Is hybrid propulsion widely used by UAVs? Is it common to have hybrid propulsion in UAVs used for terrain mapping with long endurance, short range and altitude? Is the technology and research mature enough to make hybrid propulsion?
Background: I have college project of UAV design and manufacturing and I want to be a part of it as a propulsion head.

It's becoming common, particularly for endurance.  Two such commercially produced aircraft, Skyfront's Perimeter 8 and Avartek's Boxer, have several hours of hover endurance and a 5 kg payload capacity.
Academia is also taking note: here's a 2018 master's thesis that analyzes the advantages and challenges of hybrid power.
The core motivation for hybrid power is that the internal combustion engine can be optimized for nominal power, while the battery need only handle the difference between nominal and peak power.  That weighs less and costs less than a big IC engine or a big battery, while still enjoying the greater energy density of IC fuel as compared to batteries.
Compared to purely electric propulsion, two challenges are the engine vibration that corrupts sensor readings, and the louder noise that constrains where the aircraft may be flown without annoying others.


What is the reason for the different types of valve heads (mushroom, tulip, semi tulip) in reciprocating engines? I am reading in the FAA-H-8083-32 Aviation Maintenance Technician Handbook-Powerplant book posted on the FAA site that there are engine valves with different types of head shapes, e.g. mushroom, tulip, semi tulip (image taken from flight-mechanic.com).

What is the reason for these shapes?

That part of the shape comes down to a couple of factors.

First of all, you want valves to be light. Second, the outside of the valve's head forms part of the combustion chamber.

The shape of the combustion chamber influences a couple of things. A hemispherical chamber tends to help "breathing"--the ability to get fuel-air mixture into the chamber and get exhaust out of the chamber.

Another common shape is a wedge. A wedge shape helps "swirl" the fuel/air mixture around in the chamber during the compression stroke, to ensure the fuel and air are evenly mixed, so it burns more evenly and completely.

The former tends to improve performance in terms of power per unit of displacement, while the latter tends to improve performance more in terms of power extracted per unit of fuel consumed.

A mushroom shaped head is typically (as shown above) used with a hollow valve. In this case, the valve is typically filled with sodium. Sodium is a metal with a very low melting point, so in normal operation, the valve is filled with liquid sodium. Liquid sodium is a very good heat conductor, so it conducts heat from the head of the valve to the stem, which is surrounded by oil and a valve guide, which help conduct the heat away. The mushroom head makes it relatively easy to maintain a uniform thickness of steel between the outside of the valve and the sodium inside, so the heat is conducted away relatively evenly--extreme heat gradients can lead to failure (e.g., the valve cracking). The valve's movement helps circulate the liquid sodium around to help heat circulation, and keep the temperature of the outside of the valve relatively uniform.

Another factor favoring a tulip shape (or similar) is reducing valve float. The heavier the valve, the stronger of a spring you need to ensure that the valve closes as soon as the cam lets it. If the valve stays open (floats) it can reduce power, or in more extreme cases the valve can contact the piston, which will normally lead to engine failure requiring a full inspection and rebuild at a minimum (and quite possibly requiring complete replacement). Stronger valve springs can help too, but they put more wear and tear on the drive train (e.g., timing belt) and "steal" more power from the engine to turn the cam.


What is the maximum G load a 747 can withstand during an emergency course reversal? In an emergency course reversal, passenger safety first priority, to avoid say, a column of ash from a new volcano, what it the most G a 747 can withstand with an average passenger complement?

It depends on how you define :withstand". China Airlines flight 006 experienced vertical load factors of up to 5.1 g on 19 February 1985. The aircraft could be landed safely and there were no fatalities, but there was considerable damage:

Image source

From the NTSB incident report:

  The wings were bent or set permanently 2 to 3 inches upward at the wingtips; however, the set was within the manufacturer's allowable tolerances. The left aileron's upper surface panel was broken and the trailing edge wedge was cracked in several places.

The maximum load factor an aircraft can be subjected to is a function of time. Two cases are determined for civil aircraft:

Limit load factor. For transport aircraft weighing more than 50,000 lb
(22,680 kg) this load factor is equal to 2.5, while for lighter aircraft it is a function of the weight.
Ultimate load factor = 1.5 times limit load factor, is the higher of the gust and the manoeuvre load factor.

The load factors are determined in accordance with the airworthiness regulations 14 CFR part 23.341 and 14 CFR Part 25.341. Damage tolerance of the airframe and the consequences of metal fatigue are stated in 14 CFR part 25.571.

The B747SP of the incident is an aeroplane that was designed and constructed in the 60s/70s, before Computer Aided Design and engineering were wide-spread, and the structures were designed with analytical mathematics and knowledge from experience, then tested for limit loads and ultimate loads. The more the design stresses can be modelled on a computer screen, the closer the wing ultimate load can be to 150% of limit load. Older aeroplanes did not have these modelling tools, and wings could end up to be stronger and heavier than they needed to be. Something that everybody applauds, but nobody wants to pay for with a higher ticket price.

A video of the B777 shows how the static testing for ultimate load is done: an actual wing is bent until it breaks. It is not a test of wing flex, it is a test of static strength.

The particular 747 of the incident had a wing designed for an MTOW of 378,000 kg, while actual MTOW of the 747SP was 320,000 kg: an extra load factor of 1.18. Fuel in the wing tanks provides extra bending relief, not for the ailerons though..


Where is the 'zone of reversed commands...'? There seems to be some confusion about where the 'zone of reversed commands' really is, i.e. where a decrease of airspeed results in an increase of drag.

Some references place that zone to the left of the minimum of the power required curve, while others place it (correctly, in my opinion) to the left of the minimum of the thrust required curve... Those minima are separated by a 30% difference in airspeed, hence the distinction isn't academic...

Two examples of these contradictory references, one taken from the internet, and the other from a book on gyros ('Flugphysik der Tragschrauber'):

This is commonly known as the Speed Stability, not to be confused with speed stability in the sense of static longitudinal stability. I think there's some common confusion with this phenomenon. The first part of this answer addresses the OP's question; the second part clarifies a common confusion.

1. Main Answer

In level flight (i.e. zero vertical rate), the longitudinal equation of motion can be succinctly written as:

$$m\dot{V}=T-D$$

$m$ is airplane mass, $V$ is airspeed, $T$ is thrust and $D$ is drag. Now if we express thrust and drag as first order approximation as a function of change in airspeed ($\Delta V$) from the trimmed condition, we have:

$$T=T_0+\frac{dT}{dV}\Delta V=T_0+T_V\Delta V$$ and $$D=D_0+\frac{dD}{dV}\Delta V=D_0+D_V\Delta V$$

At trim condition, we necessarily have $T_0=D_0$. So now we have a new equation of motion:

$$m\dot{\Delta V}=(T_V-D_V)\Delta V$$

This equation is a first order ordinary differential equation, and is stable if $T_V-D_V<0$ and unstable otherwise.

For a jet plane, thrust is fairly constant in flat rated conditions and $T_V$ is approximately zero. Thus, $D_V=0$ corresponds exactly to minimum drag or minimum thrust required (where $C_{D_0}=C_{D_i}$ for high aspect ratio, low Mach airplanes).
For a propeller plane, power is constant, but now the stability criterion is $-\frac{P}{V^2}-D_V<0$. This corresponds neither to minimum power required nor minimum thrust required.

2. Addendum

What does this result mean, exactly? If an airplane is trimmed in the speed unstable regime, will it decay toward stall if it experiences a speed perturbation with pilot hands-off, even if it's statically longitudinally stable?

Remember, the equation we began with only holds in level condition where the airplane is neither climbing nor descending. Thus, the pilot must be holding altitude with elevator while the speed is changing. The conclusion for speed instability is:

If the altitude deviates below the trim altitude, pulling up will decrease the airspeed, resulting in further energy deficiency. Speed will continuously decay in this manner.
If the altitude deviates above the trim altitude, pushing down will increase the airspeed, resulting in further energy excess. Speed will continuously accelerate until reaching a stable point on the other side of the polar.

Since the pilot is in the loop, it has nothing to do with the basic aircraft eigenmodes.

The same result can be obtained if we aim for constant flight path angle. In approach configuration, this is also called flight path stability, which I think is a better name than speed stability.


What is the function of the holes in splitter plates? Do the holes in splitter plates reduce the boundary layer, or do they serve a different function such as acoustic/vibration damping?

  Many splitter plates have a series of holes drilled into the surface closer to the engine side of the intake. Suction is applied to these holes, further reducing the boundary layer.

This bit is from Wikipedia, it's marked Citation Needed, and I'm unable to corroborate it.

Taking the Eurofighter example:

(Source)

If it is true, does it only work at supersonic speeds? The way I see the channel above the intake is that it is diffuser shaped. So a reduction in pressure above the intake would only work at supersonic speeds, is that correct?

A similar system can be [barely] seen on the F/A-18E.

Do the holes in splitter plates reduce the boundary layer?

Yes. Suction is applied to remove the slow-moving layer of air close to the surface, so the air entering the intake has as uniform a speed distribution over the intake cross section as possible. That this air is used for cooling before being dumped overboard is sensible, but if cooling were the single purpose for sucking it in, then better and more efficient designs exist.

The boundary layer which has built up on the forward fuselage is removed by the splitter plate itself. The perforation is only for removing the fresh boundary layer which has built up over the upstream length of the splitter plate.

On some spike intakes the designers even implemented boundary layer suction on the intake cone, ducting the flow through struts in the intake to the outside of the engine. Even this complex arrangement seems to be worth it because it improves the efficiency of the intake and allows to widen its operating limits. From this article of the Airspacemag web site:

  The SR-71 inlet rids itself of boundary layer air by sucking it in
  through slots on the spike and passing it through ducts that exit the
  nacelle.

The airflow schematics in the picture below call this particular flow "centerbody bleed". Note how it is reversed at low speed in order to suck more air into the engine while it is used to dump the boundary layer air as soon as intake pressure rises above ambient pressure at higher flight speeds.

