How to stabilize the aircraft flight after dropping boosters? I want to make aircraft model with boosters, but i don't know how to stabilize it. Аfter dropping the boosters, the center of mass of the aircraft will change, and I would also like to know whether it is necessary to stabilize it at all? Also during the flight on its own engine with fuel consumption, its mass will also change, whether in this case it is necessary to stabilize it? I need methods/ways or sources of info about it (maybe books, articles). Thanks for your attention.

The most important aspect is the vertical distance between the booster's thrust vector and the center of gravity. Once the booster stops working or is dropped, a pitch trim change will be needed to compensate for the upset in the pitching moment balance.

Next, the horizontal distance between the boosters and the center of gravity will tell you how much the pitching moment is changed upon dropping the boosters.

If control authority is sufficient to trim all those pitching moment changes, stabilization should not be much of an issue, especially if a sufficiently sized tail gives sufficient pitch damping.


When flying into an area with colder air, how will the altimeter read compared to true altitude? When flying into an area with colder air, how will the altimeter read compared to true altitude?

Air expands as the temperature increases and it compresses when the temperature decreases, shall I expect it is flying to a higher pressure area ?

There are lots of questions on the FAA Knowledge Tests about altimeter readings at non-standard temperature and pressure settings. The non-standard pressure questions are fairly intuitive. We all know that pressure decreases as altitude increases. Therefore, if the pressure is lower than what the altimeter expects, the altimeter is fooled into thinking it is higher than it actually is. Likewise, if the pressure is higher than standard, the altimeter reads lower than it should.

Dramatic and dangerous effects most commonly occur when crossing a frontal boundary from an area of high pressure to an area of low pressure. If the altimeter is set to the local altimeter setting in the area of high pressure and the aircraft flies to an area of low pressure, without changing the altimeter setting, the altimeter reading will be too high, possibly by hundreds of feet. Less dramatic changes happen all the time when you fly. As an example, right now the altimeter setting at KSBP is 30.08. The setting at KSBA (62 nm away) is 30.01. If you fly to KSBA you would notice a change in altitude of -70 feet when you get the new altimeter setting from approach control. The aircraft is 70 feet lower than you thought. That is the origin of the memory aid, “HIGH TO LOW LOOK OUT BELOW”.

AIM 7−2−3. Altimeter Errors

  b. Once in flight, it is very important to obtain frequently current
  altimeter settings en route. If you do not reset your altimeter when
  flying from an area of high pressure into an area of low pressure,
  your aircraft will be closer to the surface than your altimeter
  indicates. An inch error in the altimeter setting equals 1,000 feet of
  altitude. To quote an old saying: “GOING FROM A HIGH TO A LOW, LOOK
  OUT BELOW.”

Instrument Procedures Handbook

  When cruising below 18,000 feet MSL, the altimeter must be adjusted to
  the current setting, as reported by a station within 100 NM of your
  position. In areas where weather-reporting stations are more than 100
  NM from the route, the altimeter setting of a station that is closest
  may be used.

You will notice that, when using flight following or flying IFR, after checking in with a new controller, the controller will usually respond to your check-in with the altimeter setting he is using for flights in your area.

Non-standard Temperature
The same thing happens with temperature changes but it is less intuitive because we don’t have day-to-day experience with changing the altimeter with variations in temperature. We also learned about combustion in the engine and associate high temperatures with high pressures and that can lead to us getting tangled up in our thought process. Lets start from scratch and build a mental model of how the altimeter works.

Think of the aircraft as siting in a column of air. The altimeter measures the weight of the air above the aircraft. It doesn’t really know how much air is below it or how high it is above the ground. It just knows that at a certain pressure from the air above it makes it read a specified height. It has been calibrated for standard temperature and pressure lapse rates. If the temperature or pressure don’t change as expected, the weight of the air above it will not be the same as it is calibrated for, and it will give erroneous readings.

Let’s start our thought experiment with temperature and pressure at standard (29.92″ and 15°C) and the aircraft at 5,000′ MSL and AGL. Now imagine that the entire air mass being heated to be warmer than standard temperature. We know from high school physics that air expands as the temperature goes up. The entire column of air now expands and as the column of air expands, the aircraft moves higher in the column. Note that the weight of the air above the aircraft hasn’t changed, it just takes up more room. Since the weight of the air hasn’t changed, the altimeter reading hasn’t changed. The aircraft is now at a higher AGL than it was before but the altimeter still says 5,000′ MSL.

The reverse occurs when air temperature is lower than standard. As shown on the graphic above, the altimeter will read lower than it should if the air temperature is colder than standard. The same memory aid we used with pressure applies, “HIGH TO LOW LOOK OUT BELOW”.

People often get the impact backwards when they try to relate it to knowledge of engine combustion. In an engine, higher temperatures indicate higher pressures. So they think that if the temperature is higher, then pressure is higher. Normally higher pressure means altimeter reading is lower. The reason higher temperatures equate to higher pressures in an engine cylinder is because of the ideal gas law PV= nRT. For a fixed volume, higher temperature means higher pressure. But the volume is not fixed in the atmosphere, so the reasoning doesn’t translate directly to this problem.

For those who care, the standard corrections for temperature are: For each 10° C the OAT is warmer than ISA increase the indicated altitude by 4% to give true altitude. For each 10° C the OAT is cooler than ISA decrease the indicated altitude by 4%.


How does airfoil thickness affect the Mach number at which it experiences transonic flow? Roskam (book: Airplane Aerodynamics and Performance, Subparagraph: 2.5.2 HIGH-SPEED AIRSPEED INDICATORS (COMPRESSIBLE FLOW)) affirms:

  [...] Airplane with no sweep and thick airfoils become transonic at much lower flight Mach numbers than airplanes with significant sweep and thin airfoils. [...]

Question: How does airfoil thickness affect the Mach number at which it experiences transonic flow?

The displacement effect causes the flow around a thick body to speed up more than around an equivalent but thinner body. The thicker body pushes the air aside and around itself more, causing the flow to accelerate relative to the vehicle and to become supersonic at a lower flight Mach number.

Wing sweep has a similar effect, as only the orthogonal speed component will be affected by the wing and the effective Mach number of all thickness-, camber- and angle-of-attack-related effects is reduced by the cosine of the sweep angle. Again, the displacement effect will be reduced by sweep.


Is flap-asymmetry protection on airliners still active even on the ground? On aircraft with flaps, a general certification requirement is that it must not be possible for the aircraft to suffer a loss of lateral control secondary to an asymmetric-flap condition.  This requirement can be satisfied in one of two ways (many aircraft go for both):

Demonstrating that the aircraft remains controllable without requiring exceptional pilot strength or skill, even if the flap(s) on one side are fully up and those on the other side are fully down.
Including a flap-asymmetry protection mechanism, which automatically cuts off power (hydraulic or electrical, whichever the aircraft uses) to the flap-drive motors if the flaps on both sides fail to move in synchrony.

The benefits of flap-asymmetry protection in flight are obvious; however, in addition to their roles as high-lift devices in flight and high-drag devices during landing and rejected takeoff, flaps are also used to allow airliner passengers and cabin crew to safely egress through the overwing exits in the event of an emergency evacuation on land1 (low-slung aircraft, like the 737 and most regional jets, simply have the passengers slide down the flaps to the ground, while taller aircraft have inflatable slides that deploy over the lowered flaps).  In such a situation, as optimal use of the overwing exits depends on the flaps being fully down, having flap-asymmetry protection active would be counterproductive, as a jammed flap on one side would prevent a properly-functioning flap on the other side from providing a good evacuation path, and flap-asymmetry-induced lift asymmetry is harmless on the ground anyways (it produces a rolling moment to one side or the other, but an aircraft is physically incapable of rolling to either side when on the ground unless it’s already at or very close to flying speed, because the ground is in the way).
Is airliner flap-asymmetry protection inactive when on the ground (like many other systems protecting against things that are only a hazard when airborne, such as stall protection [stickshaker/stickpusher] and GPWS warnings) to prevent it from hindering an evacuation, or does it remain active even then?

1: In the case of a water evacuation, the buoyant characteristics of most airliners are such that the water generally comes all the way up to the wings, making the degree of flap deployment, or lack thereof, irrelevant.

It is active whenever the flaps are being moved by the hydraulic system on the 737; it isn't available when using the backup (electrical) system.
Allowing asymmetric flap extension before takeoff could have consequences as serious as asymmetric extension in flight. Plus, tying the protection to an air/ground sensor is one more way that the system might fail.
Evacuations are pretty rare events; the chance of having one along with a malfunction as described in the OP is extremely remote. Such an unlikely occurrence doesn't overcome the reasons to keep asymmetry protection available full-time.


Are transponders required for hot-air balloons in the US when operating within a Mode C Veil? I've taken some people flying recently for fun, and they noticed some hot-air balloons and asked me if they had transponders. In San Diego (Southern California) I often see hot air balloons well within the Mode C Veil (a 30 NM radius circle surrounding from a Class B airport). Do hot-air balloons need transponders when they are within the Mode C Veil?

I've gotten this question enough times to look up the regulations and answer my own question :)

14 CFR 91.215 lays it out quite nicely.  Particularly section B(3):

  (3) Notwithstanding paragraph (b)(2) of this section, any aircraft
  which was not originally certificated with an engine-driven electrical
  system or which has not subsequently been certified with such a system
  installed, balloon or glider may conduct operations in the airspace
  within 30 nautical miles of an airport listed in appendix D, section 1
  of this part provided such operations are conducted -

  (i) Outside any Class A, Class B, or Class C airspace area; and

  (ii) Below the altitude of the ceiling of a Class B or Class C
  airspace area designated for an airport or 10,000 feet MSL, whichever
  is lower; and

Where I see the balloons is definitely outside of the Class B airspace and below 10,000 feet MSL, so the answer is, no, they don't need a transponder.


Is there an increased concern for stall safety in a 1G steep turn? Most people know the standard, "At a 60 degree bank, an aircraft experiences 2G's" checkpoint for load-factor on an airplane.

I assume that this statement has an underlying, unspoken assumption that the aircraft is maintaining a constant altitude in the bank.

This leads on to matters of increased stall speed during turns.

My question is, if no pitch adjustment is made while in a coordinated turn, which I think means that no additional load-factor is acting on the aircraft, is the stall speed still affected?

