiring diagram in the POH:

You can see the alternator about 2/3 of the way up the diagram, just to the left of center. The 'B' lead coming out of it goes directly into the ammeter and to the DC power bus (the big bar along the top).

The path back to the battery is a little more complicated, but there where it says "Battery Feeder", you can see it goes through a 70A fuse, down to where it connects to the battery contactor. So when the alternator switch is on and the battery switch is on (closing the battery contactor), the alternator can charge the battery. [In small Cessnas this is a split master switch, but I'm not sure what it looks like in an Archer.]

To answer your question specifically about what the ammeter shows, this will give you the amount of current the alternator is providing in amps. After startup you should see slightly more than usual as the battery recharges. You will probably see the number go up while you are transmitting on the radio. If you see this number get really large for no reason, there may be a fault in the electrical system somewhere. If you see this number go to ZERO, it's possible that the alternator has stopped working completely and your lights and radios are now running on battery for a limited amount of time!

I hope that is helpful. Note that nothing here should suggest a specific course of action -- this is just general electrical system information. Consult the emergency procedures or an experienced flight instructor for specific information about your airplane.


How should airline pilots find out about military activity along their route? What sources of information should a commercial pilot review before takeoff to become aware of military activity that might pose a risk to their flight?

Military operations are conducted in certain airspace blocks that should be well marked on your charts.  If you are IFR your route should avoid such areas, if you are VFR, hope you know what you are doing.


Why is TOGA thrust 'never' 100% N1? The title is voluntarily the opposite of this question: Why isn´t TOGA thrust always 100% N1 or N2?.
I explain to you with an example: I'm looking to the FCOM of an A320 which states that the "maximum in normal operation N1" is 100%, and "the maximum N1 permitted" is 105%. But then, by looking at the THRUST RATINGS - MAXIMUM TAKEOFF page, where there is the table with altitude on the columns and OAT on the rows, the maximum value of N1 that I can find is 94.5%. Any other N1 table (climb/go around/etc.) has clearly lower values for N1. In A320 this value is enforced by the FADEC and cannot be exceeded.
So what's the point to have a 100%-105% N1 limits if the enforced N1 limit in TOGA can be at maximum 94.5%? Why not just set 100% as the RPM value of 94.5%? Or the 94.5% can be exceeded?
Clarification:
My question is not related to maximum thrust (which it doesn't correspond to maximum N1). I cannot publish the table, however it's the same of this table, but for another engine and for the maximum takeoff mode instead of maximum climb.
I reformulate the question in a more direct way:

Why FCOM says N1 can reach 100% and 105%, if then no N1 mode allows any N1 value over 94.5%?

Why not make 94.5% read as 100%?
94.5% tells you the margin you have. It will be closer to 100% in high-thrust versions – or even higher, i.e. the limit is pushed as the thrust ratings increase. The PW1100G you're asking about, its thrust varies from 110 to 150 kN based on the installed version.
For example, the A321's 150 kN CFM56-5B3 exceeds 100% near sea level and in cool to moderate temperatures, unlike the 120 kN -5B4 (both tables below).
Note that nowadays whatever the installed takeoff N1 limit is, it's rarely limiting – meaning the engine will reach the EGT limit before it reaches the N1 (barring major failures).

Source: A321/A320 FCOM


Would removing heat from the compressor and adding heat to the turbine increase a gas turbine's efficiency? Since a compressor compresses air, the air temperature increases due to P1/T1=P2/T2. When the temperature is increased it would be harder to compress - so would removing the heat from the air inside the compressor reduce the energy required by the compressor, and increase the efficiency of the engine? Also, since the turbine accepts hot air and uses the expansion of the gas to do work, would adding the heat extracted from the compressor to the turbine area increase efficiency even further?

Heat removal like that is sometimes done in gas turbine engines, not by cooling the compressor during operation, but by the use of an isobaric intercooler between the low and high pressure compressors.  These kinds of configurations are common in industrial gas turbines used for electric power generation, marine propulsion and other applications where extra weight is not as large of an issue as is with aviation gas turbines.