SR-71 engine airflow patterns (picture source)


When would a pilot request a "VMC descent" or "VMC climb"? Can anybody explain what's the point of this and in what kind of situations a pilot needs it?
ICAO Doc 9432 "Manual of Radiotelephony"

8.3.3 An aircraft may request a clearance to climb or descend maintaining own separation while in VMC, in daylight, below 10 000 feet in airspace classes D and E. The clearance shall include information on essential traffic.
Example:
P: FASTAIR 345, REQUEST VMC DESCENT TO FL 60
C: FASTAIR 345, DESCEND TO FL 60, MAINTAIN OWN SEPARATION AND VMC FROM FL 90 TO FL 70, TRAFFIC WESTBOUND FRIENDSHIP FL 80, ESTIMATING WICKEN VOR AT 07.

It would be needed when otherwise the flight path would not be cleared because ATC cannot ensure separation with the conflicting traffic. The pilot agreeing to maintain their own separation in VMC allows the level change to be cleared despite the conflicting traffic.


If a winglet disconnected from an airplane mid-flight, what would be the consequences? I've always knew that the winglets on an aircraft provide lift. But what would happen if a winglet disconnected from one wing while a winglet was on another wing?

this actually happened to the Rutan around-the-world flight, where one winglet was shaved off against the runway pavement during the takeoff roll. the pilot then stabbed the controls to overstress the remaining winglet, causing it to break off during climbout. the purpose of the winglet is to increase the apparent span of the wing by blocking airflow out from under the wing and over & into the airflow over the top of the wing, so an aircraft with one winglet on and one off would perform as if the wingspans on either side of the fuselage were several feet different.


Is Autopilot engaged when an engine failure above V1 occurs on modern airliners? If you lose one engine during takeoff in a multi-engine aircraft, you're trained to follow a standard emergency procedure: Pitch for Vy, use bank and rudder for zero side slip, mixture full rich, props full forward, power full forward, gear and flaps up, and then run an emergency checklist for one engine failure.

When one engine failure happens in a modern jetliner, what do the pilots do? Do they engage the autopilot right away so that the computer can take care of pitch, bank, and rudder for the best performance for a single engine operation? Or do they basically follow the same procedure as the one written above, which is used by GA multi-engine pilots?

Most non-normal ops in an airliner follow the same concepts for light aircraft - make sure the aeroplane is under control and following the desired flight path before you do anything - which includes engaging an autopilot.

So for the Boeing aircraft I fly, ensure you've got a positive rate of climb, speed should be V2 - V2+15, ensure the aeroplane is in trim (pitch first, then rudder) and your following any prescribed engine failure routing.

In terms of autopilot use, part of our company SOP is to engage the autopilot at 1000ft AAL, this is not to relieve the pilot of the responsibility of maintaining control - it won't control the rudder - more to reduce the immediate workload and allow more effective management of the non-normal situation. Once the autopilot is engaged the aeroplane can be accelerated, flaps retracted and any non-normal checklist completed.


Can helicopters have engines running while rotor blade stays stationary? So, I was skiing sometime ago, and unfortunately, another skier got injured. So, a heli was flown up to get the injured skier. The rotor blades were stationary, but I still heard some sort of engine like sound. Does anyone know any helicopters capable of doing this sort of thing?

Yes, many helicopters (but not all) can do this. There are 2 main methods:

a clutch between engine and rotor system
for turbines only: a rotor brake on the engine output shaft. The engine contains a free turbine (i.e. a turbine stage that isn't attached to the compressor) that drives the rotor system. If you brake this shaft, the engine can run without spinning the rotors.


Can jet airplanes operate on 'rough' airfields? A question I've always wondered is: can jet aircraft - not turboprop in this case - operate from 'rough' airstrips. My definition of 'rough' is grass or maybe a smooth mud airfield. I know that there's the dry lake at Edwards AFB that is used by aircraft for testing.

Are aircraft such as the Boeing/ Airbus passengers series - 737, 777, 747 etc - designed for or have the capability to regularly handle these airfields?
Are there currently any operators using these kinds of fields?

I can understand that tarmac/ concrete makes for a much smoother and more pleasant surface along with the reduce risk of FOD, hidden cracks that wheels could get caught in etc.

Any other examples such as executive jets, fighter aircraft that regularly handle rough fields are more than welcome.

The main problem with most jet airliners is that the engines are placed close to the ground. If the field is not paved, this increases the risk of damage to the engines from ingesting FOD (foreign object debris).

There are, however, jets that can operate from un-paved fields.

The 737-200 has an option called a "gravel kit." Despite having engines close to the ground, this kit helps to prevent the engine from sucking in FOD, which allows the aircraft to operate off of gravel runways. It also includes an addition to the nose gear to help prevent gravel from getting kicked up. For this reason, airlines like Canadian North still operate multiple 737-200 aircraft in order to provide service to airports with gravel runways.

High-wing aircraft are better suited for this type of mission (see What are the pros and cons of high-wing compared to low-wing design?). The C-17 is a military airlift designed to operate from remote airfields. Its wings are placed above the fuselage rather than below, which allows the engines to be mounted further above the ground. The IL-76 is a Russian aircraft of a similar configuration that also operates out of unpaved fields.

Aircraft with engines mounted on the rear fuselage also protect the engines from ingesting FOD. The new Pilatus PC-24 jet advertises the ability to operate from unpaved strips. Older aircraft like the 727 or the Tu-154 have this engine placement as well, and both can operate from unpaved fields.

737-200 landing on a gravel runway. The gravel kit is visible as the small extension just below the front of the engine, and the addition to the rear of the nose gear.


What is the 'fuel cost' of a pneumatic braking system vs. a hydraulic one, in terms of weight? A pneumatic braking system has indisputable advantages over a hydraulic braking system for large and heavy aircraft in terms of braking force and secondary perhaps in terms of weight (pipes and hydraulic fluid). However it requires engine input to compress the air and generate the pressure and force required for braking, all of which use fuel.

What is the cost of a pneumatic braking system from a fuel point of view as opposed to hydraulic in relation to weight for a large Jumbo like the Airbus A380 or Boeing 747 if one had to consider using hydraulic brakes for any of these aircraft?

There is an existing triple redundant hydraulic system in those planes required used for the control surfaces. Those are not something you want controlled by pneumatics.

This means the weight of the compressor, buffer tank etc. is already there and using it for brakes means you only need to pull a line from a supply/return line (which would also be near the gear to actuate it anyway) and adding a servo valve.


Are squawk codes used with ADS-B? When using ADS-B are squawk codes still necessary or does the unit provide a discrete code (like a tail number or something) that is recognized by ATC?

Yes, squawk codes are still used and required with ADS-B.
It's possible that in the future this may change, but because of the bifurcated implementation the FAA has chosen for ADS-B I don't think it's likely.

ADS-B operates by giving additional information beyond the Mode A (squawk) code - this is either encoded on the 1090MHz transponder frequency (Mode S "Extended Squitter" - what most of the world uses) or sent on a separate frequency (the 987MHz "Universal Access Transceiver" frequency, which is a US/FAA only thing mainly intended for light GA aircraft).

The Mode S and Extended Squitter data includes things like the aircraft's unique Mode S address (a 24-bit number assigned to the aircraft by its national registry, in the US this is linked to the N number), and may also provide the tail number or flight number. The ATC radar system will accept and display this information for the controller.
At the moment the Mode S/ES data is not used as a substitute for the Mode A transponder code, but because Mode S data is coming through the ATC secondary radar system it's all immediately available with the secondary radar return the same way a Mode A squawk code is, and it's possible that the FAA may use the Mode S ID in place of the Mode A squawk code for these aircraft in the future.

The 987MHz UAT transmissions also include the Mode S address (because it's a useful unique identifier for a given aircraft) and flight ID/tail number can be sent as well. Information from UAT transmitters is received through separate ground stations and recombined with the radar return in the ATC radar back-end.
There's some magic to that recombining which is out of my depth, but my limited understanding is that since the UAT data isn't directly correlated with a radar return squawk codes are part of how the two are matched up (the radar computer knows your N number by your discrete squawk code, the UAT ground station knows it from your Mode S ID - if they match and the position data is within reason the ADS-B information can be associated with your radar data block).


Is it legal to overload a plane in Alaska? Is it true that it's legal to fly a plane over max gross weight in Alaska? If so, what are the rules?

Under some conditions, FAR §91.323 allows for a 15% increase (up to 12,500 lbs) in max gross weight in Alaska:

  (a) Notwithstanding any other provision of the Federal Aviation Regulations, the Administrator will approve, as provided in this section, an increase in the maximum certificated weight of an airplane type certificated under Aeronautics Bulletin No. 7–A of the U.S. Department of Commerce dated January 1, 1931, as amended, or under § 91.325 14 CFR Ch. I (1–1–05 Edition) the normal category of part 4a of the former Civil Air Regulations (14 CFR part 4a, 1964 ed.) if that airplane is operated in the State of Alaska by—

  A certificate holder conducting operations under part 121 or part 135 of this chapter; or
  The U.S. Department of Interior in conducting its game and fish law enforcement activities or its management, fire detection, and fire suppression activities concerning public lands.

  (b) The maximum certificated weight approved under this section may not exceed—

  12,500 pounds;
  115 percent of the maximum weight listed in the FAA aircraft specifications;
  The weight at which the airplane meets the positive maneuvering load factor requirement for the normal category specified in § 23.337 of this chapter; or
  The weight at which the airplane meets the climb performance requirements under which it was type certificated.

  (c) In determining the maximum certificated weight, the Administrator considers the structural soundness of the airplane and the terrain to be traversed.

  (d) The maximum certificated weight determined under this section is added to the airplane's operation limitations and is identified as the maximum weight authorized for operations within the State of Alaska.

One possible reason for this is that Alaska Statute Section 02.35.110 requires aircraft operating to/from and within Alaska to carry additional emergency and survival equipment:

  (a) An airman may not make a flight inside the state with an aircraft unless emergency equipment is carried as follows:

  the following minimum equipment must be carried during the summer months:

  rations for each occupant sufficient to sustain life for one week;
  one axe or hatchet;
  one first aid kit;
  an assortment of tackle such as hooks, flies, lines, and sinkers;
  one knife;
  fire starter;
  one mosquito headnet for each occupant;
  two small signaling devices such as colored smoke bombs, railroad fuses, or Very pistol shells, in sealed metal containers.

  in addition to the equipment required under (1) of this subsection, the following must be carried as minimum equipment from October 15 to April 1 of each year:

  one pair of snowshoes;
  one sleeping bag;
  one wool blanket or equivalent for each occupant over four.