To fly a 60-degree bank coordinated turn with 1 G on the aircraft, you'll be dropping like a rock.

But your stall speed with 1 G on the aircraft is what it is, regardless of how you obtain that 1 G -- straight & level, or coordinated turn with deceleration down to reduce the G loading, or inverted & pulling 1 G... the angle of attack that stalls your wing will be reached at the same speed in all of those cases.


What are the disadvantages of a Tail providing lift? (tandem-wings included) Most aircraft feature the tailplane configuration, which requires the main wing to provide higher lift than the weight of the plane itself, what seems counterproductive.

Few aircraft have featured tandem-wings like the Rutan Quickie, Proteus or the Maurice Delanne 10 with limited success. I usually read that the main disadvantage of the tandem-wing is that the rear wing is subjected to the downwash from the front wing, what greatly reduces its efficiency Q&A link.

My point it that standard tails suffer from the same downwash effect, and on the other hand this effect can be reduced by:

a T-tail
Making the front wing much larger than the aft-wing.

Thus, which are the flaws of this design (top)?
Given that it could still have the CoG in front of the Neutral Point.
Why are lifting tailwings refered as unstable? Q&A link
Is this layout suitable for VTOL aircrafts, in which horizontal take-off and landings are not an issue?

Thank you for your attention. I would appreciate remarks of any wrong assumptions.

Extra

Why the Proteus features a tandem-wing, and why this layout unusual?

Edit:
The reason for considering a lifting tail is that to make VTOL possible for this layout, the CG must be moved between the rotors, thus far aft.

A lifting tail just means lower static stability.

You are correct, flying in the downwash of the wing makes the tail less efficient. But that does not mean it is all bad: Since downwash increases with lift coefficient, the tail effectively sees a smaller angle of attack variation than the wing, so it maintains a healthy margin from stalling even while the wing is stalled. This helps to keep the aircraft controllable well into a stall (within limits, of course).

The reduction in downwash as the center wing stalls will even add a positive AoA gradient during the incipient stall which adds some more stability, making it harder for the pilot to command a further AoA increase.

And performance-wise, the download on the tail in conjunction with downwash actually might create a bit of thrust on the tail.

Concerning Rutan Designs: Their different looks are not for better performance but merely the expression of an oversized ego and his way of self-promotion.

EDIT:

The many downvotes I now collect for speaking my mind freely shows how well the self-promotion of Burt Rutan works. But it cannot bend the laws of physics which guide my opinion. Which I will continue to express freely.


When does a passenger plane leave the cruising phase? I have Flightradar24 data here. Which are in the following structure.

latitude    longitude   altitude    lastUpdate      velocity
....
49.34240    8.77556 19650   2018-11-01 09:38:09     415
49.35040    8.77490 19550   2018-11-01 09:38:15     414
49.35617    8.77442 19475   2018-11-01 09:38:20     414
49.37045    8.77308 19275   2018-11-01 09:38:24     412
49.38132    8.77208 19125   2018-11-01 09:38:30     411
49.38964    8.77132 19025   2018-11-01 09:38:34     410
49.39796    8.77054 18900   2018-11-01 09:38:39     409
....

This is data for a specific flight. My goal is to split the flight in two parts.

Pt 1: Airborne -> Exit cruise time / start Approach
Pt 2: Approach Start --> Touchdown

Well, I am not to deep into aviation. My naive approach would be:

  If the flying altitude has dropped continuously over the last k
  points, then the first timestamp is the approach start.

Now I ask myself, in what period of time does a passenger plane leave the cruising phase?

This is different from the pilot calculating the top of descent explained here: What is a rule of thumb for when to begin a descent?

How much ft altitude does an airplane usually lose when it comes out of the cruising phase? And how long does he need for that?

The background is that I want to understand how long a passenger plane usually takes to land at a specific airport from the approach phase. Optimally, I then see that on days when the airspace over the airport is fuller on other days, times, the approach phase takes much longer. Or do you think there are other factors playing an important role? Or is the approach not so correct. Maybe it would be better if I do not look from Approachphase, but from the time where a certain radius was reached. 80km before arrival?

The distinction when you cross from one phase to another is somewhat arbitrary.  However, you can slice this multiple ways depending on your goal:  The approach phase could be considered to start at the first point of a published STAR, (Standard Arrival) the initial approach fix of a published instrument approach procedure, when the pilot descends below the class A airspace, i.e. <18,000' MSL, or the first ATC assigned vector off the filed route.  (Or some radius as you suggested...)

If you really want to dig into this my recommendation is to go to a local pilot shop and purchase a set of Approach Plates/STARS and a high altitude IFR chart for the area you are interested in.  You will likely need help interpreting them if you are unfamiliar with these publications.

If you then have a specific question on your airport of interest there are plenty of experienced people here who are willing to help, but a full education on high altitude IFR enroute and terminal arrival procedures is beyond the scope of this forum.

In lieu of that, if you don't want to take that kind of time to learn something foreign but have a specific airport you are interested in analyzing, please let me know.  I have a full set of United States procedures loaded in my iPad EFB and would be happy to post a screenshot with an interpretive description of where I might consider the approach phase to begin for a specific airfield/runway combination.

There are many many different arrival procedures nation wide, into airports of varying levels of traffic congestion.  Without a better understanding of your actual goal the question as stated is almost to broad to answer, especially for someone who is admittedly "not too deep into aviation".  Good luck with your project.


Why are static ports external on a non pressurized aircraft? Seems like the intruments in a non pressurized aircraft have access to the static pressure right where they are. It seems like external ports would be subject to more error being exposed, and lines that can be obstructed.

What benefit is having the external ports?

Remember Bernoulli? Lift is created on the wings because the air pressure decreases as the air moves faster. Same thing around the fuselage, but to a much lower extent. The air flowing around the fuselage creates a low pressure area so air in the cabin moves from inside the cabin to outside causing a lower pressure inside the cabin. Opening the cabin air vents can cause the pressure to be higher inside the cabin than outside.

Many general aviation airplanes that are used for IFR flying have two external ports that are placed well back along the fuselage. The  ports are usually positioned in areas where the boundary layer of air tends to be very stable, which lessens the effect of the static ports being hampered by turbulent flow and the Bernoulli effect.

Barry Schiff has an interesting explanation of how the pitot-static system works and shows how the placement of the static ports affects the reading of the instruments.


Why does increasing thrust on one engine make the plane roll and not yaw? If you increase the thrust on the right engine, the plane rolls to the left. But why? And why does the plane roll to the right and not yaw to the right?

Note: I am not a pilot, nor have I followed any aerodynamic studies whatsoever.

Increasing thrust on the right engine only will make the aircraft yaw and then roll to the left.

The thrust does generate a yawing moment to the left, but the secondary effect of yaw is roll. In simplistic terms, a yaw to the left means that the right wing not only momentarily moves through the air faster than the left, but more of the wing is exposed to the airflow. This generates extra lift from the right, putting the aircraft into a left roll.

A more technical explanation of this effect can be found here.


Where do aerodynamical contrails get their condensation nuclei from, when forming at high altitude? On this website, it is explained how aerodynamical contrails form:

  It’s formed by the reduction of pressure in the air as it moves over the wing.  When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator).  The reduced temperature cause small drops of water to condense, which then may freeze.  The (frozen) drops get larger as more water condenses on them.

Here is a photo of aerodynamical contrails forming at high altitude:

Source

As can be seen, the aerodynamic contrails start right at the wing. What I want to know is, where the oversaturated air condenses. "Normal" contrails coming from the engines use the soot from the engines as condensation nuclei. But what about the aerodynamic ones?

I think the water vapour condenses on the wing, but I need to know it for sure.

(airliners.net)

Those are called aerodynamic contrails, and when they are colorful as the picture above, they're called rainbow contrails.

They form due to the lower pressure above the wing in cruise when temperatures are above ISA, which favor more water content in the air. The nuclei in this case are tiny water crystals that form due to the pressure (and temperature) drop above the wing. Those crystals then facilitate the persistence of the contrail behind the plane.

Source: contrailscience.com


If propeller-driven planes with electric engine became the norm, would descent for landing with the engines turned off become a routine procedure? To elaborate on the question: I know that in modern fossil fuel-driven aircraft turning the engines off during descent and/or landing approach isn't feasible for safety reasons (which makes total sense to me) - but, given that electric engines are quite easy/quick to turn on & off  compared to toboprops and turbofans, is it  likely that lowering altitude with the engines turned off for a big portion of the descent (let's say, untill an arbitrary altitude would be reached, e.g. 500m or something along those lines) would become a norm in the airline industry?

It's unlikely the pilot would be switching the "propulsion master" off, but it's very probable that "idle power" on the throttle or thrust lever would in fact be a stopped or stopped/feathered motor.  It's likely that regen braking would be electrically available, but it's probably something you wouldn't want to use in flight, for the same reasons you don't engage reverse thrust on jet engines before touchdown (in fact, there are some aircraft where you do, for a subset of the engines, in part because of the time needed to engage reversers).

Having instant throttle response available is very important, so "off" shouldn't be the same "off" you'd use when parking and tying down -- but having genuinely zero thrust could be useful for certain kinds of approaches (others, you want/need to carry some power to ensure you stay enough above stall to maneuver without steepening the glide slope).


Why would a Cessna 182T not enter a stall? When I took delivery of a new Cessna 182T last year, I did a test flight for certification purposes. During the test flight we had to perform a power off stall but that didn't go as planned as it was simply impossible to stall. What happened is this: when the airspeed dropped well below the power off stall speed we simply started to sink slowly with a nose-high attitude at about 35 KIAS. This "mushing" went on for what seemed ages before I eventually applied power and pushed the nose down to gain airspeed again. We tried it again after that and the same thing happened. I had an instructor with me who has thousands of hours in a C172 and he had never experienced something like this before.

The odd thing is that this only happened during that flight. During later flights this didn't occur anymore.

There was a passenger in the back seat, fuel tanks only half full so the CG was more aft than usual, but well within limits

Ever since that flight I've wondered:

What could cause this to happen? (My guess is it is CG related)

And most importantly:

If I would have continued this "mushing" flight, would it be possible to have entered a flat spin or a simple "drop out of the sky" like the infamous Air France 447?