As far as adding this extracted heat energy back into the gas flow through the turbine section, this would not be possible as compressor outlet temps are in the neighborhood of 450°F where exhaust gases exit at around 1000°F, preventing a return of the compressor heat to the turbine section for extra power.  Typically intercooler exhaust is simply vented into the atmosphere.

More commonly, gas turbines are used in combined cycle powerplants which make use of the gas turbine exhaust heat to power a water boiler driving a steam turbine and secondary generator.  Some of the thermodynamic cycles used in these systems can be quite elaborate and can achieve an exergetic efficiency of over 60%, making them the most efficient heat engines on the planet.


How do airlines plan their payload for multiple flights in a day? Of course, this isn't specific to a single airline or airplane. As a base rule, it is needless to say that airlines want to profit as much as they can and the payload is a strong factor to influence the profits.

That said, let's take an example of a B737 of ABC Airlines that will be making stops to 3 different destinations before reaching its final destination.

Assuming 2 of the 4 legs have sold all seats, while the other 2 have sold only half:

How will the fuel on-board be planned? Will it benefit to carry enough for 2 legs so that they spend the least amount of time at the stop-over?
Will it benefit to only have the required fuel for one leg (including the reserves and alternate), and thence allow more luggage from passengers which would otherwise be declined?
Are these decisions made by dispatchers, pilots or a different department? Are they made right before each flight or pre-planned well in advanced?

I'm asking these because I want to understand how airlines plan their flights as to, you guessed it, maximize profits while still maintain inflation schedules and passenger satisfaction.

The only real answer we can give to this is it depends

It depends on the airline, the nature of the legs (long/short), the specific airports/countries involved (re-fuelling costs can vary significantly) and route profiles used.

If the aircraft is doing 4 very short legs, the cost of carrying the fuel may be less than the cost of refuelling three times. Equally it's certainly faster to fill the plane once, rather than 1/4 fill it 4 times, because so much time is spent in setting up etc, and you can't usually re-fuel with passengers on board.

On the other hand, if legs 1 and 3 are nearly at the endurance of the aircraft, you physically can't carry fuel for legs 2 and 4: your fuel tanks aren't big enough. Equally on longer routes, the cost of carrying extra fuel increases dramatically.

Flip that around and say that legs 1 and 3 are short, but legs 2 and 4 are long (without being at the aircraft's endurance) and you have a problem that during legs 1 and 3 you're carrying a lot of un-neccessary fuel.

All airlines will have different policies and procedures, and those policies will vary wildly depending on the fuel costs at different airports. But I'll try to summarise at least some of the options here

For lots of short routes, fuelling once for multiple legs can be viable, but may not be the best option depending on fuel costs at each location. You wouldn't necessarily do a full day on one re-fuelling instance, but you may fuel twice for 4 legs, for example
For a short leg followed by a long one, it's rarer but the above will still reply
For a long leg followed by a short one, it's unlikely to be cost effective, although in certain situations your second leg's fuel load could be close to the reserve needed for the first leg: as long as you don't have to eat into your reserve on the first leg, you don't necessarily need to refuel.
For multiple long legs, you almost certainly don't have the fuel capacity, and carrying so much extra fuel for so long makes little sense: that's expensive.

In short, you usually want to fly with as little fuel as possible - a light aircraft is cheaper to run, can hold more cargo/luggage etc.

In a few situations, the cost of refuelling can make it worthwhile to fuel up in one location, where the cost of fuelling there makes up for the cost of carrying that extra fuel around. That's a niche scenario though, and wouldn't be the "usual" mode of operation for most airlines


What's the relationship between aileron deflection and roll moment? I'm wanting to build a laterals-only kinematic model in MATLAB considering only the ailerons for control inputs and was wondering if anyone has a rough guideline for the relationship between their deflection and the resulting roll moment.