  (b) However, operators of multi-engine aircraft licensed to carry more than 15 passengers need carry only the food, mosquito nets, and signalling equipment at all times other than the period from October 15 to April 1 of each year, when two sleeping bags, and one blanket for every two passengers shall also be carried. All of the above requirements as to emergency rations and equipment are considered to be minimum requirements which are to remain in full force and effect, except as further safety measures may be from time to time imposed by the department.


Where can FAA Letters of Interpretation be found? The FAA issues Letters of Interpretation and Chief Counsel Opinions in order to clarify existing rules and regulations.  Where these can be found and searched when looking for clarification of a rule?

The FAA Regulations Division maintains a webpage which includes a searchable database of "Legal Interpretations & Chief Counsel's Opinions".  It includes the following disclaimer:

  Legal Interpretations and the Chief Counsel's opinions are now
  available at this site.  Please note that not all interpretations or
  Chief Counsel's opinions are available at this time.  This database
  consists of legal interpretations issued from 1990 to the present and
  will be updated on a regular basis.

The letters can be searched by year and/or keyword.


Does a prone position for the pilot minimize g-force effects? Knowing that the Wright Flyer was the first successful heavier-than-air powered aircraft, I'm fascinated by its operation requiring a prone position.

Searching further, I was surprised to find out that the recent EMG-5 motorglider also puts the pilot in a "prone" position.

My question:

Can someone expand on the alleged minimizing g-force effect of the prone pilot position. Why isn't it more common, especially in fighters?

It is true that the body can withstand higher g-forces in the prone position. The reason is that in the prone position, g-forces don't pull blood down to the lower body and away from the brain, as happens in the seated position.

The concept was explored for combat aircraft with the Italian Savoia-Marchetti SM.93 and the British Gloster Meteor F8. These aircraft revealed some practical issues with the prone position:

It is much less comfortable for the pilot than a seated position, especially during long flights.
It is much more difficult to design an ejection/escape system. The Meteor F8 required the pilot to jettison the rudder pedals and crawl backwards to an escape hatch.
It affords much less visibility to the pilot.

In light of these issues, it was found more practical to stick with seated pilots. In modern times, g-suits and advanced training techniques like the "hook" maneuver have allowed fighter pilots to withstand higher g-forces than in the past. Both involve reducing blood pooling in the lower body by squeezing or flexing leg and abdomen muscles to restrict unwanted blood flow and pooling.


What happens if Solar Impulse II encounters un-expected head wind and runs out of battery power? I noticed that battery charge level dropped to fair levels in some occasions in this article. It dropped to around 20% at the end of day 5 before morning of day 6.

To what extent will a flight be affected by unexpected strong head wind that results in lower Eastward ground speed and delaying time to next sun shine and battery runs out?

I tend to think weather prediction is quite good nowadays, what is the plan if this happens on long ocean flight? Does the ocean have thermals where a glider can stay up 'forever' while waiting for morning sunshine?

On Solar Impulse, or similar 'experimental' (at the edge of limits) flights, like the solo circumnavigation balloon flight, do they usually have a 'chaser/spotter' plane flying along?

Solar Impulse has two means to store energy: Batteries and altitude. Weather permitting, both will be fully charged at sunset, and mostly depleted over the night. The rate of depletion at sunrise depends only on the time between sunset and sunrise, not on the direction and strength of the wind.

Headwind only means that less ground is covered. Solar Impulse is optimized to fly at a certain speed and will not fly much faster or slower, regardless of wind. If the wind is strong enough, it will simply not cover any ground but be blown back. But the wind will not cause the batteries to be depleted earlier, and the next day they will be recharged if the sky is clear.

Wind speed and direction vary with altitude, so the aircraft might well overcome a wind that had prevented it to make any progress the night before by flying at a different altitude the next day. After all, flight duration is limited by the pilot, not the power source.


Is a single engine failure on the HondaJet recoverable? Is it possible to recover from a single engine failure at cruising altitude on the HondaJet and safely land the aircraft?

There would not be much point in having two engines if a single engine failure would be unrecoverable.  In fact, that would make the plane twice as likely to have an unrecoverable event!

If one engine has a 1/1million chance of failure, then the chance of failure of at least one of 2 engines is 2-in-a-million.(technically, slightly less than 2-in-a-million... but close enough)  And if either engine failure is catastrophic, then you've doubled your chances of losing the plane.

I don't know about the HondaJet in particular, but by logic and statistics, I can say that a single engine failure must be recoverable.


Why are there two red lines on the G-meter in the MiG-29? Why are there two red lines on the G-meter on the MiG-29 aircraft? One at 7.5G, another one is at 9G. Also on the AOA scale the red line is at only 15 degrees. That is max AOA. It looks very low to me. Or the max G must not have more than the 15AOA? Now it's about the 7.5 G at 15AOA or the 9G at only 15AOA?

@George already gave a correct answer, but I'll expand it a bit and show the source.

Indeed, the lower load limit is for trans- and supersonic flight (formally M > 0.85) and the higher limit is for subsonic conditions.

The AoA redline at 15° also relates to M > 0.85. At subsonic speeds, the max AoA is 26°. I can only speculate why it is not shown on the gauge. Normally, the control system will enforce these (AoA) limits automatically.

Here are the load and AoA graphs from the MiG-29 flight manual (or more precisely, its 'Practical aerodynamics' part).

One might notice that the official supersonic load limit is 7 rather than 7.5. I'm not sure why is the gauge showing 7.5.

The curves indicate loads and AoA that are actually achievable at different altitudes (in km). The boundaries are labelled 'structural limits' (on the top) and 'control system limiter settings' (for alpha).

The text explains:

The load applies to the mass 14200 kg, whether with or without missiles. For higher mass, the load limit is reduced by 1 g.
The lower supersonic limit is explained by the lift losses for trim. (MiG-29 is marginally stable at low speed, but (like nearly any aircraft) is highly stable at supersonic speeds, and part of the wing lift must be used to counteract the strong tail downforce).
At M > 0.85, the leading edge droop flaps are stowed, which limits AoA at 15°. At lower speeds, the flaps expand the AoA limit significantly. (There may be further limits; e.g. 13° with a failed SAS (stability augmentation system)).


What is the aircraft loss-rate from Nimitz-class aircraft carriers? What is the aircraft loss-rate from Nimitz-class carriers?   That is, what proportion of sorties result in the loss of the aircraft for reasons other than enemy action?

I was prompted to ask this after the Russian carrier Kuznetsov lost two aircraft in the space of a couple of weeks.

The Naval Safety Center publishes some useful stats here.

(Rates are per 100,000 flight hours per year. Source.)

(Source.)

A few more details are available in the summaries. One example:

Other examples:

E-2C: Electrical arcing at vapor cycle power receptacle led to aircraft fire in the hangar bay.
A single F/A-18C crashed 6.6 miles northwest of Lakenheath, England. Event Cost \$72,000,000
MV-22: While recovering to LPD, MV-22 landed short of spot 2, main landing gear in deck edge netting
Engine fire leading to ejection. Event Cost \$71,000,000
Two CH-53 helicopters crashed while on training flight. All 12 crewmembers deceased.
AV-8B experience catastrophic engine failure during takeoff. Event Cost \$62,800,000
AV-8B experienced loss of thrust, pilot ejected safely. Event Cost \$62,800,000
2xF/A-18F Collided during Air-to-Air training. 2 aircraft lost. No fatalities. Event Cost \$173,580,662
EA-18G inflight engagement during night CV landing caused extensive damage to aircraft
F/A-18 impacted ground during air show practice. Event Cost: \$75,400,000
MH-60S crashed into James River while performing SAR training flight. Event cost: \$25,508,816
KC130T EXPERIENCED HAIL DAMAGE WHILE PARKED ON THE RAMP AT PETERSON FLD, CO
(KCOS)
F/A-18C crash during night air-to-ground training mission. Event cost: \$76,500,000
[snip]
T-45C: ACUTE PHYSIOLOGICAL EPISODE RESULTS IN ACTIVATION OF THE EJECTION SEQUENCE BY CREw. Event Cost \$37,800,000
A single T-45C impacted the ground at high energy. \$37,800,000
[snip]
Aircraft impacted a tree during low level training. Aircraft recovered safely.
[snip]
MV-22 ditched off Okinawa during NVD training mission. Event Cost \$80,600,000

Class A mishap definition:

  \$2,000,000 or more and/or aircraft destroyed [or] Fatality or permanent total disability

Just note that these are just Class A's. It doesn't include B's, C's, or D's. Some of these aren't carrier related.

On a completely unrelated note, notice the costs. Modern a/c are expensive (although not all that is flyaway cost).


Does kerosene have a higher octane value than gasoline? One of my coworkers says his chemistry teacher told them (a long time ago) that kerosene has a higher octane value than gasoline. My understanding was always that aviation gasoline has octane value above 120, but it is really gasoline, while kerosene is more like diesel fuel. What is the truth?

Octane rating is a measure of how well a fuel resists detonation, which is the premature explosion of fuel in an engine cylinder before the spark is applied. This is relevant for spark ignition engines which use gasoline type fuels like avgas.

Kerosene has a very low octane rating, somewhere around 15, and isn't a good fuel for spark ignition engines as it would detonate on compression. This is why kerosene and diesel are used in diesel cycle engines which rely on the spontaneous detonation of fuel under pressure. Kerosene is not typically used in internal combustion engines, however in aviation there are turbo-diesel piston engines which run on Jet-A1.


How does a delta wing deal with supersonic boundary separation? The question Why most of the supersonic or fighter aircraft use all-moving control surfaces? emphasis the importance of all moving stabilator for supersonic aircraft.

Many supersonic aircrafts fitted with delta wings (Concorde, mirage III, mirage IV, saab draken, sr-71,...) rely on elevons only for pitch control and thus cannot use a all moving stabilator.

How do they overcome the reduced controllability due to the boundary separation of the airflow at the trailing edge of the wing?