I think this is more common than you might think in a C182. The 182 is much more nose-heavy than a 172 (which is particularly noticeable in the flare) and this seems to limit the amount of upward pitch authority the elevator has at low speeds. The only stalls I've done in a 182 are much as you described - you can hold full back-elevator and you just sort of sit there sinking.

Spinning could be an issue if it becomes uncoordinated, but a 182 is a very stable airplane and unlikely to enter a flat spin unless the CG is beyond the aft limit or severely uncoordinated. Perhaps someone else can weigh in further on that possibility, though.


How did the 737 become a 2-crew plane? In 747-400 documentaries it's often said that the switch from 3- to 2- crew cockpit is attributed to the digital displays that present only the relevant information, combined with the FMS.

Little to none about the control systems is ever discussed. Take the MD-11 for example, its avionics manage the systems even in an emergency, e.g., automatically managing the fuel cross-feed valves after an engine failure.

When compared to the 727, what changed in 4 years that made the 737 not require the flight engineer position?

Excerpt from "First-Hand: Evolution of the 2-Person Crew Jet Transport Flight Deck"

  Early in the preliminary design effort, Boeing had taken a careful look at the in service performance and accident history of previous airplane designs. The conclusions were interesting:

  In flight troubleshooting sometimes led to more serious problems, occasionally jeopardizing the safety of the flight.
  The flight engineer’s intense focus on systems problems often distracted one or both of the pilots, sometimes resulting in no one “flying the airplane.”
  Jet engine reliability was much higher than piston engines and they required little or no in flight troubleshooting or adjustment.
  Certain high workload periods during departure and arrival left little time for outside watch nor much time for handling abnormal events.

  In support of industry work to establish rational external vision requirements for transport airplanes, Boeing developed a computer model that determined the relative collision threat value of the visual space around the aircraft. Because airplanes are in motion and at a positive angle of attack, the computer model showed that the collision threat is concentrated generally forward and below the longitudinal axis of the airplane.

  In response to these findings the design team took a different approach to the 737 flight deck:

  A concerted effort would be made to simplify systems designs to increase reliability and to eliminate or greatly reduce the need for in flight trouble shooting.
  A Flight Deck design group would participate in all flight deck design decisions and would have considerable authority in the design of controls, indicators, and operating concepts across all airplane systems.
  Workload assessment would be used as an integral element of design to adjust functionality and location of equipment and controls.
  Operating procedures would be simplified and shortened to reduce distractions and provide ample time for outside watch.
  The autopilot would be improved to better support operations in the terminal area not just in cruise.
  The pilot’s stations would be designed to maximize external vision in the primary collision threat zone.

So it seems that a lot of the changes that facilitated the movement to a 2-crew flight deck were procedural ones rather than engineering challenges. Jet engines by that time didn't require constant attention and adjustments. It seems that this movement towards a reduced cockpit count was the result of "too many cooks in the kitchen", and while not explicitly mentioned it seems like Eastern Air Flight 401 was a driving factor (Edit: or at least they wanted to prevent something like that, turns out EA401 didn't happen until 1972 when the 737 was introduced in 1968).

I'm guessing though that this won't completely answer what you are looking for, I'm sure you'd want to see the specific input that the "Flight Deck Design Group" offered for the placement of controls and indicators. I guess one way of figuring this out would be to compare the cockpit layout of the 727 and 737, as far as I can find, those design decisions aren't published.


Could a blown wing ever be powerful enough to lift an aircraft at zero forward velocity? Theoretically, could an aircraft, using any manner of self generated air flow over the (fixed) wing, ever gain enough lift in order to take off (or just simply lift off the ground) at zero forward velocity?

I am guessing that if an aircraft and its engines were light enough, you might be able to provide a high enough airflow across the wings using the propellers (akin to forward motion) - although obviously I am fully aware that most of the important control surfaces would not be functional. This is just theoretical.

To what end.. well... a more efficient understanding of self-generated lift (possibly?) might result from research into extreme blown flap/wing super-STOL utilization. High-lift, small transport aircraft operating out of small congested city air/heliports with very small runways.

I am aware of the limitations of such designs, such as inefficiency at higher speeds and, in the absence of variable form and geometry (which would add too much weight) the fuel usage would become prohibitive. It's purely an idea for speculation.

Technically speaking, yes, it's possible, but wildly impractical.

Of course, as usual, Jan Hudec's explanation of the physics is completely correct. Whatever is blowing enough air over the wings to fly would generate an insane amount of forward thrust, meaning that you wouldn't stay at zero forward velocity for long.

However, it's completely possible to apply another force in the opposite direction equal in magnitude to what would otherwise be the net forward force produced by the engine(s) blowing the wings. This would yield a total net forward force of zero, thus no forward acceleration would happen, allowing you to take off at zero forward velocity. Of course, you'd want to make sure that whatever is applying this force isn't also blowing the wings in the opposite direction, otherwise you lose your lift.

Some possibilities for providing this force:

Normal jet engines mounted backwards
Normal propeller engines with negative pitch
Rocket engines mounted backwards
A (really strong) tether
Wheel brakes (this only works as long as you're on the ground, of course... you'd start accelerating forwards very quickly as the wheels break free from the ground, but you could technically leave the ground with zero forward velocity.)
A really big machine gun, such as the GAU-8 Avenger or Gryazev-Shipunov GSh-6-30
Lots of AK-47s ('lots' being a number roughly equal to the net forward thrust of the engines in pounds divided by 13)

This is roughly what the backwards-mounted rockets would look like:

C-130 Rocket Assisted Landing
Source: YouTube

Another possibility to technically meet your requirement of taking off at zero forward velocity would be to simply start off rolling backwards fast enough that you would accelerate forwards to zero forward velocity at the moment the tires leave the pavement. :)

Of course, as you've hopefully figured out by now, that all of the above are wildly inefficient ways to take off. They would, however, technically work. And, of course, in the case of the GAU-8, whatever is in front of you during the takeoff is going to have a very bad day.


The latest turbofan aircraft feature an exposed exhaust cone, does it come with some benefits? Aircraft like Airbus A320neo/A330neo/A350 series have a visible exhaust cone, which is not visible in the older versions of Boeing 747, Airbus A330/A321/A319 etc. Why is this design preferred over the other?
Image: Exhaust Cone not exposed
Image: Exhaust Cone exposed

The « cone exposition » is a result from nacelle engine design, and the choice to mix flows or not. The two main strategies are called:

Mixed exhaust engine resulting to long nacelle hiding exhaust nozzle and plug.

Separate exhaust engine which is shorter and then allows a view of nozzle component.

The reason to choose one type of exhaust is made by considering overall engine performance (mainly efficiency and fuel consumption), noise but also visual pleasing (for business jet for instance).
Exhaust mixer is generally more efficient (secondary flow is slightly accelerate producing more thrust) and less noisy (primary flow is slow down). They might be also an advantage in material resistance in cooling hot primary flow. See this article for details about advantage of mixed exhaust.
Nevertheless, long nacelle has the overcome to increase weight and drag of the engine and this is worst as the engine is bigger. Note that it is related to turbofan by-pass ratio (BPR). More the BPR is high and bigger the engine diameter is. For commercial aircraft, the trends is to increase more and more the BPR parameter because it is directly correlated to propulsive efficiency. Therefore, there is a kind of « BPR limits » where mixed nozzle benefits do not balance its drawback.
That's why, modern turbofan (high BPR and high diameter) have likely to integrate a separate exhaust nacelle.


Why is the GP7200-powered A380 fleet not grounded? After the Qantas A380 engine failure, all Trent-powered A380s were grounded.1

After the Air France A380 engine failure, the GP7200-powered A380 fleet was not grounded.

Why were these two incidents handled differently? And what are the primary drivers for grounding a fleet?

The one that decided to ground the fleet was Qantas. Qantas was having a new airplane model and potentially made a trade-off between having a not known design flaw and keep flying and the risk. At that time the A380 entered in service three years before and around 20 aircraft were in service with a mix of engines.

Today, there are 200 A/C in service for 10 years. The possibility of having a design flaw or an unknown problem is really low.

It was not a airworthiness directive as the directive was created 2 months after in January 2011. Qantas starting to get back in service the airplanes on 27th November (incident was on 4th Novemeber)


Could a Personal Locator Beacon or EPIRB carried by a passenger aid in locating a downed airliner? There've been a lot of questions lately about tracking aircraft, and after a conversation with a friend of a friend I started wondering:

Could a PLB or EPIRB carried by a passenger or crew member aid in locating a downed or missing airliner? There are a dozen or so brands out there, most using similar technology, like this ACR ARTEX unit:

Now, I'm well aware of the FAA and FCC regulations about electronic devices in flight, and that's not what I'm interested in. I also realize that airliners already carry an ELT.

If a paranoid passenger brought one on board, would these devices function at all from inside an airliner cabin? Some feed GPS location data to a 406MHz locator beacon, and those would potentially be less useful (as they'd have to be at hand and probably triggered manually. Others, like the Spot GPS tracker, have the ability to upload GPS track data on the fly to satellite communications networks such as Iridium.

I'm personally skeptical, given that GPS generally needs a clear view of the sky and that a fuselage seems likely to block or reduce a 406Mhz ELT signal, but I'm curious to know if there's any information out there.

Relatively short answer

Cospas-Sarsat: Feasible but...

It's possible, but there are many individual elements involved. It's a matter of probability that most of the time there will be a combination preventing the link to work. If you know what to do and take care of ensuring a good direct line of sight (LOS) you may be able to avoid some of these combinations. It cannot work underwater.

Parameters that can attenuate the signal reaching the satellite receiver are of different origins:

The aircraft design itself (composite materials or aluminum, number of windows, ...)
The location of the accident (canyon, trees, ...)
The transmitter use (position, window proximity, metallic surface or debris interference, body proximity, ...)

I'm not aware of experiment results for using a PLB within a metallic container like a commercial aircraft cabin. It's still possible to evaluate the RF channel loss in different configuration and see if the signal can reach the receiver with the minimum required level.

