Is fuel balancing done automatically? Is the balacing of the fuel in the wings of an aircraft like A320 done automatically or does it have to be done manually by the pilot. If it is done manually does the pilot eyeball the balancing ?

The A320 has an auto mode that will do the following:

Center tank will empty first.
The inner wing tanks will empty until they reach 750 kg in each.
The outer tank transfer valves will open and remain open allowing fuel to flow to the inner tanks.
The center tank pump will stop 5 minutes after low level is reached and remain stopped until the inner wing tank reaches the underfill sensor (approx. 500 kg) at which point it will restart.

The center tank pumps can be switched to manual mode.
The wing tanks can be turned off manually.
The cross feed valve is also operated manually.

source: A320 FCOM


Turbulent flows over 6 series airfoils 6 Series Airfoils were developed to enhance the laminar flow region over the airfoil. But, the Reynolds number at which planes fly are high for the flow to be Laminar. Then what's point of introducing 6 series airfoils when laminar flow is not possible?

Flow at the forward stagnation point of an airfoil is laminar, because the local velocity will be zero. The boundary layer originating form the stagnation point will usually not remain laminar for long, but some airfoils like the NACA 6 series were optimized to conserve this laminar flow as far down the chord as possible.

A laminar boundary layer is thinner and offers less drag, so it is desirable for aerodynamic efficiency. One of the more famous laminar wing designs was the P-51 Mustang, which used the NACA 66-(1.8)12 (a=0.6) airfoil at the wing roots.

In reality the boundary layer will transition at some point before reaching the trailing edge, and become turbulent. In the case of the P-51, a shot down example was analyzed by H. Schlichting. He found that the transition point was at the 50% chord point for $Re=4\cdot10^6$ and as far forward as 20% of the chord for $Re=7.5\cdot10^6$, which suggests that the Mustangs performance had more to do with the polished finish and flush rivets of the wings than with the airfoil used.


Why is vortex state impossible in autorotation? An article at Skybrary states that for a vortex ring state to occur:

  The aircraft has to be in powered flight. If the engines are not producing power, the aircraft is in autorotation and the upflow of air (rather than engine power) is being used to drive the rotor motion. It is not possible to enter the vortex ring state whilst the helicopter is in autorotation. [emphasis mine]

The FAA Rotorcraft Flying Handbook.pdf lists autorotation as a method of getting out of VRS.

  In a fully developed vortex ring state, the only recovery may be to enter autorotation to break the vortex ring state.

Why is a VRS impossible during autorotation?

OK -- first, a quick review of what vortex ring state is.  I'll be brief, but there's more detail over here: What is "settling with power" and how does it differ from vortex ring state?

Vortex ring state (VRS) occurs when the rotor's shed vortices begin to recirculate into the rotor system, effectively stalling out the rotor and significantly decreasing the total thrust produced.  It is one of 4 working states of the rotor, 3 of which are described in the picture below (note: descent rates are approximate.  What really matters is the ratio of the velocity induced by the rotor to the helicopter's descent speed).  The propeller working state is the normal working state of a helicopter.  VRS is the second state shown, where descent rates are relatively low (and forward flight speed is typically very low/zero), and the windmill-brake state is what you'd see in autorotation.

In the following plot (from Leishman's Principles of Helicopter Aerodynamics), these working states are superimposed on a graph of climb velocity and induced velocity at the rotor disk.  In short, VRS occurs when you're not descending fast enough to let the downwash be blown upward and you're still trying to produce thrust.  One example would be a helicopter attempting to hover above its hover ceiling (so, even at max power, it cannot maintain altitude and has a small, negative climb velocity ratio if referenced to the below plot).  As a result, it begins to descend at an increasing speed (and, by corollary, it is falling "into" its own downwash as its downward speed increases).

So why is autorotation a way out?  Autorotation is what happens when, in a sense, you stop trying to hold yourself up.  In autorotation, the thrust produced by the rotor is minimal (depending on how you do your bookkeeping of forces acting on the aircraft) during descent.  As a consequence, the aircraft is descending so quickly that the rotor wake is actually above the helicopter!  Also, while the rotor is producing some lift, and hence some downwash, the induced velocity of the downwash in the vertical direction is very small compared to the velocity of the relative wind going through the rotor disc (in the opposite direction of the downwash).

Putting this all together in different words: vortex ring state is sustained by the pilot trying to keep pouring power into the rotor system while descending into his or her own downwash (without changing forward flight speed or tail rotor thrust, as mentioned in this neat article: http://www.rotorandwing.com/2015/09/01/flying-through-the-vortex/).  He or she is producing downwash by producing thrust, and, as the descent speeds more or less balance with the downwash velocity, the same aerodynamic forces that produce the thrust end up recirculating the downwash through the rotor.

To go into autorotation, the pilot drops collective and throttle to zero and lets the aircraft fall (more or less -- it's a little more nuanced, but that's a separate question), which, in turn, drops the induced velocity of the downwash such that it is much smaller than the vertical speed of the aircraft, allowing the vortices move with the relative wind away from the rotor (upwards) as the rotor enters the windmill-brake state.  Since there is no mechanism to hold the vortices close to the rotor (or produce them in the first place), the vortices cannot recirculate and induce VRS.


Why did this 737 not take off on first attempt? This video shows a 737 'near miss' takeoff. I'm curious as to why the aircraft did not take off as expected, i.e. is there anything obvious from the video?

I'm assuming the speed and payload would have been within the expected limits, so wondering if someone can explain what happened.