My question is focused on aerodynamic solutions (I know some aircraft can shift there CG thanks to trim tank, and some fighter are not aerodynamically stable in pitch but I want to focus on the boundary separation issue).

1. Aeroelasticity
Unlike a tailplane, a delta wing is more rigid due to its much bigger chord and multiple spars, so control reversal due to aeroelasticity isn't a special concern.
Big subsonic jetliners typically lock the outer ailerons at high speeds. Concorde featured a similar function for the outer elevon, but only if $V_{MO}$ is exceeded by 25 knots as a safety measure (flight manual page 09.01.03).

Concorde's structure also makes it much more resistant to the distorting forces of supersonic flight than aircraft built before her. (...) If Concorde gets substantially above its design speed, the outboard elevons are automatically neutralized, so that no twisting is possible (heritageconcorde.com).

2. Transsonic separation
Delta wing aircraft rely on thin airfoils (vs. blunt) where the pressure gradient is more forgiving. And due to the bigger chord, the elevons deflection doesn't result in a big camber change as happens on the smaller tailplanes. The big sweep angles of delta wings also make the airflow see thinner airfoils, making the transition to supersonic easier.
3. Supersonic separation
Not an issue since the shock wave moves to the trailing edge at supersonic speeds.

Source: https://history.nasa.gov/SP-367/f87.htm

Must reads:

Why is the coefficient of drag for straight wings at supersonic speeds lower than for swept wings?
What is the pressure distribution around a supersonic wing?
What causes the reduction in speed across the shockwave?
Why does supersonic flight detach airflow from a wing?
How does aileron reversal work?


Why does propeller slip (efficiency) change during the flight (fixed-pitch)? I was wondering how the efficiency of a fixed-pitch propeller changes during different stages of flight and why it does so. I suppose when we are talking about propeller efficiency we are actually talking about propeller slip. So my follow up question is what causes propeller slip to
change, what conditions make a propeller lose efficiency. I am aware that it has to do with Pitch, but I can't figure out why efficiency changes throughout the flight. THANKS

The reason that efficiency is reduced during phases of flight that are flown at high airspeeds is because the angle of attack between the propeller and relative wind is reduced. If an aircraft is stationary on the ground, its propeller is taking a larger "bite" of air because there is no relative wind. When there is more air moving past the prop, it takes a smaller "bite" of air, therefore producing less thrust and being less efficient. The advantage of a constant speed prop is that you can change the pitch of the prop depending on your airspeed, so it maintains a constant angle of attack, and is more efficient in every phase of flight. I'm not totally sure that's what you are asking, but that's what I got :)


Is it possible to reuse water extracted from the PACKs Water extractors? During the Process inside of the Air Generation Unit (AGU) there is a water extractor behind the condenser separating the water from the air which will flow to the Re-Heater.

Is it posible to filter this water and re-use it for lavatory flush or hand wash water?

Since Humidity at FL300 is around 3%, is there even sufficient water being extracted to refill the tanks?

Water comes from condensing water vapor present in the cabin, itself coming from human breathing and perspiration, and, for a small part, from atmosphere.

The question mentions an air generation unit which is an Airbus terminology, so let's go into the A380 air system:

A380 Engineering training, source (page 101)

The two green labels are my addition to show the extractor and a spray nozzle. Water separated by a cyclone in the extractor is sprayed into ram air just after the ram air inlet and before entering the heat exchangers of the air-cycle machines. Spraying water droplets creates evaporation, hence a temperature drop in the exchanger receiving fluid, increasing air exchanger capacity.

Bleed air from the engines/atmosphere is colder and dryer at high altitude. Air exchangers work better, but cabin atmosphere may be dry for human well being. A fraction of condensed water can be returned to conditioned air going to the cabin. This is done automatically by the air-conditioning system.

To answer your question: Water is already fully reused.


What is a Second Officer? I've seen the position of Second Officer being advertised for some short-haul airlines, but I don't entirely understand what roles and responsibilities they have on the flight deck.

I know a second officer exists on long-haul flight, which allows for crew to take rest breaks through the duration of the flight. So what do they do on short-haul flights? Are they still under line training?

Definition

A second officer (SO) is third in line of command on an airplane. The purpose of a second officer is:

  a Second Officer is used on international or long haul flights where
  more than two crew are required to allow for adequate crew rest
  periods.

I have never seen anyone besides a captain and first officer on a short-haul flight. Such flights normally don't even require either the captain or first officer to take a break to rest.

Duties

Second officers are qualified pilots and also type-rated for the airplane they are on, similar to the captain and first officer, but only with lesser experience. I do not know the exact career path for them, but most probably they are promoted to become first officers. This is same as first officers are promoted to become captains.

Jobs

There are certainly jobs available for second officers, but only on airlines which operate long-haul flights. Regional airlines with flights only for a 2-3 hours don't require a second officer1.

Cathy Pacific is hiring (also here). The experience of a second officer on Cathy can be read here. Aviation Job Search is listing many positions.

SO on Short-Haul

Each airline can have its own definition of second officer. What I wrote above is the general meaning. You mentioned JetA and Thomson (in comments to the deleted answer) but I couldn't find jobs for SO.

Flight Engineer

Their role is different than pilots and now they are a diminishing breed. Most of them were not pilots. Again, an airline may label a flight engineer as second officer, though Wikipedia did not mention that.

1: The aftermath of Germanwings crash can make some changes in regulations.


How is fuel vapor dealt with at large airports? When I fill my tank at a gas station, I can see fuel vapor coming out of the tank.

I imagine the space above the aircraft tank has fuel vapor mixing with air (at lower oxygen than normal thanks to the inerting system). During refueling, the fuel will replace the space used by this fuel-air mixture and force the mixture out of the tank. If unmanaged, this is a big fuel source and fire hazard.

How is this mixture managed in order to minimize risk of fire or explosion?

Large airplanes are pressure refuelled so vapor escaping through the fuel port is not an issue as it is in smaller airplanes.  You are correct, however, that the fuel/air mixture existing in the tank will be displaced by the incoming fuel.  The wings will have vents installed that will allow the air mixture in the tank to escape (and fuel in the case of over fuelling or excess expansion of the fuel after it is loaded).

The fire risk is minimized in three ways I can think of:

The airplane is grounded to the fuel truck, minimizing the risk of ignition via sparking.
The wing vents have flame arrestors so that if the air mixture leaving the wing did ignite, the flame would not make it into the tank.
Atmospheric dispersion -- once the air mixture is out of the tank it will become more dilute with time.

In the cases there is more than vapor released, e.g. liquid fuel is expelled, these spills will be absorbed to minimize fire risk.  In the ultimate case of fire the fuel truck has fire extinguishers and there is a fire station on the airport that can apply foam and water as needed.


How does single-sided deafness affect the different pilot licenses? About 12 and a half years ago I lost the sense of hearing in my left ear due to borreliosis. However, I have always wanted to get a pilot's license at some point in my life. I am aware that for some (or even all?) licenses, a medical examination must be passed and one-sided hearing loss is essentially a no-go.

So my question is: What are my possibilities with my disability? What kind of license could I still be allowed to get or will I have to bury this dream?

I live in Germany.

If you were willing to travel to the US (which is not all that uncommon to get a pilots license) and get your license under the FAA regulations you could do it as per the regulations,

  §67.305   Ear, nose, throat, and equilibrium. Ear, nose, throat, and
  equilibrium standards for a third-class airman medical certificate
  are:

  (a) The person shall demonstrate acceptable hearing by at least one of
  the following tests:

  (1) Demonstrate an ability to hear an average conversational voice in
  a quiet room, using both ears, at a distance of 6 feet from the
  examiner, with the back turned to the examiner.

  (2) Demonstrate an acceptable understanding of speech as determined by
  audiometric speech discrimination testing to a score of at least 70
  percent obtained in one ear or in a sound field environment.

  (3) Provide acceptable results of pure tone audiometric testing of
  unaided hearing acuity according to the following table of worst
  acceptable thresholds, using the calibration standards of the American
  National Standards Institute, 1969:

The way this is written if you can score at least 70% in one ear on the audiometric speech discrimination test you should be good to go. Note this is for a class 3 medical with will not allow you to fly commercial operations. I am not sure how the new basic med requirements fall into all this but your best if you have specific questions is to contact an AME directly.

From a practical standpoint, the cockpit of most small planes is fairly defining already and the headsets take mono signals from the radios so I don't really see how the lack of binaural hearing would present any issue.


What to do if trapped on top VFR? In the event that, despite his best efforts, a VFR-only pilot has gotten trapped on top of a solid cloud layer, what is(are) the safest course(s) of action?

For example, consider a long cross-country flight where the destination was forecast to be clear, few or scattered but upon arrival turned out to be broken or overcast.

You should have an alternate planned that is forecast VFR, with appropriate fuel reserves in place, before you left.  But if you didn't, you'd call an ATC unit and tell them your predicament.  Maybe they can send you to VFR conditions either reported by PIREP or by calling around to other tower units in range.

Otherwise, you're going to have to do an instrument let down.  If you're in contact with an ATC unit, let them provide you with a location to do let down that gives obstacle clearance, and hope your instrument skills are up to the task. In Canada VFROTT is not allowed unless you've had at least the commercial pilot level of instrument training (there is a special rating if you don't). Up here it would be considered crazy for a VFR pilot with no instrument training to do VFROTT in the first place, even if it was legal.

Say you didn't have contact with ATC for some reason.  Well than I'd locate a decent size body of water with my moving map GPS and let down over that, and stop descending if I see water.

That's another planning factor.  You want the cloud you are going to be flying over to have a decent VFR ceiling under it, so you only have to let down through a layer into nice VFR below.


Where can I find the standards for airborne systems software design? Is there a central repository for this information? In reading about software and equipment design for aircraft I'm finding a lot of references to various standards like "DO-178B, Software Considerations in Airborne Systems and Equipment Certification", but I'm having a hard time finding copies of these.

Is there a central repository or location where I can review or purchase these standards?

Many of these are Radio Technical Commission for Aeronautics (RTCA) standards.
You can get these documents from the RTCA website. They are provided free of charge to RTCA members, non-members have to pay.

Alternatively you can acquire the European equivalents from EUROCAE (RTCA DO-178B is ED-12B in the EUROCAE eShop).