As explained in the next sections about the Sarsat satellites of the international SAR service on 406 MHz:

The threshold for signal reception by the receiver is 161 dBW.
With an initial power of 7 dBW (5 W), the attenuation must be at most 168 dB
In good conditions we can expect an attenuation of "only" 148.1 dB. Such signal 12.9 dB stronger that the threshold can be detected easily.
In degraded conditions we may experiment an attenuation of 177.5 dB. Being 9.5 dB below the threshold, this signal is not detected.

Indoor transmission put the system at its limits of detection capability (a S-meter unit is 6 dB).

Weak signal on a S-meter, S-unit is 6 dB (source)

Many small attenuation causes that, when summed, can prevent the link to work must be taken into account, hence this answer is a long one.

SPOT LLC

SPOT LLC solution you mention is a commercial messaging initiative working on 1.6 GHz, out of the Cospas-Sarsat system. Higher frequency means higher attenuation. The limited 0.4 W RF output also makes the signal weaker. It's very unlikely that SPOT could work within the cabin.

Details

Introduction: PLB principles

Emergency PLB operate on 406 MHz. The previous 121.5 MHz civil frequency is not monitored anymore by SAR satellites, but can be used (in addition of the actual signal at 406 MHz) as a homing guide for the SAR team. They are informed about whether this homing feature is active or not by the message sent by the PLB.

PLB output power is 5 W (equivalences: 7 dBW 37 dBm) at 406 MHz, in vertical polarization.
From C/S T.004, the performance of the satellite system is:

A "probability better than 95% to provide a valid solution (15 hexa identification provided) for a beacon transmitting with a 37 dBm output power (with a whip  antenna) and for satellites passes with elevation above 5°)"
A "probability better than 95% to provide a Doppler location with an accuracy better than 5 km"

There are no satellites entirely dedicated to the SAR service, rather distress signal receivers are carried as one of the payloads aboard satellites. The system is made of two constellations:

GEOSAR constellation, Geostationary Earth Orbit SAR. These satellites are at fixed positions, 36,000 km above the equator, and are covering the globe except the polar regions.
LEOSAR, Low Earth Orbit SAR. Satellites in near-polar (about 85°). LEOSAR is made of two sub-constellations: Sarsat satellites orbiting at 850 km, and Cospas satellites orbiting at 1,000 km.

(Source)

A third constellation is planned to operate at a higher altitude than LEOSAR (MEOSAR, currently not fully deployed).
While GEOSAR are likely to be first to detect the PLB activation due to their permanent LOS, LEOSAR will provide the best detection due to the shorter distance to the device.

I'll focus on LEOSAR / Sarsat payload which provides the best chances for the detection of a PLB signal partially blocked by RF impermeable obstacles.

Sarsat payloads

Satellites carrying a LEOSAR Sarsat payload are NOAA 15 (Sarsat 7), NOAA 18 (Sarsat 10), NOAA 19 (Sarsat 12). Other are scheduled to be active in the future (some may be already, I haven't checked). They are fitted with a SAR receiver (search and rescue processor, SARP). Sarsat satellites can be tracked on N2YO.com.

According to Cospas-Sarsat LEOSAR Space Segment Commissioning Standard - C/S T.004, a SARP-2 has a sensitivity of -161 dBW, and SARP-3 -164 dBW (that's about 60 attowatts to be compared with a cellular phone cell: -134 dBW, 1,000 times less sensitive).

Can this small quantity be delivered to the SARP from the inside of a cabin?.. This is a matter of knowing the initial power, and determining the loss along the wave path until the signal reaches the receiver.

The power generated by the transmitter amplifier is 5 W, or 7 dBW. 161 dBW on the receiver means receiving a 1 W signal that has been attenuated by 161 dB. If we have already 7 dBW at the source, we can bear a maximum attenuation of 168 dB.

Main loss inventory

Propagation loss in direct view

The most important part of the signal will be lost in the atmosphere between the antenna of the PLB and the antenna of the SARP.

Loss due to distance

Loss is dependent on two factors: distance and wavelength. Both factors limit the area of the receiving antenna compared to the whole sphere of the wave front, and in consequence the portion of energy received is smaller too.

Using the propagation model ITU-R P.525 (free-space propagation).
Distance = 850 km, f = 406 MHz.
Assuming isotropic antennas for the time being.
According to the model, the loss (dB) is:
Lbf = 20 log(4πd/λ)
.   = 32.4 + 20 log(f) + 20 log(d)
.   = 32.4 + 20 log(406) + 20 log(850)
.   = 143 dB

The signal that left the PLB antenna reaches the SARP antenna -143 dB weaker, assuming the best antenna alignment. But this is never the case, because when in a polar orbit, the satellite is unlikely to be exactly above the beacon.

Misalignment of the receiving antenna

There will be an angle between the nadir of the SARP and the beacon. This off-nadir angle, depends on the actual track of the satellite for the current orbit:

Target off-nadir

The gain of the antenna varies according to this angle. Sarsat-TIROS SARP Receive Antenna (UDA) Gain Pattern:

UDA antenna gain at different off-nadir angles (source)

Assuming the polarization is favorable (RHCP), the gain may varies between +2 dB and -10 dB. Note that:

The gain is better when the off-nadir angle is large. This is likely to compensate the higher distance to the beacon.
I'll assume a link length f 850 km, but the actual length is larger when the beacon is not directly at the satellite's nadir.

We should now work with the probability distribution of the misalignment for the LEOSAR antennas to have the full range of possibilities and their likelihood (e.g. Monte-Carlo simulation). To simplify, let's just use a bad case: -4.4 dB (this is not the worst possible). Loss so far: -147.4 dB (-143 - 4.4).

Attenuation due to feeder and duplexer

On the satellite, the SARP is connected to an antenna via a duplexer to isolate the SARP uplink antenna from the SARR downlink signal (SAR repeater used on local mode, not detailed here):

Hardware on the Sarsat satellite (source)

The duplexer + feeder loss is -1.6 dB. Loss so far: -149 dB (-147.4 - 1.6).

Loss margin left

We had a budget for -168 dB loss, we have used -149 dB. The margin until the PLB signal is lost by the SARP is 19 dB, assuming we are in this configuration:

PLB is outdoor.
The LOS is completely clear, no foliage, no roof, etc.
PLB antenna has no gain/loss (0 dB gain).
PLB antenna is vertical, this is the normal use of the SAR device.
PLB antenna and RF output stage impedances match (no SWR).
There is no atmospheric loss (free space)

Additional losses

It's time to re-look at the assumption made until here, and take into account the additional effects encountered when the PLB is used from the cabin of a crashed aircraft.

PLB is outdoor.

Actually the PLB is behind a cabin window.
From Arkema:
"Most formulations of colorless Plexiglas sheet readily transmit standard broadcast, television and most radar waves of the electromagnetic spectrum". It can been assumed that windows (including the internal protection sheet) don't block the PLB signal. Loss = 0 dB.
In the cabin, numerous reflections will occur on the wall, and due to the closed space with only small windows, it's likely the reflected signals will globally interfere in a destructive way (phase mismatch) with the original signal.
If the PLB is displaced, even by a small amount, the reflections are changed. The RF energy actually vary continuously, creating a fading effect. Reflections has been studied in somehow comparable environments, for UHF frequencies. While the frequency was a bit higher, and the environment was a simple one, we can use the results for a rough order of magnitude:

RFID signal (915 MHz) loss due to reflections (source)
The spatial fading is visible, at some distances reflected signals add, at other they subtract. This loss is also known as multipath effect. The signal may be attenuated by 10 dB or more compared to free space.
Each reflection has also the potential to change the polarization. See next point.

PLB antenna is vertical, this is the normal use of the SAR device.

The transmitter and receiver antenna polarization must match or an attenuation occurs. The polarization of an antenna describes the plane in which the electric field is transmitted. For a simple antenna, made of a single active element, the electric field is sent in the plane of the element:

Vertically and horizontally polarized waves (source)
Vertical polarization creates less reflection on the ground, which is usually what is desired, so a PLB creates a vertical polarization, but only if the antenna is kept vertical, else the polarization will follow the antenna orientation.
To cop with both linear vertical and horizontal polarizations, and everything in between, the SARP antenna is designed to be circularly polarized, and is able to receive both in good conditions, but not optimal conditions:

  linearly polarized antennas will work with circularly polarized
  antennas and vice versa. However, there will be up to a 3 dB loss in
  signal strength. In weak signal situations, this loss of signal may
  impair communications (source).

The antenna on the satellite should be like this:

Helical, circularly polarized, antenna (source)

The LOS is completely clear, no foliage, no roof, etc.

Indeed, if there is a metallic obstacle in the LOS and there is no possibility for reflections (no hole in the metallic cabin), no signal is transmitted. This could happen if the PLB were surrounded by metallic debris from the airplane wreckage.
Else there is a volume (the Fresnel zone) between the transmitter and the receiver that must be free of obstacles. This zone is an ellipsoid around the LOS. If obstacles are present there will be diffraction and reflection. Diffraction scatters the signal (no need to have knife-edge corners), and reflection deviates it, which could be a positive effect when there is no clear LOS.
Dense vegetation or heavy foliage on the LOS: There is water in plants, a loss occurs, typically larger than 0.1 dB/m at 406 MHz. We can use models to evaluate the attenuation:

(Source, page 155)
Example: The aircraft crashed in a forest. The aircraft is surrounded by 25 m trees, except in the direction from which it came.
Satellites low on the horizon will not be accessible: GEOSAR satellites over the equator are usually low on the horizon when viewed for medium latitudes, and some orbits of LEOSAR will be hidden by the trees due to the loss exceeding -15 dB as soon as the depth of foliage is 50 m (slant depth). This depth can be quickly encountered, leaving a small free field of view from a window (it would be larger for the windshield though):

Tree trench created by crash-landing in a forest

PLB antenna has no gain/loss (0 dB gain).

This is likely the case for many PLB. PLB antenna seems to be a shortened 1/4 λ whip. Antennas are measured against an hypothetical perfect antenna (isotropic antenna), and the ratio of amplification is expressed in dBi (dB comparing to isotropic). dBi and dB are equivalent in the current discussion as the isotropic antenna has a gain of 0 dB.

PLB antenna and RF output stage impedances match (no SWR).