With the help of the clearly visible livery (Royal Air Maroc), registration (CN-RNV) and approximate time (before August 2016, probably not by too much), I managed to find the specific Incident: Royal Maroc B737 at Frankfurt on Jul 23rd 2016, three takeoffs for the price of one on The Aviation Herald. It even links to the very same video that you do.

Unfortunately:

  On Aug 30th 2016 the BFU responded to an inquiry by The Aviation Herald of Aug 25th 2016 stating, the BFU had neither received any notification by the crew, airline or airport involved nor by the person taking the video (see below) and became aware of the occurrence only through the release of the video into the public more than a months after the occurrence. The BFU argued that as result it will not be possible to establish sufficient facts and evidence needed for a detailed investigation, hence the BFU decided to refrain from initiating an investigation.

So we won't have anything better than guesses anyway.

The article does speculate that the wake turbulence of the landing aircraft crossing above the runway just before might have affected it, but it was too close to the starting point and is unlikely to have this effect, so what remains is that the pilot flying started to rotate too early. There may be basically two reasons for that:

The pilot flying started rotating before appropriate speed was reached, or
they calculated lower rotation speed (Vr) than they actually needed.

The take-off procedure with the speed call-outs made by the pilot not flying is the same for every take-off, so the second reason seems more likely.

The rotation speed depends on weight, density altitude and flap setting. Inserting wrong weight is probably most common reason for this type of incident, but calculating for higher flap setting than they used is a believable option here too as they used rather low setting for the actual take-off.

It should be noted that the pilot realised their mistake and handled it correctly by lowering the nose back to let the aircraft gain enough speed first.


Can ATC provide VFR flight following to a VFR aircraft in class E airspace? Just wondering if ATC can do this or not. I'd also appreciate input on whether it's useful or a good idea, and how it looks in the real world.

They can. I often pick it up here in the Northeast since the airspace is fairly busy. Generally speaking I was taught that flight following is always a good idea. It is a useful tool to you in the cockpit and can provide advisories of fast moving hard to see aircraft as well as those that may be approaching to overtake you. Generally it helps to improve your radio skills and may reduce your work load ultimately.

As for who to contact, that can change. I had this very question during my training (asked here) and you can find the answer here. If you are departing from a controlled field you can sometimes ask the tower to set it up which they may or may not do. The Airport Facility Directory will have an "approach" and/or "departure" frequency listed for most fields which is generally who you call. If you are in the wrong area they generally send you over to the right controller.


Why do small aircraft rentals charge per flight hour? Why do airplane rental companies charge per flight hour (using hobbs time), instead of either MX hours (tach time) or based on the amount of fuel used?

Wouldn't a 3-hour long cross-country flight be much cheaper than three hours spent in pattern work?

In my opinion, rental rates would seem more logical if they are based on MX time.

In the US most General Aviation rentals are "Wet Hobbs Time" -- You are billed based on the actual clock time the engine was running (in tenths of an hour), and fuel/oil are included in the rental price.

This is attractive to renters because of the simplicity: You don't have to pay the FBO separately for the fuel, oil, and aircraft, and you're basically paying for the amount of "clock time" you spent in the plane.
It's easy to understand, especially for new students who may not quite grasp the concept of how the recording tachometer measures "time".

Hobbs billing is attractive to the FBO as well, for different reasons:

Rentals are typically blocked out by clock hours ("10 AM to 2 PM").
Since booking is clock time billing based on clock time (Hobbs hours) makes sense.
In practice the two numbers are usually close (and when they're not the error tends to favor the FBO).

Let's look at your "Tach vs. Hobbs" scenarios (for the sake of simplicity let's assume both flights are three "clock hours" chock-to-chock):

Pattern Work

Pattern work consists of three basic phases:

Takeoff (full throttle, high RPM - Tach runs faster than Hobbs)
Circuit (low power, low RPM - Tach runs slower than Hobbs)
Descent & Landing (near idle power, very low RPM - Tach runs MUCH slower than Hobbs)
There might be a taxi in there for full-stop practice, but you'll be close to idle there too.

If we average this out over three hours of pattern work (and assuming you and your instructor don't commit suicide to break the monotony) the Tach hour meter will be reading a number much lower than the Hobbs meter.
In this scenario you might have 2.25 tach hours for your 3 Hobbs / Clock hours.

3-hour Cross Country

A cross country consists of 5 phases:

Takeoff/Climb (full throttle, high RPM - Tach runs faster than Hobbs)
Cruise (moderate throttle, moderate RPM - Tach and Hobbs run about the same speed)
Descent (moderate-low throttle, moderate-low RPM - Tach may be a bit slower than Hobbs)
Circuit/Approach (low power, low RPM - Tach runs slower than Hobbs)
Landing (near idle power, very low RPM - Tach runs MUCH slower than Hobbs)

If we assume Phases 1 and 3 take about the same time they roughly cancel each other out. Phase 2 (Cruise) is a wash too, so we're left with two brief periods where the tach is running noticeably slower than the Hobbs (the pattern and landing at the destination).
In this scenario you might have 2.85 Tach Hours for your 3 Hobbs / Clock hours.

  In both of these scenarios the FBO comes out ahead of the game -- they're billing you for 3 hours of time, but their maintenance cycle is based on the tach. This might be enough to squeeze another day or two of profitable flying out of an airframe before it's next annual or 100-hour inspection.

There are scenarios where you can "beat" the Tach clock, but most of them involve being downright abusive to the aircraft/engine. For example if you flew that 3 hour cross country at low altitude and full throttle (very high RPM) you might rack up 3.3 tach hours for your 3 Hobbs hours. The FBO is basically stuck eating that maintenance expense.
Along similar lines when renter pilots abuse an engine (for example by leaving the mixture knob full rich for the whole flight) the tach time doesn't always tell the whole maintenance story - the mechanic's time cleaning the plugs because the next renter couldn't get through a runup has to be paid for somehow.