If you want to be up-to-date (for example if you're building a new avionics system), you should buy the latest version of the standard, such as DO-178C / ED-12C, which is the latest edition of your example above (published in 2012).


What is the purpose of this interesting pattern of class E airspace in northwest Alaska? Perusing SkyVector today I noticed this airspace pattern:

It can be viewed in more detail here.

What immediately jumped out at me is that

there are no charted airports in any of this airspace;
the RNAV routes crossing through this area are also class E, so in reality the airspace is not actually divided, but the VOR airway crossing in the northwest of the area gets different treatment; and
there is little discernible pattern to the airspace based on terrain, landmarks, or politics (e.g., it is more extensive to the north than to the south of the intersection, there is a very small extra bit in the southeast corner).

My best guess is that this airspace exists to facilitate IFR/VFR separation where three RNAV routes intersect, but that doesn't necessarily explain all the points above. I'm hoping that someone familiar with the area and/or better at searching FAA publications can lend some insight.

This is easier to explain in reverse: rather than why there is so much class G airspace there, why is there so much class E airspace  everywhere else?

Remember, all airspace is class G unless it has been designated something higher. Wind your clock back to the early days of aviation, and the entire country was class G!

However, that isn't very safe, particularly in bad weather. Pilots were getting lost and running into each other.

So we got navaids and radios, and narrow strips of airspace along between navaids became controlled airways, i.e. class E. If you scroll over to Canada in SkyVector, you can see what this probably looked like.

Fast forward several decades, and there are now so many airways criss-crossing the US that they've merged into one big class E blanket, so it's easy to forget how we got here.

But there are still a few areas where the airway density never quite reached full saturation, mostly in mountainous terrain (of which Alaska has a lot), and those pockets still remain class G--and reveal the past.

As to why the V airway in your example has a different floor than the T airways, I'd assume that comes down to the minimum VOR reception altitude due to the terrain. Other than the floor, though, it's the same.


What is the maximum range of a BAe 146-300? Can auxiliary fuel tanks be added? What is the maximum range of a BAe 146-300? I saw one on Flightradar24.com which started in western France and is still flying non-stop to Greece (flight in progress at the time of posting).

I was really surprised, I guess they probably have an additional fuel tank? I know that some airlines do add extra tanks  to their planes (e.g. on a B737) to increase the maximum range; but is it possible on on a BAe 146-300?

According to wikipedia it has a range of 2909 km and the distance is around 2500 km


How do blade cuffs help cool the engine? How does the blade cuff (a fairing of suitable airfoil shape used to cover the shanks of propeller blades in order to reduce the aerodynamic losses) help cool the engine?

Source: An Illustrated Dictionary of Aviation

The cuff is used to change the pitch of the blades (or to create an airfoil when there is none) near the spinner. The cuff increases the airflow near the engine cooling intakes. The cuff is large, and larger at the blade root because the linear speed in this area is very small compared to the tip.
Lockheed P3 Orion

Cuffed blades of a Lockheed P3 Orion. Top: Photo by palmerphoto at RedBubble. Bottom: Photo by MDLPhotoz at Flickr

North American P-51 Mustang, cuffed vs uncuffed

Awesome photo by Allan Grant/LIFE Magazine, via This Day in Aviation

Captioned:

  Joe De Bona and Jimmy Stewart with Thunderbird, their P-51C Mustang
  racer, April 1949. Placed on the ramp in front of the airplane is
  equipment that has been removed or replaced. Note the four “cuffed”
  Hamilton Standard propeller blades along the right side of the
  photograph. They have been replaced with un-cuffed and polished
  Hamilton Standard “paddle” blades.

More aviation photos by Allan Grant


What is a metering frequency and why would an airport have it? I just read a post from a pilot complaining about how he thought a metering frequency was useless.  It has something to do with being on the ground at an airport.

What is it, what purpose does it serve, and what causes an airport to adopt one?

Use of a metering controller and frequency at an airport is one strategy used to prevent congestion on ramps and taxiways. When ready for push back from the gate, pilots call metering. The metering controller will typically either give them permission to push with a ground frequency to call when ready for taxi, or they will tell them to monitor a particular ground frequency for permission to push. They may even tell the pilots to stay on the gate and call back after a specified delay. The use of metering allows the airport controllers to manage ramp and taxi space by sequencing aircraft push and taxi.

Metering control can also help manage potential congestion due to ground delays for situations such as poor weather, runway closures, deicing, flow times into particular destination airports or any other situation that increases taxi time or slows down departures.

Some airports uses different strategies such as discrete ramp areas and frequencies to coordinate push back and taxi flow or internal coordination between controllers.


How can I see all commercial destinations to a country from a particular airport? I live in Las Vegas, which obviously has a large airport with a broad array of international flights. Is there a quick way for me to see all the non-stop commercial flights that are available to Mexico? I don't need to necessarily know the airline, just which cities are available for non-stops from Las Vegas.

There can be a number of websites which can provide this information. Wikipedia and the specific airline's website can be a great resource too.

OpenFlights gives you a graphical representation of direct flights from a particular airport. It appears that this service is for all major airports across the globe.
SkyScanner can provide the information you are looking for. As for your example, you'll enter Las Vegas in the From text area, and then leave the To area blank. It will search all destinations (grouped by countries) where there are direct flights (or with one stop) available.


Center of Pressure(CP) The center of pressure is:

A)The point at which the Total Aerodynamic Force acts
B)The point where the lift force acts
C)The point where the resistance force acts

Only one answer is correct,i think is B or A but the question is really strange.

The fact that Total Aerodynamic Force is capitalized indicates it is a term that can be researched:
http:// www.dynamicflight.com/aerodynamics/aero_force
Total Aerodynamic Force includes drag, therefor the answer is B)
the point where the lift force acts.


Why does a stall decrease lift, rather than increasing it? A stall occurs when the angle of attack of a wing or other airfoil becomes so high that the airflow over the upper surface of the wing separates from the wing, rather than remaining attached to it; this causes the wing to produce less lift and more drag, making it harder to maintain level flight and more sluggish to respond to control inputs.

However, shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing, and, thus, greatly increase lift?  What am I missing?

For a parcel of air to generate a lift force as it flows over the wing requires the wing to tip that air parcel's momentum vector downwards slightly; the reaction force that the wing experiences as it does this is what we measure as lift.

In the case where the airflow over the top of the wing separates from it, the parcels of air flowing by do not get their momentum vectors redirected downwards anymore and the wing hence "stops flying".

Meanwhile, the region of separated flow constitutes a zone of turbulent air which forms a stirred-up wake in the rear-facing "shadow" of the wing at its high angle of attack and the only work performed by the wing in this case is to stir up that wake- and that constitutes lots of drag.


In what emergency situations would a pilot bail out of a helicopter? The Hiller UH-12 POH actually mentions “Emergency Exit in Flight” as an emergency procedure. What emergencies would make the pilot do this and why don’t other helicopter POH’s mention this?

One example of a helicopter bailout that I am aware of was the ditching of a USAF Sikorsky HH-60G during the infamous "Perfect Storm." After several failed attempts to refuel from a tanker in the extreme weather the pilot decided to ditch. He hovered to allow the other occupants to bail out, but he was unable to bail out himself as the engines flamed out from fuel exhaustion and he autorotated to the water. After it hit the waves it rolled over. At this point he realized that he had not jettisoned his door as per the procedure for bailout or ditching so he was trapped. He managed to escape, but one of the other crew was lost at sea.


What does the behaviour of water on the skin of an aircraft in flight tell us? Sometimes in damp conditions I notice some surprising (surprising to me, anyway) behaviour of water.

I'll see moisture glistening in the heads of rivets or bolts on the top of the engine pylons - that stays there for a long time (many minutes) after take-off.

Similarly, at the front edge of my window I will see a pool of water that only very slowly diminishes, seeding little drops (that become smaller and smaller as airspeed increases). Again, this pool remains far longer than I'd expect.

I'm surprised because I'd expect the evaporative and sweeping effect of the high-speed airstream to clear this moisture extremely fast, and yet it doesn't.

I suppose that despite the speed of the airstream, there are numerous small areas on the surface of the plane that are sheltered from it very effectively.

I'm interested to know what the implications of the causes of this effect are are for aerodynamics, for example how they relate to drag on the fuselage, or - where it happens on lifting or control surfaces - how their operation is affected.

This effect is likely caused by something called the "boundary layer," first defined by German engineer Ludwig Prandtl in a paper he presented in 1904 at the third International Congress of Mathematicians in Heidelberg, Germany. From Wikipedia:

The effect of the paper was so great that Prandtl became director of the Institute for Technical Physics at the University of Göttingen later in the year.
Over the next decades he developed it into a powerhouse of aerodynamics, leading the world until the end of World War II. In 1925 the university spun off his  research arm to create the Kaiser Wilhelm Institute for Flow Research (now the Max Planck Institute for Dynamics and Self-Organization).

The math in the paper is complicated and frankly well beyond my paygrade, but the way I understand it, what I think you are seeing is caused by a laminar boundary layer that forms very close to almost all surfaces of the aircraft. It is a layer of air with high viscosity, low turbulence, low speed relative to the air farther out from the surface, and causing an increase in drag. That's why the water doesn't blow away or evaporate.
The image below is one I found which does a good job of presenting the two types of boundary layer -- laminar and turbulent. Look at how smooth the laminar boundary layer looks compared to the turbulent boundary layer. This might help to understand the concept better.

The boundary layer is also something I experienced with an experimental aircraft I restored that had an opening on the top right of the fuselage to feed air into the fuel injection system intake. As it was, air would essentially stop flowing into the system when the aircraft accelerated, and the engine would stop. Not good. In order to stop that from happening, a raised scoop had to be installed that extended above the laminar boundary layer and into the turbulent boundary layer. That solved the problem.


Is adverse yaw ever desirable? Is there any situation during flight where adverse yaw (yaw due to roll) is desirable?

Yes. For deliberate cross control situations, adverse yaw helps.  This means during side slip or forward slip maneuvers, where adverse yaw means less rudder is required to keep the nose pointed in the opposite direction to the lowered wing.

If you are in a glider or an old classic aircraft with lots of natural adverse yaw, if you lower a wing without any rudder to keep the ball centered, you end up in a mild side slipping turn automatically thanks to adverse yaw.