In order for the current out of the PLB RF amplifier to be radiated entirely by the antenna, PLB output impedance and antenna impedance must be equal. As soon as there is a mismatch, a part of the RF current is not converted into an electromagnetic wave and returns to the amplifier, mixing with the current flowing in the other direction. The result is the appearance of standing waves in the PLB device. The ratio of SW is smartly named SW ratio (SWR). PLB are stated having a SWR less than 1.5 or 2 in their specifications. The corresponding loss is 0.2 and 0.5 dB. Not so much.
The impedance of the antenna varies greatly as soon as the antenna is not in a free space. A common cause is that a part of the body (containing water) is close to the antenna, or touching it. This could easily lead to a loss of 10 dB. The radiative efficiency of the antenna (gain) also suffers: Touching the antenna is like deforming it. Its characteristics are changed. For example is is no more optimized for the signal frequency to radiate. So in addition of SW appearance, the current that reaches the radiator is also poorly converted into waves. In your scenario it is important to use the PLB as intended, the antenna being unobstructed.

There is no atmospheric loss (free space).

This is another effect difficult to estimate, as it varies with time of the day, weather aspects and Sun activity.
Oxygen and water vapor play an important role. From ITU-R P.676-10:

Attenuation in dB per km (source)
It seems for UHF the absorption is rather limited, so let's ignore it.
Other losses are due to sporadic electrical activities in the ionosphere. They are more difficult to estimate, and they translate into noise rather than attenuating the signal received by the SARP. Other elements like the capability of the SARP to reject noise would also need to be considered.
Ionosphere anomalies could potentially introduce errors in the Doppler localization.

Bottom line

Let's recapitulate the losses we have discussed:

 - Impedance mismatch / SWR:                 -0.5 dB
 - Transmitter antenna gain:                 +0 dB
 - Reflection in the cabin:                  -0 to -10 dB (center).
 - Cabin window:                             -0 dB
 - Fresnel obstruction and trees:            -0 to -15 dB
 - Free space loss:                          -143 dB
 - Absorption by atmosphere gases            -0 dB
 - Other propagation losses:                 Not evaluated.
 - Off-nadir angle:                          -0 to -4.4 dB
 - Linear to circular polarization mismatch: -3 dB
 - Satellite duplexer:                       -1.6 dB

                               Acceptable:   -168   dB
                               Best case:    -148.1 dB
                               Worst case:   -177.5 dB

The worst case is encountered when:

The PLB experiments the higher level of reflected waves. This can be eliminated by being very close to the window, and shielding the rest of the cabin with something metallic, but not too close.
The Fresnel ellipsoid is blocked by trees. Not much freedom here. LEOSAT will hopefully have orbits with sufficient high elevation to be out of the obstacles. If the aircraft is really covered by deep foliage, there is no much chance for a successful link.
The aircraft is at the nadir of the satellite. The SARP antenna is optimized for off-nadir locations (no sure off-nadir locations will be better served, due to the increased slant distance, that would need to be checked). Too bad if the aircraft is in a forest.

Note that carrying a personal locator doesn't cause a safety problem, as it doesn't transmit before being activated. Activation will be either manual (usually for PLB) or by sensing a large acceleration (seems limited to ELT). I don't know if the security guys at the airport will let you board with a transmitter. It should be easier to persuade the pilots.

Case of a SPOT device

Compared to the numerous documents that describe governments-financed SAR service on 406 MHz, SPOT is a more confusing solution.

This is not related to SPOT, the observation satellite family built by Astrium. GEOS IERCC (International Emergency Response Coordination Center) has no link with the GEOS satellites family launch by Nasa some decades ago. It is not either GOES, the geostationary satellites that support GEOSAR.

Globalstar is a large satellite phone company owning satellites. SPOT LLC is a subsidiary of Globalstar providing satellite data links and related sets for messaging and voice communications. GEOS-Alliance has launched a privately-owned assistance service based on SPOT capabilities. GEOS-Alliance services are sold online, and by retaillers.

From SPOT website:

  SPOT Gen3 provides location-based messaging and emergency notification
  technology that allows you to communicate from remote locations around
  the globe. It offers custom tracking interval options,
  motion-activated tracking, long-life battery and more.

In short:

Instead of buying a SAR transmitter and then having the service for free, the GEOS-Alliance subscriber receives a transmitter and pays an annual fee.
Instead of having the SAR device connected to the SAR organizations worldwide through the LUTs and MCCs/RCCs, the subscriber is connected to GEOS which calls the local emergency services on your behalf.

The technology behind:

SPOT/Globalstar seems to use LEO spacecraft at 1,414 km instead of 850 km for Sarsat, with a 52° inclination instead of 85°, meaning their coverage could be reduced.
SPOT messengers are limited to 0.4 W instead of 5 W for Sarsat, maybe because of regulations, as they can be used without actually signaling a distress situation.
Globalstar frequency used is 1.6 GHz, instead of 406 MHz for Sarsat.

It seems that these three technical differences are not going to make SPOT devices as efficient as Sarsat:

Using the same ITU-R P.525: Lbf = 20 log(4πd/λ) gives a free space loss of 159 dB upfront, to be compared to the 143 dB for Sarsat (difference: -16 dB).
0.4 W = -4 dBW instead of 7 dBW for Sarsat (difference: -11 dB).
We may add a loss of 1 dB for the atmosphere absorption at this frequency.

If I'm not wrong, that's already a difference of -28 dB at the receiver. It must be very sensitive, or be liked to hi-gain antennas to compete with the governments-operated SAR. That's not going to make it from within the cabin of an aircraft with a limited LOS, and sometimes this seems to even not work well in the open space.

Reference documents

C/S A.003: Cospas-Sarsat System Monitoring and Reporting
C/S G.003: Introduction to the Cospas-Sarsat System
C/S G.004: Cospas-Sarsat Glossary
C/S G.006: Cospas-Sarsat System Assessment
C/S G.008: Operational Requirements For Cospas-Sarsat Second Generation. 406 Mhz Beacons
C/S T.001: Specification for Cospas-Sarsat 406 MHz Distress Beacons
C/S T.003: Description of the Payloads Used in the Cospas-Sarsat LEOSAR System
C/S T.004: Cospas-Sarsat LEOSAR Space Segment Commissioning Standard
C/S T.011: Description of the 406 Mhz Payloads Used in the Cospas-Sarsat GEOSAR System

More Sarsat documentation: Cospas-Sarsat site


Keeping the engine under the wings, doesn't hurt the lift as the airflow speed is higher under the wings? To create a lift, the airflow speed on top of the wings should be higher than the airflow speed on the bottom of the wings.

But when you keep the engine on the bottom of the wings, wouldn't it hurt the lift as air flows faster on the bottom of the wings?

To create a lift, the airflow speed on top of the wings should be higher than the airflow speed on the bottom of the wings.

No, that's not true.

In order to create lift, the pressure on top of the wings must be lower than the pressure on the bottom of the wings. The airflow speed doesn't matter.

I'm guessing your thought process is something like this:

To create lift, the pressure on top of the wings must be lower and the pressure on the bottom must be higher.
According to Bernoulli's principle, faster moving air has lower pressure and slower moving air has higher pressure.
Therefore, to create lift, the air speed on top must be faster and the air speed on the bottom must be slower.

However, point 2 here is wrong. Bernoulli's principle does not say that faster moving air has lower pressure. What Bernoulli's principle says is that given two points a and b, if

the fluid flow is steady, and
the fluid flow is incompressible, and
viscosity is negligible, and
gravity is negligible, and
the points a and b are on the same streamline, and
the fluid is moving faster at a than at b,

then the fluid has a lower pressure at a than at b.

Since the air above the wing and the air below the wing are not on the same streamline, Bernoulli's principle doesn't tell you anything about the speed of the air going above and below.


What is the purpose of sum voltage monitor? I was looking at the design diagram of actuators.
Every channel of the actuator is connected to a voltage differential transducer. The transducer outputs are fed to a sum voltage monitor. I could not understand its purpose.

How can summed voltage exist if there is a single sensor? Or am I missing something here?

It compares the commanded position with the actual position, it is the standard feedback loop.


Trying to identify large WWII aircraft by aileron position on rudder So I have this artifact, a man's slide-on tie clip with the figure of a large, overwing, possibly four engine aircraft, with possible pontoons. Any idea what this baby is? It looks like a Pan Am Clipper, but the position of the aileron, midway up the rudder, has my search skills stumped so far. Thanks.

The Boeing 314 Clipper had a very distinctive triple tail, but it also had (what appears to be) a trim tab half way up the rudders, so maybe that's it?


Can the wheel brakes hold an airliner in place if its engines are at full thrust? Are wheel brakes on most commercial jet airliners capable of holding the jet stationary on a dry surface, if all the engines are given full thrust? Could doing this be potentially unsafe or damaging to any of the aircrafts other structures,  such as pylons, wings, etc.

Yes they are. In fact this is required for a static takeoff. A static take-off means setting the brakes and running up the engines to takeoff power then when ready they release the brakes and start the takeoff roll.

Besides that one of the test Airliners have to withstand is a worst-case V1 abort.

This means starting a takeoff and at the last second slam the brakes, come to a stop and then taxi around for 5 minutes. This all needs to happen fully-loaded and with worn brakes that are still certified safe to use.

Between hitting the brakes and coming to a stop the engines will not have time to spool down and stop providing takeoff power.

Airliner mechanics also do engine run-up tests essentially powering up the engine and going through the power range and making sure it works as specified. This is usually 1 engine at a time.

In these run-up tests loose debris (including cars) can be thrown around by the engine blast. So it's important to keep the area clear.


What is the best method for an emergency takeoff? (YouTube)

Before the collision at Tenerife the pilot of KLM Flight 4805 caused a 22-meter (72-foot) tail-strike by rotating early (I can't say that I blame him given the circumstances).

Is dragging the tail on the runway the best course of action in scenarios where a liftoff must happen as soon as possible?

Near as I know there are no emergency takeoff procedures and for good reason:  if you do have an emergency, stay on the ground. Period.  As for emergencies which happen during a takeoff roll, this one really isn't covered among them as there really is no emergency regiment to get airborne.  The KLM captain in the Tenerife accident made a series of judgement errors and failure to follow ATC instructions combined with bad weather and a crowded airport which contributed to the accident.  Once he was rolling, the best he could do was just hope the plane would reach Vr and climb over the PanAm jet, which didn't happen.  He panicked at the end with the tail strike, only making things worse.  But at that point he had painted himself into a corner with no escape.