How to identify load paths? How would you go on about identifying the load paths on an empennage structure, wing and fuselage?

The simplistic explanation is that you design the aircraft shape, run computational fluid dynamics (CFD) simulations to get loads, then run finite element analysis (FEA) on the underlying structure to size the components.

CFD might look like:

FEA might look like:

Then when you have finished this sequential process you do it over again because after resizing the components the weight of the structure changes, so you have to change wing area to match the weight change, which makes the loads change, which then requires another component resizing.

You might wonder if it wouldn't be better if all these objectives were solved at the same time, and you'd be right. A newer design process is called multidisciplinary design analysis and optimization (MDAO) and is just becoming possible with modern, high power computing systems.

MDAO might look like:

where all the red buttons are control points that the optimizer uses to refine shape. You have to give the optimizer rules on how to proceed, which might look like:

If you want to read about MDAO one good source is Univ Michigan at

http://mdolab.engin.umich.edu/

Read a couple of their published papers and you'll get a pretty good sense of the complexity. MDO is the same thing as MDAO.

If you are interested in the optimizer, OpenMDAO is open source software. It is not aviation specific, though it was developed in conjunction with NASA. It is at:

http://openmdao.org/


Help identifying airfield and aircraft from movie "My Spy" Here's a couple of stills from the movie "My Spy" - the climactic scene takes place at a single-strip airfield with a paved or tarmac 09 runway.

My tentative identification of the (clearly unflyable) military planes in the background, from left to right: an Avro CF-100, a F-86 Sabre, then a two seat F-104, and next (judging from the tail) another single seat F-104 in unusual light green camouflage.

The red and white plane in the background, from a later shot, is an F-5 in unusual livery.

The second picture points to the F-86 being RCAF, which would point to a Canadian airfield. On the other hand, the furthest F-104 looks very much to be in Belgian colours.

There's not much more to go on! I wondering where this is, and how these planes ended up there.

"My Spy" 2020 Amazon Films / STXfilms

The film was shot in and around Toronto, so a search of local airfields on Google Maps led me to Markham Airport. From there, Wikipedia provided this list of military aircraft on the field:

CT-128 Expeditor transport

CT-133 Silver Star

CF-5 Freedom Fighter

CF-104 Starfighter (Various Airforces)

Canadair CL-13 Sabre

Schweizer SGS 2-33A Royal Canadian Air Cadets[citation needed]

Noorduyn Norseman - shell only

The aircraft belong to the Canadian Air, Land and Sea Museum. Looking at the satellite image on Maps it would appear that a number of other aircraft have been stored there, but have now been removed.

There's a clutch of rather more complete airframes in amongst the buildings in the south west corner of the airfield.

Google Maps


Why does an aircraft have to be flying straight and level to facilitate a jump? In the other questions about parachutes on this site, it has been stated that the aircraft would have to be flying straight and level to facilitate a jump. However, there were quite a few pilots during WWII whose planes were not flying straight and level and who still managed to escape.

Is there a difference between then and now that I am not catching?

One of the reasons why many WW2 pilots did not make it out were the G-forces and acceleration in directions other than the door when they had to get out. The bailing out survival rate appears to be correlated to the ease of exit.

If you're going to have a smooth and safe departure, you're happiest doing it during level flight when there's no interference from plane motion.

(source: lonesentry.com)


How can you do preflight fan inspection when it's windy? While sitting, bored to tears, intermittently scanning the 'Delayed' display next to my flight, I've looked out of the airport windows.

Out on the stand, amongst the baggage and fuel truck activity, I've seen a technician examining the engine fans with a torch (flashlight), presumably part of a mandatory pre-flight inspection procedure, looking for FOD, cracks etc.

It occurred to me: What does the techie do on a windy day when the fans are spinning too fast to examine by eye?  Is there some way of braking the shaft to stop the windmilling?

(YouTube) Reversers would slow down or stop the windmilling by blocking the majority of the bypass.

If repositioning the aircraft abeam the wind is not possible, then deploying the reversers does the trick. It's the same method used when starting a jet engine in strong tailwind.

Windmilling can be very fast. The clacking sound you hear is normal.

Jetliners don't have shaft brakes, but some turboprops do.


What kind of liquid can be seen 'leaking' from the upper surface of the wing of a Boeing 737-800? Can anyone advise what the liquid seen on the wing of a 737-800 on approach as highlighted on the attached images is? The liquid was seen "pulsating" from the bases marked by the arrows. As the plane came to land, the intensity of the flow decreased until it eventually stopped altogether.

The pictures show water condensation on the wing upper surface due to very cold fuel in main tank number 2.

The fuel is cold because the air temperature at cruise altitude is significantly colder than the air temperature encountered during the descent.

The condensation appears to be oozing from the wing but it is in fact flowing slowly over the surface, filling and overflowing every seam and imperfection.

Since you have provided photos and not a movie, I can't tell you what the "pulsating" really was, but it probably was either a slight boundary layer turbulence, or changes in the seam geometry due to wing flexing.

EDIT: There seems to be some confusion regarding the "pulsating" nature of the observed flow, with at least one correspondent1 insisting that the uneven flow proves that the fluid originates from inside the wing.

There are many mechanisms that could account for a pulsing or uneven flow of fluid over the wing, without resorting to the theory of a bleeding airplane.

The most likely reason depends on the fact that the wing surface is not a single sheet of metal, but contains discontinuous pieces such as spoilers, flaps, access panels, ailerons, etc. Because the wing flexes, the volumes of the spaces between these pieces, and the voids under some of them, continually change in flight.