EDIT: The answer from @PeterKämpf on the following question helped a lot (Thankyou Peter) but I couldn't understand the difference between p, dimensionless roll rate and wx, rolling speed in rad/sec. How can a roll rate be dimensionless?

His answer is here: How I determine numerically the roll rate of an aircraft?

Sorry, I couldn't comment on the actual post, I don't have enough reputation.

Using a linear theory, the roll moment coefficient can be expressed as:

$$
C_l =C_{l0}+C_{l\beta}\beta+C_{l\delta_{\alpha}}\delta_{\alpha}+C_{l\delta_{r}}\delta_{r}
$$

, where:
$\beta$ : slip angle
$\delta_{\alpha}$ : aileron deflection
$\delta_{r}$ : rudder deflection

This doesn´t really say much, as we are simply defining this coefficient in a way that is convenient. This equation just says that $C_l$ is equal to a linear composition of the effects of assymetry, slip angle, and aileron and ruder deflections, each weighed by a stability derivative.
Some of these stability derivatives have proper names or are trivially discarded:

$C_{l0}=(C_l)_{\beta=\delta_{\alpha}=\delta_r=0}=0$ , for symmetric aircraft without engine torques
$C_{l\beta}$ is the dihedral effect, or the effect of slip on roll. It is the sum of contributions from the wing-body assembly, the HTP and the VTP. The wing-body effect can be further split into: the geometric dihedral of the wing, wing sweep and vertical position (high or low).

$C_{l\beta} =(C_{l\beta})_{WB}+(C_{l\beta})_{HTP}+(C_{l\beta})_{VTP}$

$C_{l\beta}=-\frac{a_w \Gamma}{4}-\frac{C_L}{4}sin(2\Lambda)+(C_{l\beta})_{\text{wing height}}+(C_{l\beta})_{HTP}-a_v\eta_v\frac{S_v}{S}(1+\frac{\partial\sigma}{\partial\beta})\frac{h_v}{b}$

$(C_{l\beta})_{HTP}$ is obtained the same way as the WB contribution, although with different adimensionalizers.
For general reference, wingtips above the root are stabilizing, positive sweep is stabilizing and the wing height effect depends on the fuselage fraction ahead and behind the wing, but usually a high wing should be stabilizing.

$C_{l\delta_{\alpha}}$ is the roll control power or the effect of ailerons on roll. Is is calculated directly from the Prandtl wing theory by considering the impact of aileron deflection on the lift distribution
$C_{l\delta_{r}}$ is the effect of rudder on roll

$C_{l\delta_{r}}=C_{Y\delta_r}\frac{h_v}{b}=-a_v\eta_v\frac{S_vh_v}{Sh}\tau_r$

Your original question asked for the roll rate, and Peter Kämpf does indeed provide an answer, but it is probably no more useful to your particular need than my very first equation.

If you are interested in the roll rate, you will need to solve the lateral-directional system. You can simplify it somewhat by allowing fixed controls and level fight, thus obtaining a stability cuartic. This has, for most configurations, two real and two complex conjugate solutions,  corresponding to the three lateral-directional eigenmodes: Roll Subsidience (large negative real root), Spiral Divergence (small real root, sometimes even >0) and Dutch Roll (complex conjugate root). Since the large negative real eigenvalue has a much greater modulus than the rest, its mode (Roll Subsidience) will dominate for short periods, and you can obtain a good approximation of the roll transfer function by doing $\Delta\beta=\Delta\hat{r}=\Delta\delta_r=0$ and $C_{l\hat{\dot{\delta}}_\alpha}\cong0$:

$$
G_{\hat{p}\delta_\alpha}=\frac{\Delta\hat{p}(s)}{\Delta\delta_\alpha(s)}=\frac{C_{l\delta_\alpha}}{\hat{I_x}}\frac{1}{s-(C_{l\hat{p}}/\hat{I_x})}
$$

...in the Laplace plane. Keep in mind the parameters are non-dimensional.