What is the difference between the airspace's classes (for non-pilots)? When I'm reading about aviation, it's very common to see references to Class B or Class C airspace and airports.

What are the different classes and what are the differences between them?

Although Lnafziger's answer is correct, I'd like to elaborate on the purpose of the airspace classes.

Class A: This airspace is intended for high-speed, point to point travel.  That is why pilots flying in Class A must be instrument rated and in contact with air traffic control (ATC); aircraft above 18,000 feet are likely to travel quickly and may not have time to avoid each other visually.  This is the upper layer of the airspace system.*
Class B: This airspace is intended for large airports with lots of jet traffic.  The large number and size of aircraft require space for ATC to get them efficiently in and out of the airports.  Approach control services are provided.  As a result, class B areas are physically large.  It is possible for small aircraft flying under visual flight rules to operate here with explict ATC permission, though permission to enter may be denied during periods of high traffic.  Small piston-powered aircraft land at Class B airports infrequently, typically due to high landing fees and fuel prices.
Class C: Smaller airports with a mix of piston-powered aircraft and jets.  Most Class C airports are home to flight schools and small aircraft hangars as well as scheduled service by airlines.  While still busy, Class C airports have less traffic than Class B, so ATC doesn't need as much space to keep them organized.  Like Class B, approach control services are provided.  Pilots are only required to establish two-way communication to enter the airspace.
Class D: The smallest airspace type for tower-controlled airports, traffic primarily consists of general aviation aircraft.  Often these airports handle cargo, business jets, charter service, maintenance or assembly facilities, etc. as well as private aircraft and flight schools.  They also host firefighting operations, airshows, or anything else that needs facilities but would interfere with normal traffic if put at a larger airport.  Although Class D airports are small, some are very busy; others handle only a few flights an hour.  Approach control services are only provided if the airport is near Class B or C airspace.
Class E: This is the lowest level of "controlled airspace."  "Controlled" doesn't mean you have to talk to ATC, but that ATC services are available (within the capabilities of radar and radio equipment).  This is general-purpose airspace.  Aircraft flying under visual flight rules can fly more-or-less whatever they want (weather permitting), and instrument flight rules traffic operate under positive control by ATC.  There are regulations in this airspace, but they aren't onerous, and they're designed to accommodate the wide variety of aircraft and activities that are found here.  Some small airports have Class E airspace surrounding them at the ground.  Although Class E is a single class of airspace, there are tighter regulations above 10,000 feet, where there are no speed restrictions (other than the prohibition on supersonic flight over land), compared to at lower altitudes where speeds are limited to 250 knots.
Class F: See What is Class F airspace, and why is it not used in the US?
Class G: This is found near the ground everywhere except controlled airspace around airports, and occasionally at higher altitudes in remote areas (Alaska or the Rockies, usually), but never above 14,500 feet.  Most untowered airports have Class G airspace surrounding them.  This is "uncontrolled" airspace, so there is no ATC service available at all.  Radio and radar work better the higher you go, so at low altitudes you just can't expect good results.  Class G serves as a notice that ATC services are unavailable, typically due to a lack of radar coverage.  In remote areas, the same problems occur at higher altitudes (radar won't see you if you go behind a mountain, and sometimes you're just too far) so Class G is found there as well.  Most of the time, the only reason anybody goes into Class G airspace is because they're taking off or landing at an uncontrolled airport, or transiting certain areas that lack radar coverage.
Other: There are many kinds of special-use airspace, all of which serve as keep-out signs of varying levels from "please be considerate" up to "will shoot on sight."  In the US, these don't really interact with airspace classification.

* Technically, airspace above 60,000 feet is Class E, but nobody goes up there except scientific and spy/military flights.


Why would a pilot on final to a towered airport not speak up when ATC clears another aircraft to land on the same runway, without mentioning them? Every time I watch AOPA's Accident Case Study video Communication Breakdown, I'm left wondering. Here's an aircraft (in this case, '1DA) on final approach for landing on a towered airport, when another aircraft ('4SR) is cleared to enter the traffic pattern, and gets cleared to land before they turn base, without ATC mentioning the aircraft already on final. One could make an argument that the pilot of '4SR should be aware of '1DA, but somehow, they apparently weren't. Once the pilot of '4SR is made aware of the traffic already on final, they make what I can only describe as a hasty attempt at an evasive maneuver which results in '4SR crashing into the ground, killing everyone aboard '4SR.

Yes, this is a towered, controlled airport; ATC should be the one to inform the incoming aircraft about the one on final. But for some (here unimportant) reason, ATC fails to do so.

To make matters worse, '1DA is flown by a student pilot and instructor, so there's two people there, either of whom (and certainly the instructor) can use the radio.

In such a situation, why wouldn't the pilot of '1DA speak up?

'1DA was still some distance out from the airport. It seems to me that besides the obvious of maintaining situational awareness, there were several rather obvious options available here:

No later than when the pilot reported being on base, ATC could (maybe should) have instructed the pilot of '4SR to make a right turn to re-enter the downwind leg, and to then extend downwind and land behind '1DA. Yes, reentering the downwind leg from the inside would be unusual, but it seems as though it would have kept them well out of the path of '1DA on final.
ATC could have told '1DA to go around when the controller realized that '4SR was unaware of '1DA, allowing '4SR to land ahead of '1DA; things could be sorted out once both where on the ground (the video mentions this possibility).
The pilot of '1DA could have made a position report specifying being number one on final for the same runway, when ATC didn't mention them to '4SR.
The pilot of '1DA could have aborted their landing and executed a missed approach, announcing this, effectively letting '4SR land ahead of them.

Any of those (and probably a few other) options seem like it could have prevented the accident altogether, at the cost of a relatively minor inconvenience to either '4SR or '1DA. So why would no one do anything until it was (obviously in retrospect in this particular example case) too late?

why wouldn't the pilot of '1DA speak up?

The simple answer is that you are not trained to "speak up" at a towered airport, you are trained to follow instructions from the tower and there is an assumption the tower will provide separation and appropriate clearances. As an ultimate fall back you are always supposed to "see and avoid".

For the official report you should reference the NTSB report but if you listen to the ATC audio in the linked video as well as their explanation of the link the pilots had no reason to think there was an issue and the clearances given were rushed but explicit. As I note above AOPA also notes

  "it is a pilots responsibility to visually confirm final approach"

You can also hear that the pilot of '4SR was giving confusing instructions about base extension and clearance to land. The pilots was not incorrect in his approach actions but made a critical go-around error and may have been a low altitude, dirty config, stall.

Ultimately the lower aircraft on approach had the right of way so '1DA had no reason to speak up as they were in the right. AOPA states that the pilots may have thought "he was overtaking the aircraft from above" which is incorrect, or at least procedurally incorrect as '4SR would have had to yield to '1DA anyway.

In the end the controller should have simply issued

  '4SR Go Around

Any time, you as a pilot hear that you should build the habit to just go around. As PIC you can ALWAYS execute a go around, controllers should be prepared for it and more importantly you as the PIC should be prepared for it.


How should we think of 'layers' of air flow? Do a search for "boundary layer" or "laminar flow" on this site, and you'll find plenty of things to read.

They'll talk about "the boundary layer", "flow separation" and so on, as though it were completely obvious what they actually were. Sometimes, they'll even be illustrated with diagrams representing the strata in the flow of air.

I'm no physicist, and this talk and these diagrams make me imagine the fluid equivalent of plywood, or expensive paper handkerchiefs, constructed of discrete layers of material that can be readily identified as such and peeled away from each other.

Is that really how these layers of moving air work? Is the boundary layer over an aircraft's wing like a discrete sheet, with a clear and distinct boundary between the next discrete layer? Or is there a smooth continuum, rather than a distinct step, between the layers and their properties?

Do distinct layers exist more in the conceptual analysis of aerodynamic behaviour than they do in readily identifiable physical entities?

If you were to plot a graph representing some property of the air flow (say its velocity, direction, pressure or turbulence) against distance from the surface of the wing, what sort of shape would the curve have?

In short: how should we imagine these layers?

To understand what a boundary layer is think about air moving above a flat plate. Before the plate and very far from it the air has everywhere the same speed and direction. However, directly on the plate its speed is zero: molecules directly on the plate do not move at all.

Between the plate surface where the air isn't moving and some distance from it where the air has a constant speed, there is a zone where the air changes its speed a lot. We call this zone boundary layer.

There isn't really any clear and distinct boundary delimiting it. That's why we have many definitions for its thickness. The dashed line in the image below is probably the 99% boundary layer thickness.

Generally we give a name to a layer when the physical effects and properties of the fluid in this concrete zone can be differentiated from the rest of the flow. Most of them are on fluid boundaries (body surfaces, places were fluids get gaseous,...) but you can have them inside a fluid too (shock waves).

For example, instead of looking at the velocity you could look at the temperature of a fluid flowing around a very hot body and define a temperature boundary layer.

Some known layers are:

Laminar boundary layer
Turbulent boundary layer
Blasius boundary layer
Stokes boundary layer
Ekman layer
Shock layer
Knudsen layer
Merged layer


What is the difference between Hobbs Time and Tach Time? Over in this answer there's a description of Hobbs Time versus Tach Time, and how the Hobbs counts hours while the tach time is equivalent to "hours at cruise RPM".

How do these two "clocks" work, and what do these numbers mean in practical terms for flying and maintaining an aircraft?

Hobbs Time

In most planes, the Hobbs clock is started and stopped based on an oil pressure switch, so it starts when the engine starts, and stops when the engine is shut-down. While it's running, it just ticks off a tenth of an hour every 6 minutes, based on "regular wall clock time". So a tenth of idling on the ramp is the same as a tenth at cruise.

Tach Time

The tach clock isn't really a clock at all, it doesn't actually measure time, it really measures engine revolutions. But it's calibrated such that a tenth of an hour of tach time is clicked off when the engine is at cruise RPM for 6 minutes. In other words, if the plane is at cruise RPM, the tach clock will be clicking off tenths of an hour at the same rate as the Hobbs clock, and the same as the watch on your wrist. But if the engine is idling at an RPM speed that's half of what cruise RPM is, then the tach clock will be running at half the speed of the Hobbs clock.