Water flowing over the wing will fill some of these spaces and voids. Once a void is full, changes in volume will alternately squeeze out some water and suck some in. This could certainly look like a pulsating leak to someone who has not seen it before.

Another effect that can lead to pulsating does not require changing volume, but just a trough formed by the butting of two panels, roughly perpendicular to the airflow, filled by the trickling condensation. The air does not always flow smoothly over the wing2, and minor turbulence can press down on one section of the trough, causing water to spurt out of another section.

Seconds later the turbulent flow shifts, and the water hurries along the seam to pulsate in a different place.

The question can be completely resolved only by someone who can consider these theories while observing the effect.

The original poster, actually.
In spite of the large arrows painted on the wing surface.


Can a plane with air-breathing engines in level flight achieve enough velocity to make a single orbit? A rocket uses engines that carry both fuel and oxidizer. Most are composed of multiple stages and lift a payload to orbit. They are able to burn their engines anywhere in or out of Earth's atmosphere since they carry both fuel and oxidizer. This is necessary to achieve orbital elevation and velocity.

Can a plane with air-breathing engines achieve a high enough speed in level flight such that it reaches orbital velocity? The idea being the plane could use the fuel in its tanks and the air in the atmosphere (at whatever altitude makes the most sense) to gain enough kinetic energy that the plane could then "climb" to an orbit. This would be similar to a zoom climb, but resulting in the plane entering a stable orbit. The plane could burn its engines until flameout as it ascended through the atmosphere. At that time it'd be dependent on kinetic energy alone. The engines, fuel tanks, and other equipment for air breathing flight can be jettisoned at this point if need be.

For the purposes of this discussion, orbit is defined as a trajectory having a minimum altitude of 100 kilometers. Once at this altitude, the vehicle would need to be able to make at least trip around the Earth without requiring additional delta-v from any onboard source. If the path taken was highly eccentric or even whatever it's irrelevant to this discussion as long as the minimum altitude is maintained. There is no requirement for re-entry at a specific location or even that re-entry is survivable by the craft.

The craft could use esoteric materials, fuel sources, engines, and even construction so long as those have been used in at least one prior sub-orbital flight.

If the plane needed a Reaction Control System to control its trajectory once it reaches high altitude this would be acceptable. These systems don't have enough thrust to actually send the plane into orbit.

No.

Currently the altitude record for a jet is 37,650m (This is a zoom and climb record, not sustained flight).

Ralph J sum up the issue pretty well, but let me add some data:

Ramjets are the kind of engines that looks the closest to what you want to achieve. They top out at 2km/s Orbital speed would be 7km/s. And that's with a minimal payload.

There is an inextricable issue with forgoing O2:

You need significant O2 to have thrust
You cannot get thrust if there is atmosphere (Drag+heating)

Lastly; to achieve an orbit, you need a circularisation burn, that can only be done outside of the atmosphere.

The lowest point of your orbit cannot be higher than the lowest burn performed.
Using only atmospheric O2; your orbit will be bound to reenter on achieving your first revolution.

(If you intend to achieve escape velocity however, this is not an issue anymore).


What aircraft model is on this old photograph? I would be grateful if anyone could identify this - Copyright Real Photographs Co. No. 750

It's a Carden-Baynes Bee... A British plane built in the 30s...

https://en.wikipedia.org/wiki/Carden-Baynes_Bee


Why are European low-cost carriers not running long haul flights to destinations like the USA, Asia, or the Middle East? Does this benefit them somehow, by only travelling within Europe?

Some are, Norwegian Air runs a flight from Stewart airport in New York to Dublin Ireland and back.

One the benefits that LCC's get is from buying only one airframe for their fleet, which can, depending on where they are based limit their range as many fly A320's or 737's.

The benefit is not within Europe per say but with short, fast turn around, flights that benefit from the general implementation of the hub and spoke system. There are also LCC's domestically here in the USA like Spirit and plenty in Asia as well.


Airliner - "barking dog" after take-off and before landing I've always wondered what is that noise. It sounds like a barking dog or an air drill.
Sometimes you hear 4-5 "shots" but last time I flew the noise went on from before take-off to long after we were in the air.

It looks like they are tightening something. Am I correct?

This sounds like the PTU, used to pressurize the hydraulic system when only one engine is running.

http://www.askthepilot.com/questionanswers/the-barking-dog-airbus-noises/


What is the difference between centripetal and centrifugal force? In a homework problem I was asked:  In a balanced banked turn an increase in angle of attack will :?

Answers are :
1/ reduce indused drag.
2/ increase centripetal force .
3/ increase centrifugal force .
4/ have no effect on the turn.

Obviously answer 1 and 4 are wrong but what about centripetal vs centrifugal force?  What is the difference between the two?

Two of the given answers - choices (2) and (3) - are correct in different reference systems. It depends on the reference system of the observer:

In a non-accelerated reference system, centrifugal forces do not exist. Increasing angle of attack increases the centripetal force, that increases the turn rate. The reaction force of the centripetal force is the innate force of the plane.
A passenger in the plane observes the scenario from within an accelerated reference system. The plane is turning without obvious reason. The passenger's body wants to move in a straight line and seems to exert a force on the seat. This virtual force is called centrifugal. The seat seems to react by supporting the body with the same amount of force. When the pilot increases the angle of attack of the balanced turn, the centrifugal force will increase. The observer inside the rotating system is not aware of the centripetal force. The centripetal force does not exist for him.

Centripetal and centrifugal forces never co-exist in the same reference system! Even though they might appear like counter forces to each other, they should not be understood like that.