What is the asymmetric feature on the rear of a Sukhoi Su-47? I saw this part on the rear of the Sukhoi Su-47 Berkut, and I can see that it is asymmetric. Is it maybe for interference, or is it a take-off aid because the Su-47 is a STOL plane?

What is it and why is it asymmetric?

Edit:Second picture is mirrored

The Su-47 has two tailbooms of unequal length outboard of the exhaust nozzles. The shorter boom, on the left-hand side, houses rear-facing radar, while the longer boom houses a brake parachute.

Source: Wikipedia

Related: What is this part between the Su-35's jet exhaust nozzles?

Note: Your second image is mirrored. Here's a real photo with the right (starboard) boom being the longer one:

  Su-47 (aka S-37) in lead, also notice the tail numbers of the formation (no mirroring); source: wikimedia.org


Did any metal airplane use a Maxwell slot? The Douglas DC-8 used leading-edge slots. Unlike slots in light planes, this one was door operated.
I thought Douglas must have patented it. But as far as I searched, they did not. So it must have been an older design. More researching later, I arrived at a patent, inconveniently titled "Airplane wing", by Frank R Maxwell (image below). Which is the same as the DC-8 minus the pop-out upper part. Earlier designs by Maxwell for this design date back to at least 1928.
And indeed a 1937 NACA report, Wind-tunnel tests of a Clark Y wing with 'Maxwell' leading-edge slots, uses the term Maxwell slot to describe them.

For a given slat size the Maxwell wing with the narrow slat has approximately the same aerodynamic characteristics as the Handley Page type of slotted wing.

Did any metal airplane use a true Maxwell slot? One example would suffice. I haven't been able to find any. I think the 'pop-up part' is important; the DC-8-style slot did not work on the DC-9 without vortilons (during development).

Finally! The Vought OS2U Kingfisher (first flight 1938). At least the OS2U-2 model...

Production variant with minor equipment changes and powered by a 450 hp (336 kW) Pratt & Whitney R-985-50, 158 built. [emphasis mine]

... had default Maxwell slots, thanks to a wartime NACA report.[1] The report makes no mention of the plane's model, but another report does in the references section.[2]
I thought, it can't be true, it must have been a NACA modification. But no, the first reference explicitly lists the modifications done. The first of which is what I needed:

Modification 1.– Auxiliary slats were mounted ahead and above the normal leading edge of the wing root, overlapping the inboard end of the original Maxwell slot arrangement by about an inch (fig. 11). [happy-emphasis mine]

And that's not all. The PDF is a 90 MB color scan, so, photos, naturally:

Photos are hard to come by, but I found one that shows the popped-up leading-edge from behind:

Source: ww2db.com; the plane was available in land/sea configurations.

1: Adams, Betty. "Flight Measurements of the Effects of a Wing Leading-edge Slot and Other Modifications on the Stability, Maximum Lift, and High Speed of an Observation Airplane." (1943). https://ntrs.nasa.gov/citations/19930093568
2: Adams, Betty. "Flight measurements of the effect of various amounts of aileron droop on the low-speed lateral-control characteristics of an observation airplane." (1943). https://ntrs.nasa.gov/citations/19930092611


Airframe parachute on commercial airlines My question, would a multi-deployment airframe parachute system be viable on a commercial airline? For example, Grumman & Cirrus already use this system.

As with the automobile industries, they were found to be culpable of many road deaths prior to implementation of seatbelts (refused by all the automakers at the time), so is it the airlines' responsibility for not making flight safe enough when the technology is there to implement?

You cannot buy a new car without it having airbags, why not airframe parachute on every new plane as a standard?

Cirrus builds very small planes that fly at a very low altitude and at very small speeds, hence the parachutes (CAPS) can be used.
General aviation aircraft are HUGE, BULKY and designed to fly at very high mach numbers. They are not designed to "withhold spin" as required by the CAPS.

Moreover, if this was possible, this would already be in action, because an Automobile accident is not much of a Headache for auto companies, but an Aviation disaster is for sure a PR nightmare for an Aviation Company.


In this 2nd brace position, should your head lean against the seat in front? Air Crash Investigation: M1 Plane Crash (S14E01) motivated this question, the relevant timestamps for which I greened. Neither the audio or picture below (in which the hands are hiding the front of the head) explicitly resolves: For maximal safety, in the 2nd suggested brace position (on the right, timestamped at 42.10), should the head touch or rest on the seat in front?
I know nothing of physics or aerodynamics, so please explain in as simple English as possible. The original video appears too dark, so I did enhance the originals (using Gamma Correction):

The head should be against the seat in front of you. The reason is that if the plane crashes, your head and upper body will be thrown forwards until they hit something. If you're in a normal sitting position, your head will be moving pretty quickly by the time it smashes into the seat in front of you, leading to serious injury or death. Moreover, as your torso and head are moving forwards in an uncontrolled way, you can get injured by your head essentially flopping around. Since your head is going to end up against the seat in front of you anyway, putting it against that seat before the crash means it doesn't smash into it during the crash, and that it's not flopping around on its way to the seat.


What is the relation between altitude and vertical speed? I recently saw 2 graphs on a flight path on BBC, shown below. For those interested, it's the path of the recently crashed Boeing 737 max flight ET302, but that's irrelevant to my question.

My question is about the relationship between the two graphs. I would expect the bottom graph to be the first derivative of the top one - with altitude increasing when vertical speed is positive and decreasing when it is negative. However, this does not entirely hold. At around 5:38:45 for example, the altitude clearly drops, while the vertical speed remains non-negative.

Am I missing something about the definitions? Like, vertical speed being defined relative to the wind and the plane stuck in a down-gust. Or, if or example, is there some well-known error caused by the way these quantities are measured by the instruments?

The graphic is attributed to Flightradar24. Flightradar24 relies on transmitted data from the plane (via ADS-B usually) and it is not always perfect. See: Why do aircraft on Flight Radar 24 jump around randomly sometimes?

Back to your question, decreasing vertical speed (V/S) is not descending, it's just the rate of climb is becoming shallower, but still positive.

Factors that can cause the illustrated drop:

Bad data from Flightradar24.
Insufficient reading resolution (thanks @bogl).
Imperfect reading by the plane (see below).

V/S and altitude are measured by the static ports, if the plane's attitude is not ideal, say the plane is moving sideways slightly, each static port on each side will read differently.

The measured quantities are also different, one is a rate of change, and one is absolute reading, this may cause the V/S to lag behind the altimeter.


Why tap the brakes on take-off in fixed gear aircraft? The Socata TB10 checklist indicates that you should touch the brakes after lift-off. I assume this is to stop the wheels from turning, but why?

In a retractable gear aircraft this would make sense to avoid using anything in the wheel-well to stop the wheels and potentially causing damage or generate heat (as a side note, I think some airliners actually have brake-pads in the wheel wells for this purpose). However, the TB10 is not equipped with retractable gear.

The TB10 and TB20 are very similar, and it might be a case of copy & paste between the two POHs (the TB10 POH also lists V-speeds for "flaps and landing gear retracted").

Do spinning wheels have an adverse effect on aerodynamics (or handling in general)? I would assume, if anything, they'd produce less drag. And as a cherry on top, I'd also assume they'd start spinning again fairly quickly, as only the lower part of the wheels are protruding out in to the air flow.

This answer to another question has a link suggesting that the spinning wheels may be sufficiently large gyroscopes to affect handling of the aircraft. That was however on a larger aircraft and higher speed (Lockheed Constellation). I don't know whether that effect would be observable on TB10.


Can a Pilot hold multiple foreign licenses and practice the privileges of it in each country? Is it possible for a CASA CPL holder to convert to both an FAA and CAAUK license and hold all three at the same time?

Would the pilot then be qualified to exercise the privileges of the CPL in Australia (with a CASA registered aircraft), USA (With an FAA registered aircraft), and the UK (With a CAAUK registered aircraft)?

Yes, a pilot can hold multiple licences from different authorities, but it's not about operating in a country. A license from an authority applies to aircraft registered with that authority, wherever that aircraft is in the world.

An FAA license lets you operate an N registered aircraft anywhere in the world, it does not let you operate a foreign registered aircraft on American soil unless that foreign authority gives permission to do so.


Is it possible to use refrigeration technology to cool the skin of supersonic aircraft? The fastest air breathing aircraft, the Blackbird SR71 flew at speeds above Mach 3.  Its engines could achieve much higher speeds but were limited by the aircraft's skin which would melt due to friction with the surrounding air should it fly faster.

Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

Is it possible to use refrigeration technology to cool down the aircraft's skin so it can reach much higher speeds?

Fundamentally: no.

Refrigeration technology moves heat energy around, it doesn't get rid of it. Your fridge moves heat from inside to the outside.

To get rid of it, you have two means, broadly speaking:

Heat up some mass and throw it away
Radiate it away as light/IR

Air cooling fundamentally heats up the surrounding air, and moves the now hot air away.  I believe the SR-71 used it's fuel as heat sink as well, heating the fuel before combusting it, and ultimately throwing it away.

Radiating it is inefficient at low temperatures. And it's directly dependent on surface area, which is at a premium on a plane.

So in essence you'd have nowhere to put the heat energy you removed from the skin of the aircraft...


How would a new "clean sheet" General Aviation piston engine with all the latest technology differ from existing Continental/Lycoming types? My understanding is that current GA piston engines from Continental and Lycoming are based on a design from the 1960's. There have been many evolutionary improvements, but only within the limitations of those 1960's designs. They are large displacement (~1.5L/cylinder), run at RPMs in the mid 2000s range so that they can spin the prop at optimal rates without using a gearbox, and they are air cooled to save the weight and complexity of a liquid cooling system.

What would a completely new, designed from the ground up GA piston engine look like today? Factoring the massive improvements that have been made in all sorts of engines over the years in non-aviation applications- for autos, trains, boats, generators, etc.

Would they still operate at such low RPMs? Yes reliability and durability are still major concerns, but manufacturing tolerances are so many times smaller now then they were in the 1960s (possibly allowing for reliable operation at higher RPMs), and they are now mounting fixed gearboxes to commercial airliner turbine engines so adding a fixed gearbox to a GA piston engine should be trivial.