Relation

So, for the tach clock, less "time" is clocked when the plane is idling on the ramp, or flying at low RPM. For short flights (where ramp idling time is a significant percentage of total time), and flights where you're doing a lot of pattern work (and thus operating at low RPM), tach time will be significantly less than Hobbs time.

Maintenance

When an engine is replaced or overhauled, that engine is considered to be "zero time". Whether they actually reset the tach gauge to 0 when the engine is overhauled, or if there's a way to put a time on a brand new tach or Hobbs meter when the old one dies and is replaced, I dunno, but in general, I'd assume that the aircraft's engine and airframe log books are really the official recording of the progression of airframe/engine times, so I don't know that it's particularly important whether the tach is set to the engine's time, or if it's really just used as an incremental meter.

AOPA has a nice (and long) article about this.


What's the minimum legal height in Europe for a low pass at an airport? Does anyone know in which document I can find the legal minimum height for a low pass at an airport, assuming the pass has been approved by ATC? I have been discussing this topic a lot with several flight instructors but we haven't found any official document where a specified height is set.

I'd like to know about EASA regulations if possible, but FAA or others are also appreciated.

Official Record Series 4 No 1174 General Permissions
Standardised European Rules of the Air – Exceptions to the Minimum Height Requirements
1) Definition
In these permissions:
a) ‘SERA’ means the Annex to Commission Implementing Regulation (EU) No. 923/2012 (‘the Standardised European Rules of the Air’ (and references to SERA followed by a number mean the corresponding provision of SERA)).
b) ‘licensed aerodrome’ means an aerodrome licensed under the Air Navigation Order 2009 or an aerodrome certificated under Commission Regulation (EU) No.139/2014 of 12 February 2014 and
c) ‘Government aerodrome’ means any aerodrome in the United Kingdom which is in the occupation of any Government Department or visiting force (as defined in article 255(1) of the Air Navigation Order 2009).
d) ‘notified’ has the same meaning as in article 255(1) of the Air Navigation Order 2009.
2) General (SERA.5005(f)(2))
a) The Civil Aviation Authority (CAA) permits, under SERA.3105 and SERA.5005(f), subject to the condition set out in subparagraph (b), an aircraft to fly elsewhere than as specified in SERA.5005(f)(1) at a height of:
i) less than 150 metres (500 feet) above the ground or water; or
ii) less than 150 metres (500) above the highest obstacle within a radius of 150 metres (500 feet) from
the aircraft.
b) The aircraft must not be flown closer than 150 metres (500 feet) to any person, vessel, vehicle or structure except with the permission of the CAA.
3) Approaches to Landing or Forced Landings
The Civil Aviation Authority permits, under SERA.3105, SERA.5005(f) and SERA.5015(b), an aircraft to fly below the heights specified in SERA.5005(f) and SERA.5015(b) if it is flying in accordance with normal aviation practice and:
a) practising approaches to land at or checking navigational aids or procedures at an aerodrome;
b) practising approaches to forced landings other than at an aerodrome if it is not flown closer than 150 metres (500 feet) to any person, vessel, vehicle or structure; or
c) flying in accordance with a notified procedure or when specifically authorised by the CAA in accordance with SERA.5015


During the early design stages, what parameters are considered in determining the weight of an aircraft? What parameters are considered in estimating the weight of an aircraft in the beginning of the design process?

Any design starts with a mission profile. As a design engineer, you get the payload, range and speed, and maybe constraints like the maximum runway, and then have to come up with a configuration.

With that in hand, you look what others have achieved before. There are collections of weight data for different designs and their parts, and formulas which combine the main parameters in a way that the weights line up more or less nicely when plotted over the result of this equation. Plug in your new design's number, and you get something wich will be within ±10% of what is realistic. This is called parametric weight estimation (PDF!).

Since you can only consider what you have, you start with the boundaries of the sizing mission:

Payload
Range or endurance
Desired cruise speed and altitude
Operational constraints like runway length, operation from unprepared fields, threat scenario.

EDIT:

@Peter is right: Cost per kilo of airframe and per seat-mile (where applicable) is also a starting requirement, and cost minimization is taken care of by designing the smallest possible airframe which can satisfy all requirements while using proven technology. And parametric sizing only works when based on proven technology.


What is the difference between EPR and rotor speed as thrust setting parameter? Amongst various thrust setting parameters, Engine Pressure Ratio (EPR) and fan rotor speed are two of them. Does anybody know why GE uses fan rotor speed and P&W uses EPR as thrust setting parameter?

While the true 'reason' is probably buried in some engineering notebooks from 60 years ago, it basically boils down to a design decision by the team designing the engine controls.  Both methods work, and both companies have a list of reasons why they think their solution is 'best'.

P&W will tell you that EPR is a better indication of engine thrust, and they're right.  But it relies on pitot probes to measure the pressures within the engine.  Flow stability means response can be slow or erratic and the probes can be clogged causing problems. OTOH, it can indicate problems (unrelated to the probes) due to engine damage.

GE prefers to use N1.  This is a simple speed indication of the fan.  It's not a direct indication of thrust but is directly correlated.  The big advantage is that it is extremely reliable and stable.

A good explanation is available from the Flying Engineer.


Is the turn radius at fixed bank angle really independent of aircraft type? I'm curious about the relationship between an aircraft's turn radius and the bank angle. This formula describes the relationship between the two:
$$ R = \frac{v^2}{g \tan(b)} $$
Where $R$ is the turn radius, $v$ is the speed, $g$ is acceleration due to gravity and $b$ is the bank angle.
This formula is found dotted around the internet (with variations depending on units used) examples include here and here.
My question is, why is there no reference in the formula to the physical characteristics of the aircraft? Does the relationship between turn radius and angle-of-bank really have nothing to do with aircraft weight, wingspan or aerofoil design? Does the same formula really hold true for both glider (sailplane) and large passenger jet?
Hoping for an explanation with no maths please!

If the turn is fully coordinated, turn radius at a given bank angle will depend on the speed of the aircraft and the local value of gravity.
This is because of simple vector and geometry: the vertical component of lift must still support the weight of the aircraft (whether that's a hang glider or an AN-225, a Helio Courier on the edge of stall or an SR-71), but the horizontal component, generated by the bank, produces a horizontal (=> centripetal) acceleration that is fixed in relation to gravity for a given bank angle.
The problem is that this centripetal acceleration is determined by both turn radius and speed.  So, if your hang glider and your SR-71 want to make the same radius turn (within their usual flight envelope), they'll do it at wildly different bank angles.


Have other designs for individual gasper ventilation been tried? The air gaspers (in large passenger aircraft) above the passengers' heads are fed from the main air conditioning system, at least in all aircraft that I am familiar with (see also this answer). This design probably makes sense, as it's very simple (no moving parts other than the gaspers) and takes advantage of a system which is already installed in the aircraft to provide additional functionality.

Still, I'm curious:

have other designs for individually adjustable passenger ventilation been tried (i.e. installed in large passenger aircraft), and why were they abandoned in favour of the current design? Off the top of my head I can think of a system with little fans above each seat - although it's probably far more prone to breaking than the current system.
Some cars have individual climate control (where the air supplied to each occupant can have a different temperature), has that ever been tried on a large commercial aircraft?

Whether or not they have been "tried" is probably not answerable, as "try this" type designs typically don't make it into production aircraft.

Look at the simplicity of the existing system, its simply some lightweight ducting with an adjustable air vent above each seat. The entire system has no other electric or adjustable parts, so maintenance is pretty easy, as well as reliability. Its also probably a hell of a lot easier to get certified.

Putting little fans above each seat wouldn't have any benefit here, as the air they move from one side of the fan to the other side has to be of a significant volume anyway, so you would still need ducting. The fan speed would be controllable, but then again so is the orifice size of the existing vents, so really no benefit there other than complexity and more wiring/electronics.

The existing system also works relatively well for individual climate control. Unlike in your car, you don't have a large space between you and the person next to you, so getting "individual" control is difficult (Seat A at X degrees, Seat B at Y degrees, etc). But note that even in your car you really aren't adjusting the heat or cold temperature, but the fan speed either through the AC system or through the heater. The heater and AC are at a constant temperature all the time.


Where can I look up FAA Letters to Airmen? I was recently asked if an airport was covered by a Letter to Airmen (LTA) providing IFR separation for VFR practice approaches, and I realized I had no idea. How can I look up LTAs for an airport, and do they ever expire?

They are searchable at FNS NOTAM search: https://notams.aim.faa.gov/notamSearch/

A link is provided to a PDF, containing a link to the actual LTA.

(Filter by LTA, remember the agreement might not be with the airport, but with the ATSU, like the local TRACON/ARTCC, try searching by airport name + radius).

Expiry date is shown in the details view.


How do commercial pilots keep situational awareness of their current call sign? Air transport pilots operate flights with a variety of flight numbers, often with multiple flights in the same day. This means, as a result, the callsign that they are using changes as well. Also, according to this answer, sometimes the callsign is completely different from the flight number.

Given that, how do pilots maintain keep track of what callsign that they are listening for at a given point in time? Are there conventions for posting the callsign somewhere in the cockpit?

Some modern airliners have a place where the flight number can be displayed on the cockpit displays. This information can be entered off the dispatch paperwork when setting up the flight plan, and is displayed throughout the flight for easy reference.

For example, you can make out "DLH463" on the screen at the bottom right of this Lufthansa A380 cockpit photo, corresponding to Lufthansa flight 463.

The Boeing 787 has a similar feature:

  The two outboard displays are a fixed format that provides primary
  flight display information combined with an “auxiliary” display that
  consolidates frequently referenced information for the crew, such as
  flight number, each pilot’s microphone-selected radio and its
  frequency, and transponder code.

You can see this on this 787 cockpit photo at the top of the leftmost monitor: "FLT -> BAW787"

(Photos not embedded due to licensing restrictions.)


How to calculate the static pressure in a pitot tube? In a pitot-static tube, we have two different pressure, the static pressure $p_s$ and the total pressure $p_0 = p_s + \frac{1}{2}\rho v^2$, which comes from the Beroulli equation for incompressible flows.