How are VORs oriented near the magnetic poles? I noticed on SkyVector that, for example, the Resolute Bay VOR (YRB) and Baker Lake VOR (YBK) seem to be oriented in such a way that the 360 radial is pointing in the direction of true north (and I don't think the variation is anywhere near zero there).

I know VORs are supposed to be oriented according to magnetic north, but is it common practice close to the magnetic poles to have them point true instead? If so, is there any way to tell if a particular VOR is doing just that, except looking at the chart and noticing that the small arrow is pointing north rather than off to one side?

In Canadian Airspace, Northern Domestic Airspace (NDA) is the area of compass unreliability within which runways and NAVAIDs are oriented to true or grid north versus magnetic north.

You should not need to worry about which NAVAID is to true north; simply keep track of whether you're in Southern or Northern Domestic Airspace.


Why would sea level static thrust have a takeoff and a maximum continuous value? In the type certificate data sheet for a jet engine, there is this table
static thrust is the thrust generated by the engine when the plane is at rest with respect to the Earth and the surrounding air. So why would it have different values? And is the plane still "at rest" at these values?
Also, what is the different between thrust and sea level static thrust here?

It is from http://aerodesign.stanford.edu/aircraftdesign/propulsion/tvshv.html

The static thrust value is with the engine fixed in place in still air.  Imagine the engine on an outdoor test stand, or on an airplane with the brakes on or tied down.  As soon as the airplane starts to move, the thrust value declines and the static thrust value no longer applies.  The static thrust value at sea level is used because it provides a consistent baseline reference, because once the engine starts to move, additional variables come into play.
The lower chart is showing the relative decline in thrust with speed for the range of engines between pure jet (bypass ratio = 0) to very high bypass (5-6), to illustrate the impact of bypass ratio on the decline in thrust with speed.  "Static thrust" is implied at the speed value of 0 Mach (airspeed is zero, so you are "Static" by definition) as a value of 1 and declines in thrust from static are a value less than 1. So you can interpret the left edge of the chart as "static thrust" and everything to the right as (moving) thrust, with the curves going down and recovering (or not) with increasing Mach #.
It's just illustrating relative effects, so with 6 engines that have different bypass ratios, their decline with speed, from the engine's Sea Level Static Thrust (whatever it is, represented by the value of 1), is shown by each curve. Following the curves as an example, the pure jet has only lost about 7% of its rated static thrust with speed at Mach 0.5, whereas the very high bypass fan has lost over 40% of its static thrust value at Mach 0.5, in the same atmospheric conditions.


What is this hole on the left wing of the Eurofighter Typhoon? I came across the picture below and saw the hole near the centerline on the left which appears to have some carbon next to it. What is its purpose and where does the carbon come from?

The Auxiliary Power Unit (APU) exhaust

(Source)

The item 72 is what you are looking for and it is labelled "APU exhaust". The black residue is soot from incompletely burned fuel.

The Eurofighter Typhoon can use its APU to start the main engine or to provide power when on the ground. From airvectors.net:

  The Eurofighter is also fitted with an auxiliary power unit (APU) for self-starting and ground power.

See also:

What is the APU actually used for?
Do fighter jets have an auxiliary power unit?


Where can I find coordinates for Special Use Airspace (e.g. restricted areas) in the USA? I am looking for exact specifications for a few prohibited and restricted areas near me so I can plot these areas in other mapping software.
I can use sectional charts to estimate, but I am curious if there is some kind of listing that contains coordinates that define these SUAs.

As Pondlife says, this answer tells you where to look: All special-use airspace is defined in JO 7400.10. Version C is current as of February 16 2021.