Would they still be air cooled? Yes it can still save weight, but possibly at the cost of extra cooling drag, and definitely at the cost of much more finicky operation that requires the pilot to carefully monitor engine temps throughout their flight cycle.

Would they still be in a flat engine arrangement (flat 4, flat 6, etc.)? I would imagine so, I can't imagine an alternate piston engine arrangement with the smoothness, packaging, and scalability advantages of the flat architecture. Straight engines get too long, and V engines require many more changes to scale the number of pistons up or down.

Would they still fly on spark-ignited gasoline-type fuel, or could compression ignition based on jet fuel produce the power density and reliability required? I realize that the single largest factor in this area might be the simple availability (or lack thereof) of fuels, but I am talking about purely from an engineering perspective. Suppose we are talking about North America where both Avgas and Jet A are widely available, what would be the superior fuel for flying GA piston engines?

What other differences might there be that I haven't considered?

It would have a gearbox between the crankshaft and the propeller. That is certain.

Other very likely details would be:

overhead camshafts
four valves per cylinder
water cooling at least for the cylinder heads
electronic ignition
single lever control

This would enable it to run constantly at about 4000 RPM and create the same power from a smaller displacement. The consequence would be a more compact and lighter core engine. Compared to a car engine, the cooling system would be more capable, so the weight would still be higher than that of a car engine of the same nominal shaft horsepower. But it should be lighter and allow for a more aerodynamic cowling than the current piston aero engines.

The Porsche flight motor (PFM 3200) was like that, and for a reason. Yes, it did not outperform the older engines in terms of mass, but it was noticeably less noisy. Starting from a dated six-cylinder boxer design, the engineers could not fully optimize the engine for its intended purpose.

Once a gearbox is fitted, it makes sense to reduce propeller speed, so the diameter  of the propeller fitted to that engine would be a bit larger than that of a comparable propeller on a conventional engine.

Once Jet-A is widely available at GA airfields, a diesel version would also be very likely. Note, however, that the system weight of a piston engine (engine + accessories + fuel system + fuel) will be higher for a diesel on short ranges.


How does this IMU work and how to convert its output into meaningful information? To understand more about IMUs and IRS, I have chosen the Honeywell 6DF-1N6-C2-HWL Inertial Measurement Unit as an example:

I don't see any mention of the output of the IMU in the linked datasheet - have I got it wrong? And how to convert the output into meaningful data? How do I convert raw accelerometer data into Speed in knots, for example? Is there a specific formula to calculate the speed (velocity might be apt) while also taking into account the movements in other directions as well?
Is the raw data from this specific IMU accurate enough for use in a GA aircraft / Light Sport Aircraft?
Am I right in saying that this IMU doesn't need Pitot / Static sources?

While most details might be about this specific IMU, please feel free to also give answers that apply to all IMUs as a whole.

EDIT: I do not want any positional data. I prefer an AHRS with Airspeed and Altitude sensing capabilities.

Regarding the Specified IMU

The referenced document describes the output protocol.

  SAEJ1939 CAN 29 bit identifier communication output — the standard for the transportation segment — allows more data to be transmitted than an RS-485 output

You would need to use SAE J1939 to decode the actual meaning of the output data. It is possible that Honeywell could provide you more information as part of purchasing the product.

To use this to develop an AHRS solution you would still need pitot & static inputs to derive pressure altitude and airspeed, as you won't be able to get those from the provided sensors.

  acceleration range up to ±6 g, rotation rates up to ±75 deg/s,inclination angles up to ±50 deg and a sensor update rate from 1 Hz to 100 Hz

I think the biggest hurdle you have here is that it will only measure inclination angles up to 50 deg. Aircraft can find themselves in > 50 deg pitch attitudes, which won't be registered by the IMU.

This particular product (Honeywell 6DF-1N6-C2-HWL) seems to be designed for ground vehicle applications rather than aviation.


Why might a passenger jet fly at altitudes in a strong headwind when the winds are more favorable at a different altitude? I was a passenger on an east to west flight recently. During the flight I was playing with Garmin Pilot's new "Flight profile" feature that nicely shows the wind speeds at various altitudes, and I noticed that we were flying against an 80+ knot headwind at FL320, but if we'd dropped to FL220 we'd shave about 40 knots of headwind. That seems pretty significant. A 1,000 NM flight with 355 knot ground speed (which we were cruising at) would save 17 minutes of cruise time with 40 knots less headwind. That's a 10% increase in efficiency.

So the question is, did we stay (or even plan in the first place) to fly at FL 320 the entire cruise portion of the flight simply due to ATC needing to keep us there for traffic routing purposes,
Is there an even larger increase in fuel efficiency between FL320 and FL220 than 10%,
or is the difference negligible enough that it's not worth asking for a different altitude from ATC,
or some other factor not considered here?

You assessment is flawed in that it does not take different air density in account.  You state the following conditions:

FL320
80 kt headwind
355 kt groundspeed
1000 nm distance

and you are proposing descending to the following conditions:

FL220
40 kt headwind

At 32,000 ft, standard conditions are around 275 hPa, 225 K and density of 0.43 kg m$^{-3}$.  If your 80 kt wind is a pure headwind, a 355 kt groundspeed translates into a 435 kt TAS. If we correct this TAS for the ambient conditions we get an indicated airspeed of about 255 KIAS.  This is the airspeed that really matters because it is what the airplane experiences as it interacts with the atmosphere.

If we descend to FL220, where standard conditions are 430 hPa, 244 K and 0.61 m$^{-3}$, an airspeed of 255 KIAS yields a TAS of 363 kt TAS.  If we have a headwind of 40 its, this yields a groundspeed of 323 kt.  This is about 30 kt slower than you were flying at FL320.  A flight of 1000 nm will take about 15 minutes longer to complete.

However, it is most likely possible that the airplane you were in is capable of flying faster than 255 KIAS at FL220 and in fact is probably capable of somewhere around 310 KIAS.  Flying at 310 KIAS at FL220 will have true airspeed or 440 KTAS and with the 40 kt headwind a ground speed of 400 kt. This will get you to your destination faster, but at the expense of a much higher fuel burn.

It is also likely that you could have been flying faster at FL320 (not sure what max mach number would have been for you), but also at the expense of burning more fuel.  If the airplane wasn't dispatched for the additional fuel burn to go faster at FL220, then after descending you would probably not have enough fuel to go fast enough to make up the difference in true airspeeds.


Is there a required check before selling an airliner? In France, when selling a car, the last check (more or less a C-check) must has been done within the 6 months before the owner change.

Is there an equivalent requirement for airliners? I'm more interested in ICAO's rules.

I'm thinking of budget airlines that tend to sell their aircrafts while still young (less than 10 years) and thus having been subjected to at most one D-check.

I'm not aware of any such time limit or window requirement related to maintenance programs that apply to the legality of sale transfers, and couldn't find anything myself, so I'll proceed based on the assumption the answer is "no" but would be very pleased if someone who knows can correct the record.

Airliners or aircraft in general are not required to be sold "mechanically fit" like a, car, as a condition of sale, or fit with the last x months or years.  Regardless of who owns it or when it's transferred, it's airworthy and can be flown, or it's not, and can't (except for temporary permit to fly when you want to ferry an airplane from point A to point B, which you would do if you bought an un-airworthy but mechanically fit aircraft and wanted to get it to your own base).

So if an airliner is sold with a D coming up, there's nothing stopping the transfer from one owner to the other, even if it's at its D check interval limit and is grounded until the check is performed.  The new owner can purchase it and apply for a ferry permit, or a short term extension to the D interval (there is usually some latitude allowed for special situations).

If I was buying an airliner that was only a thousand cycles short of a D, well that is maybe 6 months or 8 months of flying or whatever, and I might specify that the seller complete a D check as a condition of sale (as well as incorporation of all outstanding Service Bulletins), or, I allow for the risks in taking it with the unknowns of a major structural inspection coming up with this risk factored into the price (this can be evaluated by analysing how the airplanes of that type are holding up at D check across the fleet - some airplanes already have pretty onerous structural patching requirements by then, imposed by surprise results during fatigue testing).


Is there any reason to use only UTC standard time in aircraft and not GMT? Why are we not using the GMT time standard in an aircraft? Why is only UTC the standard on-board?

UTC is in principle the same as GMT.

But for accurate applications there is a difference:

  Saying "GMT" often implies either UTC or UT1 when used within informal or casual contexts. In technical contexts, usage of "GMT" is avoided; the unambiguous terminology "UTC" or "UT1" is preferred. Wikipedia

UTC: Is defined by atomic clock with corrections for leap seconds added manually.
UT1: Is defined by the Earth's rotation, hence is more susceptible for change and less easy to calculate than an atomic clock.

However the difference between the two is less than one second. It might matter for your $8000 \frac{\text{m}}{\text{s}}$ satellite or for your $300 \frac{\text{m}}{\text{s}}$ aircraft, but otherwise your can consider them equal for informal applications.


What is the FAA's source for the type designators they use in IACRA? When using the FAA's IACRA system, an online portal used for creating applications for the issuance of a rating, you often have to choose an aircraft type to meet qualifications or to set which aircraft you'll be using during a practical test. When selecting aircraft in IACRA the type designators are different from what I'm used to. For example, a Cessna 172 K model's type designator in my experience is C172K, but in IACRA it's CE-172K. The Beechcraft Sierra would normally be B24R, but it's something like BE-23-BE24R in IACRA.

Where is the FAA coming up with these type designators they're using in IACRA?

Try this.

This is the FAA's Type Certification Data Base, which lists all type certificates issued by the FAA.

Note:  this only covers certified aircraft and not experimental or homebuilts.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/Frameset?OpenPage


Why do some fighter jets have movable horizontal stabilizer instead of elevators installed on the stabilizers? I have noticed that the horizontal stabilizers on some fighter jets move up and down to increase and decrease the angle of attack, but on some airliners they use elevators to adjust the angle of attack. Why isn't this kind of system used on airliners?

All parts of an airliner's horizontal surface move, not just the rear part. The rear part, called an elevator, can move much faster and is for maneuvering. The forward part, called a (trimmable) stabilizer, is for trimming and moves slowly. It is moved in response to changes in loading, speed or flap settings and positions the tail surface such that only small continuous elevator deflections are needed. It doesn't need to move fast - high load factors would upset the passengers and overload the structure.