Now, lets say we are given an inflow of air from the freestream (assume STP conditions) with a speed of $v_{\infty} = 100$ ft/s. The total pressure would just be $p_0 = p_{\infty} + \frac{1}{2} \rho v_{\infty}^2$, but what I fail to understand is how we would get the static pressure. Would it just be the freestream pressure? Or something else...

The pitot tube is pointed into the airstream and measures the total pressure. Static pressure must be measured, not calculated. It is done so at a separate location, using a static port.

Both pitot tubes and static ports are needed in order to derive air speed, as this incident sadly illustrates.


Can a GPS system be used to identify fixes on conventional instrument approaches? Is the use of a certified IFR GPS receiver approved for identifying fixes defined by VOR radials, on a conventional (non RNAV) instrument approach?
For example, in KCDW LOC RWY 22, the intersections DOWDY, FAAIR and KOLLI (IF, FAF, step down fix, respectively) are all identified by VOR cross radials (TEB VOR). Can they be identified solely by reference to the GPS database?
Another example would be KABE LOC RWY 13; IZKIP is a step down fix identified by a cross radial from ETX VOR. However, the chart states:

  IZKIP FIX MINIMUMS (DUAL VOR RECEIVERS REQUIRED)

Can a GPS be used to identify IZKIP in this case?

According to AC 90-108:

  ...operators may use a suitable RNAV system in the following ways... Determine aircraft position relative to or distance from...  a named fix defined by a VOR radial.

Therefore my understanding is that named waypoints/intersections defined by VOR radials on a conventional IAP like in the examples above, can be identified using a GPS, except perhaps when the chart requires dual VOR receivers.

Your reading of AC90-108 is correct. As long as the RNAV equipment is operated and installed in accordance with TSO-C129/-C145/-C146 it can be used as a "suitable RNAV system." (ref: AC90-108, para. 6 a.).

Also, see the AIM, 1-2-3 b. "Types of RNAV Systems that Qualify as a Suitable RNAV System."

Lastly, as far as the requirement for "Dual VOR Receivers" for IZKIP, since you are using a "suitable" (and legal) alternative method by using the GPS database fix location, no additional restriction should apply. Also, it is my understanding that when the subject stepdown fix (identified by a VOR radial) is inside the FAF, as is the case with IZKIP, the dual VOR requirement is for the purpose of presetting the missed approach holding fix.  Again, not an issue if you are using a "suitable RNAV" system for identifying the fixes.


How much fuel gets dumped and why? WRT Would the Aeroflot Flight 1492 have been able to dump its fuel first before landing?, I am wondering how much fuel gets dumped every year? gallons? litres? pounds?

Also, is it only in unplanned or emergency situations?

While according to British Airways estimates quoted in the Inter-Governmental Panel on Climate Change (IPPC) report on “Aviation and the Global Atmosphere” published in 1999 “only a very small percentage (on the order of 0.01%) of fuel used by the aviation industry each year is jettisoned”

https://greentumble.com/environmental-impacts-of-fuel-dumping/


What first aid equipment is on a commercial aircraft? I have always wondered what type of first aid equipment is on a commercial aircraft with an airline such as Thomas Cook or Ryanair. Do they have things to save your life etc.??

Obviously, first aid kits have items that save your life. The contents of the First aid kits or Emergency medical kits (EMKs) vary based on the jurisdiction. For example, FAA requires the following items

  ... on all airplanes of air carriers operating under part 121 for which at least one flight attendant is required. EMKs and AEDs are “no-go” items and must be carried as indicated on the Minimum Equipment list.

Contents of EMK; table from FAA Advisory Circular 121-33B Emergency Medical Equipment.

On the other hand, Indian DGCA requires the following items in first aid kits:

First Aid contents; table from DGCA Document No: F No. 11-690/CAR X-II/2010/AI(2) Provision of Medical Supplies in Aircraft

Most of the requirements are broadly similar, while there are some region (or country) specific differences; for example, DGCA requires rehydration solution to be carried, presumably due to heat. FAA, on the other hand requires Automated External Defibrillators (AED) to be carried for aircraft with payload more than 7500 lbs (DGCA requires it only in international flights of over 2 hours).


Helicopter Basics: How does lead-lag of blades actually help in compensating for increase/decrease in speed due to flapping? I understand the following -

Due to dissymmetry of lift, flapping up and down occurs.
Since angular momentum has to be conserved, the speed of blade increases/decreases during flapping
up/down motion.
To compensate for this blade leads front by a certain
degree in the rotor plane, in advancing half and lags by a certain degree in retreating half.

My question is - what does the "compensation means"? Even after rotating the blade front by 1-2 degrees (leading), the speed of blade will remain the same after it is reached its maximum lead angle. So, what happens after that?

Is it the case that lead/lag helps in absorbing the load/vibrations by allowing some degree of freedom for blade to move by few angles?

Blade flapping is simply the action of the tilting of the rotor disc; the blade's tip is following a particular path attached to a hub that is rigidly fixed on its vertical axis. The blade flaps up and down because its incidence/pitch is being continuously varied (or not if the stick is centered) by the swash plate.  The limits of the rotor disc's tilt range is, beyond the swash plate limits, ultimately the blade flapping travel limits.  Articulating rotors can handle this, but on a two blade teetering rotor, achieving those limits is called "Mast Bumping" and is a fatal condition.

For lead lag, the issue is you need to allow for that motion on an articulating rotor helicopter because Coriolis force makes a blade want to speed up as it flaps up relative to the other blades (actually, moving off perpendicular to the mast's axis).  When it flaps up, its center of mass moves closer to the hub than a blade on the other side that is closer to perpendicular to the mast, which makes it want to speed up, like a figure skater pulling arms in while spinning.  The lead/lag hinge provides the necessary compliance to accommodate the speeding up and slowing down tendency of the blade as its center of mass moves closer and farther from the mast as it rotates while moving up and down, so it doesn't force the rotor hub to absorb the forces.  The lead/lag damper is there to resist sudden movements of this lead/lag action that can be caused by sharp ground contact, which can lead to ground resonance.

On a teetering rotor system, you don't need lead lag hinges because you have the geometry of the teetering hinge, which is the blade flapping hinge for both blades on a single flapping axis.  The up flapping blade moves away from the hub because the hinge is above the blade axis (called an under slung hub) and this keeps the CG from moving closer to the hub significantly, eliminating most of the Coriolis forces, or reducing them to a level can be absorbed by the rotor head and blades.


What are the rules for Public-Use Operations? On another aviation board I'm on, (link), one poster is making the claim that operators of airplanes for Public Use Operations (eg. Police planes, Search-and-rescue aircraft, most government planes), do not even technically need a pilot's license, and do not have to meet FAA maintenance rules.
He even cites an FAA Advisory.

I find this extremely hard to believe. But I haven't found anything to refute this.

What are the rules for Public Use Operations, and what role does the FAA play in keeping them safe?

Short answer: public (federal or state government) aircraft operations can be exempt - but aren't always - from most FAA regulations, provided they're non-commercial operations. Any regulation that specifically applies to a "civil aircraft" doesn't apply to a public aircraft operation.

First, some definitions from 14 CFR 1.1:

Civil aircraft means aircraft other than public aircraft.
Public aircraft means any of the following aircraft [...]

The definition of "public aircraft" basically comes down to 'any aircraft the government owns or has a long-term lease on'. That potentially covers everything from Air Force One down to the Podunk Sheriff Department's C172. With that in mind, a lot of regulations apply only to civil aircraft, e.g. 14 CFR 61.3 requires a pilot's certificate to operate an aircraft (emphasis mine):

(a) Required pilot certificate for operating a civil aircraft of the
United States. No person may serve as a required pilot flight
crewmember of a civil aircraft of the United States, unless that
person:

So 61.3 doesn't apply to public aircraft. Similarly, 91.203 requires an airworthiness certificate for civil aircraft only (emphasis mine):

[...] no person may operate a civil aircraft unless it has within it the following:
(1) An appropriate and current airworthiness certificate

As for maintenance, part 43 only applies if an aircraft has that certificate, per 43.1:

this part prescribes rules governing the maintenance, preventive maintenance, rebuilding, and alteration of any—
(1) Aircraft having a U.S. airworthiness certificate;

In other words, if your aircraft is public use then you don't need a pilot's certificate to fly it, you don't need an airworthiness certificate, and therefore you don't need maintenance! See also 91.7, 91.9 and many other regulations that only apply to "civil aircraft".
Whether or not a specific flight is a public aircraft operation (PAO) or not is explained in AC 00-1.1A, Public Aircraft Operations. Pages 10 and 11 have flowcharts for determining whether a flight is a PAO but to simplify, if the aircraft is owned by the government and the flight isn't for commercial purposes (also defined in the AC) then it's a PAO. And that means many regulations don't apply to them:

What Oversight of PAO Does the FAA Have? The FAA has limited oversight of PAO,
though such operations must continue to comply with the regulations applicable to all aircraft
operating in the NAS.

Again, if you read the regulations with the civil/public distinction in mind then you can see a lot of exemptions. For example, 91.131 is the regulation for operations in class B airspace. It says (emphasis mine):

(a) Operating rules. No person may operate an aircraft within a Class B airspace area except in compliance with §91.129 and the following rules:
(1) The operator must receive an ATC clearance from the ATC facility having jurisdiction for that area before operating an aircraft in that area.
[...]
(b) Pilot requirements. (1) No person may take off or land a civil aircraft at an airport within a Class B airspace area or operate a civil aircraft within a Class B airspace area unless—
(i) The pilot in command holds at least a private pilot certificate;

That means (a) applies to all flights but (b) applies only to civil flights, i.e. a PAO flight still needs a clearance for class B airspace, but the pilot doesn't need to have a pilot's certificate. It looks to me like the regulations are generally written so that everyone has to follow the same rules of the air (for obvious safety reasons) but public aircraft are often exempt from licensing, airworthiness and maintenance requirements.
Does this all mean that US airspace is full of unlicensed pilots flying non-airworthy aircraft that haven't been maintained properly, all under the cover of government business? Not really; the government has as much interest in safe and productive flying as anyone else, and that requires qualified pilots and well-maintained aircraft. There are some plausible reasons for the PAO exemptions, like allowing military pilots to operate civil flights, and federal and state governments are often exempt from their own legislation in all sorts of areas, not just aviation.