Centrifugal force is a construct to explain our perception and sometimes to simplify calculations in rotating reference systems.

Between the two, centripetal force is the more fundamental notion. It really describes what is going on, therefore (2) is the better answer from a scientific point of view. Nevertheless, (3) centrifugal force is a valid answer if the reference to a rotating system is established. That seems to be the case in the question. The centrifugal force describes what an occupant feels who is not aware of the rotation.


Are noise cancelling headsets safe? Headsets such as the Lynx Micro System are modelled on ear defenders and physically attenuate the noisy cockpit environment by clamping hard onto your noggin. Most modern GA headsets (e.g. Bose A20) seem to be built with far less attenuation, don't clamp on as hard, and tend to rely on ANR features rather than isolating the ear.

Naively, it seems that this would pump more audio power into the ear. Granted, it will be in opposition to the original noise, but it seems it would be very tricky to get the cancellation exactly in phase. So, is there a risk that using ANR and less attenuation would not be as effective in preventing ear damage as passive attenuation?

Naively, it seems that this would pump more audio power into the ear. Granted, it will be in opposition to the original noise, but it seems it would be very tricky to get the cancellation exactly in phase.

If the headphones succeed in reducing the noise you hear, then they will also have succeeded in reducing the power delivered to your eardrum (at the relevant frequencies) -- because those are one and the same.

It is conceptually and mathematically convenient to imagine that the original noise and the canceling noise both reach your eardrums and interfere there -- but if you actually work out how energy moves in the combined situation, what actually happens is that the cancellation speaker sets things up such that much of the sound energy will get reflected away from your ear canal instead of entering it.

The core mathematical point is that the air displacements and minute pressure differences that make up the sound do indeed (to a good approximation) behave linearly -- so you can take a description of the noise and a description of the cancelling noise separately and simply add the displacements at the location of your eardrum.  But power goes as the square as the displacement, so it does not add linearly. You can't take the power transport of one sound along and add it to the power of another sound to get the power of the combination (except averaged over time if the sounds are not coherent in frequency, which is specifically not the case for noise-cancelling headphones).


Can you help me ID these two helicopters? Hey folks! Desperate to find what these two helicopters are. Any help greatly appreciated!!

A

My guess is an AW139 on the left and the AS355 on the right. Both are very widely produced models.


Off-center fan in geared turbofan - does this exist? Has anyone ever heard of a geared turbofan in which the axis of rotation of the fan is offset from (i.e. not the same as) the axis of rotation of the turbine? In all the current geared turbofans I've encountered, the input shaft from the turbine to the gearbox is co-axial with the output shaft to the fan.

There are no off-centered turbofan engines in service today. However, United Technologies Corporation (UTC), has patented an engine with an angled core (Pratt & Whitney is a part of UTC).

Image from US patent Gas turbine engine with separate core and propulsion unit
US 8789354 B2

Air from the fan is ducted around the side and back of the core to enter from the opposite direction. The hot gas will be discharged forward through a power (low-pressure) turbine connected to the fan via a gear system. The turbine, gearbox and fan will be connected through a short shaft.

Source: aviationweek.com

The design is expected to have some advantages due the unconventional layout adopted, like:

As the core and fan are separate, core can be easily removed for maintenance.
As the compressors (in cores) are at an angle to each other, the possibility that uncontained failure in one engine will damage another engine is reduced (when the engines are installed in the double bubble D8 configuration)


Are there any safety features to prevent reversible-pitch props from reversing in flight? Some aircraft have propellers capable of reversing pitch, which is also known as beta angle. The example I saw was an Aviat Husky.

(Youtube demonstration)

I want to know if there are any safety features that prevent this from activating while airborne. For example, what kind of controls activate it? How long does it take to actually reverse the blades, and how much physical effort does the pilot have to exert? Would it be impossible to force the reverse when at high cruising speed?

If it is a mechanically powered reverser, then is it possible for it to be inadvertently triggered?

And if it is activated, can it quickly be set back to original blade angle?

Note, I am interested in any propeller-driven aircraft, not just commercial ones, but smaller ones as well. Piston prop, turboprop, GA, anything.

Beta props are pretty rare on anything smaller than Twin Otter or King Air or Porter or Turbo Beaver.  Can't speak for MT's system, but most beta mode systems use mechanisms to switch control of propeller blade angle from the propeller rpm governor to the throttle.  Beta mode is the mode of throttle or power lever control of blade angle, with engine torque controlled by a governor system of some kind to regulate rpm.

Beta mode is very similar to turbine helicopter collective control, where a lever, the collective, directly changes rotor pitch and a governor system regulates rotor rpm by adjusting engine torque.

Beta mode is normally entered just above flight idle (flight beta) once the propeller speed governor is no longer regulating blade angle (blades at fine pitch stops).

There is normally some kind of detent or gate that requires a deliberate movement or unlatching to move below flight idle (the Twin Otter requires you to twist the throttle grips - on others there are finger triggers).

Also, some turboprops have beta lockout systems that prevent power lever movement below flight idle until you're on the ground.  But a lot of airplanes don't and can be put into ground beta in flight if the flight idle gate or latch is deliberately operated by the pilot.

During the Twin Otter program there was some controversy at DeHavilland Canada because demo pilots were using ground beta (coming into DISC - blades flat, like a disc) to make crazy steep descents (I've also heard of Porter pilots doing it).  A very dangerous thing to do, because if the prop doesn't want to come out of beta mode when you move the lever(s) back up, you are dead meat.


How to balance a propeller aircraft for left turning tendencies? (Source) Rudder twist on a Saab 91 Safir.