How do slats actually work? I want to know what is the difference between slats and flaps and the mechanism of the slats

This what I know and researched about flaps:

Flaps are a kind of high-lift device used to reduce the stalling speed of an aircraft wing at a given weight. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used to reduce the take-off distance and the landing distance. Flaps also cause an increase in drag so they are retracted when not needed.

This is picture of slats in real life

Flaps on a plane

Once again, I want to know the function of the slats and the difference between flaps and slats.

A slat is a retractable version of a device known as “leading edge slot”. It is basically a slot just behind the leading edge.

When an airplane is flying slow, it will probably either be somewhat nose-up to maintain level flight, or be descending, or somewhere in between. In either case, the air strikes the wing at a higher angle than when the plane is facing forward and flying straight at high speed. The angle with which the air hits the wing is called the angle of attack.

On the surface of the wing, there is a layer of slow-moving air called the boundary layer, caused by friction between the wing and air. This boundary layer makes the wing appear thicker than it is, particularly towards the back, since the air piles up toward the back. The faster airflow prefers to flow around this boundary layer, instead of through it/around the foil, meaning the fast-moving air is turned less, and so produces less suction at the top of the foil.

At the trailing edge, this boundary layer terminates in turbulence, and creates more suction at the back. Obviously, due to the curvature of the airfoil, the backward side of the top of the wing is more angled towards the back and less upwards, so having suction in that region creates more drag and less lift than having suction further forward.

In effect, due to the boundary layer, the airflow around the wing “sees” the top rear portion of the wing bulge out more than it actually is. The resulting “shape” of the wing is flatter at the top, creating less lift, and has a blunter, steeper trailing edge, creating more drag.

At the higher angle of attack mentioned above, the boundary layer over the top of the wing is even thicker, since, as you can guess, the airflow around the wing would be slowed more on the more difficult journey. Eventually, at very back, the direction of airflow in the boundary layer may reverse, (this is what is called flow separation) creating even more turbulence and increasing suction in that region even more, making something that is bad worse.

With increasing angle of attack, the boundary layer continues to thicken, and the zone of separated flow expands forward, eventually there will come a time when the wing “bulges” out so much it no longer produces an increase in lift, but produces a lot of drag. This is a stall.

The slot behind the extended slat lets in more fast moving air, speeding up and thus thinning the boundary layer.

So, what is the difference between flaps and slats?

Flaps deflect air already flowing past the wing downward. Slats enable fresh air to flow over the wing. (NOTE: there are flaps with slots in them. These slots work the same way as slats.)
Flaps can work at zero angle of attack. Slats can't.

Also, just a clarification for the OP about the “droop”. Leading edge droop is exactly what it sounds like. It gives the air a smoother journey over the wing at high angle of attack, increasing lift. But at low angles, it produces negative lift.....


What are the differences between direct law and mechanical back-up? I am learning for an avionics exam and I cannot find any differences between the direct law and the mechanical backup.

I will describe the laws that I learned such that it is easier to see if I have some misconceptions.

Normal law: Standard mode of operation
Alternate law: (at least two failures detected):

Loss of pitch and bank protection
$\alpha$-protection and high-speed protection replaced by stabilities
Load factor limitation is still active

Direct law: (at least two failures second not self-detected)

All protections lost
Manual pitch trim

Mechanical back-up: (all electric supply for control computers lost)

All protections lost
Manual pitch trim

  Is the only difference between the direct law and the mechanical
  back-up that we do not have operating computers for control?

The key piece of knowledge to understand a FBW system is that you need to differentiate between signalling and actuation.
signalling is what tells the control surface to move and to what position. In old airplanes, this was by steel wires. In FBW airplanes it's by copper wires carrying an electrical signal.
actuation is what actually moves the controls surfaces to the commanded position, the muscles. Traditionally this is hydraulics, but you can also find electric actuators these days.
For the Airbus, Normal, Alternate & Direct all use electrical signalling (whether produced by computers or not). But what to do if there's no electrics? That's Mechanical backup, where you have steel-wire signalling of specific surfaces.