A330 port tailplane root (picture source). Note the markings which show the range of incidence angles covered by the trimmable stabilizer.

Benefits of a stabilizer-elevator configuration:

Camber: The elevator deflection changes the camber of the airfoil of the tail surface and makes the production of the intended lift change more efficient. If the elevator deflection is supposed to create a downforce, negative camber is produced and vice versa. This reduces the drag which is created in order to maneuver the aircraft.
Lower control forces: By moving a smaller surface, less hydraulics power, and on small aircraft, less muscle power is needed for the same moment change than with a full-flying tail or all-moving surface. Makes sense, right?
Better tailoring of the hinge moment derivatives: Two effects are important for getting the control forces right: The change in hinge moment over deflection angle ($c_{r\eta}$), and the change in hinge moment over angle of attack ($c_{r\alpha}$). With tabs, the right elevator hinge position, nose shape and control horns, both can be tailored individually, while a full-flying tail or all-moving surface will give the designer less freedom to manipulate both independently. Note that full-flying tails only emerged with hydraulic controls, because only those systems can manage the control forces of all-moving control surfaces at high speed.

Horn balance and overhang balance on control surfaces (picture source)

Benefits of a full-flying tail surface

By avoiding a contour break due to a flap deflection, all-moving control surfaces can avoid the shocks which would otherwise occur at high subsonic speed. This is their main benefit for combat aircraft.
At supersonic speed, the uncambered surface will produce less drag, so for supersonic speed the all-moving control surface is more efficient.
By moving all of the tail surface at high speed, an all-moving control surface will produce the highest rate of moment change over time possible. To produce also the highest sustained moment change, however, it needs to be larger than a comparable stabilizer-elevator combination because it forfeits the benefits of variable camber.
The all-moving control surface has no gaps which could add radar reflections, allowing for a more stealthy design.
Lower mechanical complexity. This is somewhat offset by the need for much beefier hydraulics.


Does Lift/Drag generally improve at higher Reynolds numbers? I have been looking at the lift/drag polars for various airfoil profiles.  The L/D ratio seems to improve at higher Reynolds numbers. Obviously different wings are optimized for different speeds, and there are many other factor to consider, but in general terms do wings get more efficient at higher speeds?

I believe a 747 is more efficient than a model airplane because of the higher Reynolds number it operates at. Is this correct?

Yes, the boundary layer that surrounds the airfoil is known to decrease its thickness as the Reynolds number increases; its relative thickness to the characteristic length (the cord for example) scales as $Re^{-1/2}$ in steady flow, and the polar curve becomes more horizontal when Re increases. Meanwhile, lift coefficient increases just a little bit (check this post).

In result, lift-to-drag ratio increases with Re.


Why did the turbojet replace the piston engine? Many people say that this was because aircraft powered by turbojet engines were faster during WWII. However the Grumman F7F-4N Tigercat, a fighter powered by a piston engine, flew at a maximum speed of 460mph and the Gloster E28/39 (first British jet engine aircraft) flew at a maximum speed of 466mph, which is not that great a difference.

Others say it's because aircraft powered by piston engines cannot fly as high as jet engine aircraft, but what is the reason for this?

Why did jet engine aircraft replace piston engine aircraft?

Two more reasons the gas turbine supplanted the piston engine for aircraft use:

Power output. Aircraft piston engines have a practical limit on how much power they can put out, before becoming inefficient. This worked out to be around 3000hp. Two of the largest and most powerful piston aircraft engines that were also reliable enough for aircraft use are the Napier Sabre and P&W R4360, at around 3000hp, developed at the end of WW2. Yes, more power has been obtained from piston engines on land based vehicles, but they're either racing engines whose reliability is too low for aircraft use, or they're far too heavy for aircraft use, such as the 20k+ HP diesel engines that power cruise ships.

Gas turbines don't have those limits, one contributing factor being the lack of a reciprocating action. With a piston engine, each full stroke results in the piston accelerating to top speed, then coming to an abrupt halt and accelerating in the opposite direction... twice. Gas turbines that just spin can be made substantially larger while maintaining the reliability and efficiency that aircraft use demands.

Also, gas turbines tend to have a very high power to weight ratio, making them ideal for large aircraft use, where weight is very important.

Equating HP to thrust isn't simple as HP is raw power, while thrust includes altitude, velocity, and propeller/fan efficiency.

A simplified example was published on Aerospaceweb:

  Luckily, we do have access to data from a NASA report that does
  provide all the data we need to illustrate a sample case. The data is
  provided for a Boeing 747-200 cruising at Mach 0.9 at 40,000 ft
  (12,190 m). In this example, the aircraft's engines produce 55,145 lb
  (245,295 N) of thrust, only a quarter of its rated static thrust, to
  cruise at a velocity of 871 ft/s (265 m/s). Using the equations
  provided above, we calculate the power generated by the 747 to be
  87,325 hp (65,100 kW).

Using that very simplified example, a GE90 producing 115,000 pounds of thrust would be putting out the equivalent of around 160,000 horsepower.

Also, the maintenance requirements on gas turbines are substantially lower, especially for the high output engines. For example, the very large RR Trent series turbofans have a TBO (Time Between Overhaul) of around 15,000 hours. On the large piston engines such as the R4360, the TBO was more like 1500 hours, and the very large piston engines, especially the radial engines, had a prodigious appetite for engine oil. Plus intermediate maintenance on piston engines that gas turbines don't need, like changing spark plugs, that had to be done frequently. The Convair B36, which had six R4360's, required 336 spark plugs. Not something you could do in your driveway in an hour.

Some of the gas turbine's reliability comes from it's lack of vibration. Large piston engines, with their reciprocating action, tend to vibrate a lot, which reduces the life of engine and auxiliary components, like fuel pumps and spark plug wires.

Thus, not only did the turbojet, turboshaft, and turbofan make possible aircraft that wouldn't be practical at all with piston engines, such as large airliners flying at 30k+ feet, they also lowered the cost of maintenance and the frequency of maintenance substantially.

There is one area where the piston engine for aircraft use is still the better solution, and that is when the engine gets small, below around 500hp. Gas turbines don't scale down all that well. The small ones are not fuel efficient, nor does the cost get substantially lower.

As an example of this, consider one of my pet daydreams - a single seat Mosquito helicopter. Aside from the ultralight version, two versions with larger engines (and requiring a FAA helicopter license) are made, the XE285, with an 85hp snowmobile engine, and the XET with a 90hp gas turbine engine derived from a backup power generator. The piston engine sells for maybe 2k USD and burns around 5gph, while the gas turbine sells for 10k and burns 8.5 gph.


"The pilot must know the center of gravity and gross weight to fly" -- True or false? I've been trained to know the calculated gross weight and center of gravity for takeoff and landing for every flight for which I have acted as pilot in command.

I've had other instructors dispute that this is not a requirement and simply being familiar enough with the aircraft to reasonably be able to estimate that you're within C.G. tolerances considering the fuel and passengers is satisfactory.

Are there any regulations that explicitly define the requirements of pre-flight weight and balance familiarization for civil airplanes requiring an AFM containing takeoff and landing distance data?

If not, surely there is some catch-all so lawyers can sue us easier; what would that most likely be based on case law? 14 CFR 91.13, the "all available information concerning that flight" clause from 14 CFR 91.103, or something else?

Note that 14 CFR 91.103(b) requires familiarization with runway lengths of intended use and takeoff and landing distance data for any flight; myself and others would argue that being able to meet that condition would require, at a minimum, familiarization with certain weather metrics and knowing the airplane's loaded gross weight.

However, I am still not seeing an explicit requirement to know a specific C.G. figure.

True. It is the pilot-in-command's responsibility to determine if the aircraft is safe for flight. 14 CFR § 91.3(a) makes the PIC directly responsible for that.

The 2016 FAA Weight and Balance Handbook also states:

4. The pilot in command (PIC) has the responsibility
prior to every flight to know the maximum allowable
weight of the aircraft and its CG limits. This allows
the pilot to determine during the preflight inspection
that the aircraft is loaded so that the CG is within the
allowable limits


Are GA airports typically accessible 24/7? Is access to GA airports 24/7?

I always thought it was.   Every airport I've visited had gate-codes to open gates after hours.

But I recently started planning a flight to Salina, KS (KSLN), and the local FBO told me field access shuts down entirely at 10p.m.

As a transient flight, I'd have no way back to my plane after that.  I'm skeptical, as I believed airport access was 24/7.

Am I just completely mistaken in that belief?

The general answer is that the airport manager is in charge of ground access (unless the TSA is involved; see below) and individual FBOs can open or not whenever they like. There are no standard federal rules for the same reason that there aren't any for when gas stations should open and close: they're private (or state-owned) businesses.

So before going somewhere new it's always good to call and ask about opening hours, access etc. If there are multiple FBOs at the field, you can call them all: one might be open 24 hours a day even if another one isn't.

One specific point about Salina is that the TSA may be limiting access. The AOPA airport directory says that KSLN is under security directive SD-8G, i.e. the TSA limits access to the apron. That means you might need an authorized escort - most likely someone from the FBO - to and from your aircraft. If the FBO shuts at night there may be no one there, but FBOs in general are usually good at making special arrangements if needed.

And although your question seems to be about ground access, I guess it's worth stating the obvious: airports are accessible 24x7 from the air. Nothing stops you landing at 3am, filling up at the self-service pumps and then taking off again. But watch out for local practices like voluntary noise abatement curfews, e.g. at KPDK:

  Pilots are encouraged not to fly between the hours of 11 p.m. and 6
  a.m. Due to FAA Regulations, PDK cannot implement a mandatory curfew
  at this time.

It's still legal to land or depart there during curfew hours, but you'll really annoy the airport management, FBOs, and local pilots who have to deal with the complaints from local residents.


What plane is shown here in the movie Rampage? I'm not sure if its a real life airplane, but I feel like I have seen it somewhere:

It looks similiar to the F-117 Nighthawk, pictured below, but it is definitely not this:

Northrup Grumman B2 Spirit stealth bomber.