What are the design methods used to balance a propeller aircraft for left turning tendencies? I think it matters more the more powerful the engine(s) is.

A rudder twist seems to be one, not-so-detailed sources mention an offset engine, a slightly longer-, or differently balanced wing.

There are several ways, and the most effective vary their own yawing moment contribution with engine thrust:

A trim tab on the rudder which forces it into a correcting deflection. This trim tab can either be adjusted in flight or only on the ground. The flight-adjustable trim tab is set according to flight speed and is used on airplanes with a wide speed range.
Mounting the engine at a slight angle to the airplane centerline. This places the propeller at an offset which produces its own yawing moment.
Camber on the vertical tail. Since the tail is in the propeller slipstream, it experiences a relatively higher dynamic pressure at low speed and high engine settings, just when the p-factor is biggest.


Is it necessary to strengthen the airframe of an aircraft that is used for sustained low level flight? I was looking at the image of the A 10 and saw that its air frame looks far more stronger than the air frame of the F 15 or the F-16.

I realized that the A 10 is used for undertaking low-level flights for deep interdiction. Apart from the A 10, the French Rafale fighter is also used for low level flights.

So is it necessary to strengthen the air frame of such aircraft or are they built from scratch with a material that is different from the ones used to design aircraft like F 15 and F 16?

I've heard of designers strengthening high stress areas of air frames with fiberglass,Coffee filters etc. However, increased weight will translate into higher wing loading which means more landing and flying stresses.

EDIT:

Sustained low level flights are associated with more structural failures in fighter aircraft. This is why a number of Air Force the world over have mission specific aircraft. Basically what I need to know is whether the airframe  of a fighter aircraft that is generally used for low level flight(A 10, Rafale, Su 34) any different from the airframe of other fighters like F 15, F 16 or Typhoon?

To be sure, I got the answer from the posts below, but just wanted to clarify as this question is on hold.

Since the question you're ostensibly asking is "Do you need to strengthen the airframes of aircraft used for sustained low-level flight" let's leave aside the military aircraft in your question and just look at "sustained low-level flight" in general.
But first let's bear in mind two concepts:

The first concept is that all aircraft (indeed, all structures) have a finite fatigue life: through normal use, vibration, operating stresses from takeoffs/landings/maneuvers, etc. an aircraft accumulates a certain amount of fatigue, and at some point that fatigue will lead to failures (bending or cracking) in the aircraft's structures.
Aircraft are engineered to withstand a certain amount of fatigue (the "fatigue life" of the airframe) based on a number of factors, and when the fatigue life limit is reached the airframe is either retired (scrapped) or rebuilt/reinforced to extend the fatigue life.

The second concept is something in fluid dynamics called a boundary layer - the region of a fluid in contact with (and most influenced by) some surface.
For purposes of flight we can consider air (the atmosphere) to be a fluid in contact with the Earth's surface. This means down close to the Earth's surface there's a boundary layer with associated meteorological phenomena.
The first article in this NOAA piece talks about some of them, but for purposes of this discussion let's just reduce it to "It's really bumpy and gusty in the boundary layer. Once you climb above the boundary layer the air is smoother."

Bearing these two concepts in mind, an aircraft that spends most of its time operating down load (near the surface of the Earth) is also spending most of its time inside the boundary layer, where it gets bounced around and shaken up more by the turbulence in the boundary layer, and thus accumulates fatigue at an accelerated rate.
Because of this aircraft that spend a lot of time in the boundary layer are usually "overbuilt" to handle a greater degree of fatigue/stress, or have reduced service/fatigue life limits to ensure the aircraft is inspected and repaired/retired before fatigue can lead to a failure.

The accelerated fatigue from spending a lot of time in the boundary layer can also be dealt with by inspections rather than engineering changes. Piper Service Bulletin 886 highlights a particularly dramatic case:

  On March 30, 1987, a PA-28 engaged in pipeline patrol operations suffered an in-flight wing separation resulting in a fatal accident. Investigation revealed the wing failure was due to propagation of a fatigue crack, which originated in the wing lower main spar cap.

Piper then goes on to describe "Normal" and "Severe" usage of the PA-28 family of aircraft, broadly speaking Normal is flying "normally" (Take off, climb to an altitude generally above the boundary layer, fly to your destination, and land), while Severe usage involves spending a lot of time in the boundary layer ("below 1000 feet AGL"). They then recommend inspection intervals based on the type of usage the aircraft is seeing (around 60,000 hours for "normal" usage, and roughly half that for "severe" usage due to the accelerated fatigue accumulation).

TL ; DR

Most aircraft that spend most of their time at low altitudes (close to the ground) are "overbuilt" structurally because there is more turbulence down low, so they accumulate fatigue at an accelerated rate.
Plenty of "normal" aircraft also spend a lot of their life at low altitudes, and this is usually handled by inspecting them for signs of fatigue damage more frequently than their higher-altitude counterparts.


Was it ever possible to land big commercial jets from the missed approach point during non-precision approach? When you shoot a non-precision approach, you are expected to maintain the MDA until the missed approach point, and if you have the runway in sight before you get there, you may proceed to land on the runway, and if not, you must perform a missed approach.

My question is: Was it ever possible to land big commercial jets when the pilots spotted the runway right at the missed approach point during non-precision approach?

With the benefit of Visual Descent Point (VDP) and Constant Descent Final Approach (CDFA), most airline pilots today do not need to worry about this, but I'm still curious: Was it possible?

To add, the reason why I'm asking this question is that if a stabilized landing was not possible in some airplanes (big commercial jets), why was and still is the missed approach designated far too close to the runways for stabilized approach on most non-precision approach charts, instead of it simply being designated at the VDP point, when after all it's a "missed approach point"?

Possible? Yes, depending. Advisable? That's a matter of opinion. I suspect most these days would probably say no unless you did a circling maneuver to another runway. And then many (most?) would say that you have no business circling at less than, say, 800 ft AGL in a 747 or the like.

Restating my first answer: Possible? Yes, for some MAPs, with some aircraft, at some weights, at some airports, to some runways, by some pilots, but not if you HAVE to fly a stabilized approach.

  Was it ever possible....

What was physically possible in the past—let's say 25 years ago so it's within the context of my experience—is still physically possible today, so I think what you might be asking is whether it was ever done.

While I don't recall ever having done it myself from the exact MAP, I think it reasonable to accept that it would have been done on occasion by some. Back in the 1990s at the two 747 carriers I flew for, we flew non-stabilized approaches when convenient, especially in freighters. Thus, in-close, down low maneuvering was not the no-no that I get the impression that it is today.

Indeed, the IGS 13 approach at Kai Tak in Hong Kong in bad weather meant you'd be popping out of the clouds a few hundred feet above buildings and then having to make a 47° right turn to line up with the runway. The decision altitude was 600 feet as I remember.


What is the mathematical relation between CLmax and Reynolds number? I know that Reynolds number influences CLmax, but I am not able to find any formula with which I could plot a graph!

The Reynolds number is the ratio of inertial to viscous forces, so viscous effects become smaller with a growing Reynolds number. Viscous effects cause flow separation, so a higher Reynolds number helps to delay stall.

Details depend on the particular airfoil shape, so a general formula does not exist. However, there are many plots of how maximum lift varies over their Reynolds number for groups of similar airfoils.

Below you see Figure 18 from chapter 4 of S. Hoerner's "Fluid Dynamic Lift", giving the maximum lift coefficients of some airfoils of 12% thickness. Besides the Reynolds number, nose shape and camber also influence the maximum lift coefficient, so no simple formula is possible.

Note that the Reynolds number has a logarithmic scale and shows a linear dependency with lift coefficient above Re = 10⁷. So for large Reynolds numbers you might approximate the maximum lift as something like $$c_{L_{max}} = 0.3\cdot log(Re) - 0.5$$
but there is rather little experimental data to support or refine this approximation.


Is it possible to power a B-52 bomber with four bypass turbofan engines? Is it possible to power the B-52 Bomber with 4 bypass turbofan engines instead of the eight in its current configuration? They would have less noise and be a lot more fuel efficient.

A four-engine configuration has been proposed at least two times but would require expensive modifications to the wings.

A 2017 Flight global report, Rolls Royce offers BR725 for B-52 re-engine effort, said

  So far, the USAF has foregone plans to reconfigure the B-52s with four engines, which would have required expensive modifications to the wings, R-R says. R-R previously considered a higher thrust variant of the RB211 turbofan engine as a four-engine option on the B-52, but the service does not appear to be moving in that direction, according to R-R.

A 2014 Aviation Week report, B-52 Re-engine Resurfaces As USAF Reviews Studies, said

  Pratt & Whitney studied the idea in 1982, with four PW2000-series engines. In 1996 Boeing and Rolls-Royce jointly proposed to fit B-52s with four RB211-535s, with the government leasing the engines. The first plan was not taken up because all B-52s were to be replaced by B-1s and B-2s by the late 1990s, and the second failed because of resistance to leasing combat assets and a flawed economic assessment by the Air Force.


How to calculate Reynolds number? How do I calculate the Reynolds number of a wing with wing chord,1.2m and speed of 60km/h

One way is to see what the following NASA links have to say about it and possibly using the calculator they have at the bottom of either page.
https://www.grc.nasa.gov/WWW/K-12/airplane/viscosity.html

The similarity parameter for viscosity is the Reynolds number. The Reynolds number expresses the ratio of inertial forces to viscous forces. From a detailed analysis of the momentum conservation equation, the inertial forces are characterized by the product of the density $\rho$ times the velocity $V$ times the gradient of the velocity $dV/dx$. The viscous forces are characterized by the viscosity coefficient $\mu$ times the second gradient of the velocity $d^2V/dx^2$. The Reynolds number $Re$ then becomes:
$$Re = (\rho \cdot V \cdot dV/dx) / (\mu \cdot d^2V/dx^2)$$
$$Re = (\rho \cdot V \cdot L) / \mu$$
where $L$ is some characteristic length of the problem.

and

The dynamic viscosity coefficient divided by the density is called the kinematic viscosity and given the Greek symbol $\nu$
$$\nu = \mu / \rho$$
$$Re = V \cdot L / \nu$$
The units of $\nu$ are length^2/sec.

https://www.grc.nasa.gov/WWW/K-12/airplane/reynolds.html

The Reynolds number expresses the ratio of inertial (resistant to change or motion) forces to viscous (heavy and gluey) forces.


What is the meaning of this airport marking? This mark is identified as a plane control marking and based on an apron surface. What does it mean?

The marking indicates that vehicles must yield to aircraft. It is placed in areas where a service road crosses a taxiway.


What happens if a parachute does not open while skydiving? While skydiving we have to jump from the airplane at a certain altitude above Earth. So if for some reason the parachute doesn't open, then will that person (or any object similar to a human body) fall to Earth or will it be incinerated/vanish in the sky due to air-friction in space before coming down to the Earth?

The majority of the heat developed during reentry of a space-vehicle comes from air compression not friction. Remember, to go into space, you don't have to go very high, but to orbit you will have to go very very fast, and it's not the fall that produces the heat, it's the slowing down part.

If you had a human fall without a chute, the terminal velocity (where air resistance cancels gravity and you continue downward at a constant speed) would be around 100-200 mph, not nearly enough to cause any kind of heat (or cars would burn up by going normal cruising speeds). To get higher terminal velocities, you'd have to go a lot higher, where the air is a lot less dense, higher than any skydiver have, and as far as I can tell Felix Baumgartner did not experience any incineration during his jump.

The unfortunate skydiver will almost certainly die, but there'd definitely be a body, and it'll probably be in one piece.


Why was a CFM56 conversion never offered for the short-body DC-8s? A major part of why the DC-8 hung on well into the 21st century (albeit almost entirely in cargo service) was the late-1970s/1980s conversion of 110 DC-8-60s (out of a total of 262) into DC-8-70s; the main (or, for the DC-8-6/72 and DC-8-6/73, the only) change was the replacement of the DC-8-60's old, loud, thirsty Pratt & Whitney JT3D low-bypass turbofans, and the nacelles thereof, with new, [relatively] quiet, efficient General Electric/SNECMA CFM56 high-bypass turbofans (and the nacelles thereof).1 2
Only the DC-8-60 was eligible, though; the short-bodied versions of the DC-8 (the DC-8-10/20/30/40/50), which formed a slight majority of all DC-8s ever produced (2943 out of a total of 556, as compared to the 262 DC-8-60s)5 needed not apply, even though they would have benefitted just as much from the reengine as the DC-8-60 did, and the process would have been mostly the same (in fact, the DC-8-61 was merely a straight fuselage stretch of the last short-bodied version, the DC-8-55, and, yet, the former version got [the opportunity for]6 a reengine, while the latter did not).
Why did the short-bodied versions of the DC-8 never get offered the CFM56 upgrade that was extended to the DC-8-60?

1: Thus continuing a long tradition of up-engining older-model DC-8s to newer-version standard, although the DC-8-70 was the only DC-8 version for which this was the only possible way of getting one (without any being built new).
2: The April 1979 decision to reengine DC-8-60s with the CFM56, incidentally, saved said engine from extinction, allowing it (in multiple later versions) to later see service on the 737-300-through-900, A320ceo, and A340-200/300.
3: Over half of these were of the latest and greatest short-bodied DC-8, the DC-8-50 (142 built new, plus a total of 20 converted to DC-8-50 standard from DC-8-10s [12 aircraft], -30s [5], and -40s [3]); there were also 52 DC-8-30s (not including the five converted into DC-8-50s), and smaller numbers of -20s and -40s (but not -10s, all of which [except for two which crashed first] were converted to DC-8-20 or DC-8-50 standard).4
4: I am aware that many of the short-bodied DC-8s had either crashed or been retired by the time the 1980s rolled around; however, this was also true of many of the almost-as-old DC-8-60s.
5: Numbers from here (Wikipedia, unusually for it, has had some major inaccuracies regarding the production and conversion numbers for the short-bodied versions of the DC-8, especially the DC-8-50).
6: Surprisingly, many DC-8-60 owners and operators did not take the opportunity to upgrade their aircraft.  Out of 262 DC-8-60s produced, only 110 were reengined into DC-8-70s; some of the remaining 152 DC-8-60s were presumably aircraft that had already crashed or been retired by the time the reengine programme came around, but, even of the DC-8-60s that were still around, many went unupgraded.

I suspect it's because the business case just wasn't there. Every retrofit project I've been involved in boiled down to the money. Retrofits aren't cheap and ultimately the mods have to recoup those costs through improved revenue or decreased operating expenses.

Looking at your references, there was some structural changes to the wing and landing gear that came on the -55. I expect the earlier versions were ruled out of the mods due to the lack of those upgrades and the heavier CFM engines.

That doesn't explain why the -55 wasn't included. The Wiki article stated the -61 conversions did cost more than the -62 and -63 conversions because it had the shorter wing of the -55. So it boils down to the difference between the -55 and -61. Unmodified, the -61 had about 38% more payload capacity (72k lbs vs. 52k lbs). But the modified -71 lost almost 12k lbs of payload in the conversion due to the extra engine weight. So the -71 advantage over the -55 was only 20%.

The -72 and -73 didn't lose as much payload with the mods as they had the longer and stronger wing to start with. If you apply a weight penalty to the -55 similar to the -61 mods, it cuts the payload to approximately the same as the -43. That's about a 20% reduction in payload.

The other consideration is who was doing the mods.  I didn't do an exhaustive analysis, but the vast majority of the -7x conversions were for the big air cargo operators - Flying Tigers, UPS, and FedEx - or large leasing companies where the planes were in use in the US and Europe where noise was becoming an issue.

Many of the -6x aircraft that weren't converted (discounting those written off or broken up) were being operated in the countries where cheap trumps everything.

So most of the conversions were for cargo.  Another plus for the stretch aircraft is that cargo operations tend to max out on volume before weight. Obviously, that's not always the case, but the loads that max out weight tend to be special cargo (heavy equipment or machinery) not routine packages. The -71/-73 could carry almost 80% more volume than the short bodies.  Even the -72 had a 16% greater cargo volume. That's a big advantage in operational efficiency.

Since the long body could haul more cargo and generate a greater margin per flight, it made a worthwhile investment.


Would hydrogen fuel tanks on an airplane increase lift? Since hydrogen is lighter than air, would hydrogen fuel tanks help provide lift for the airplane?

Let's do some quick math. Air has a density of about 1.3 g/L and hydrogen has a density of about 0.09 g/L. The best case for lifting force would be storing the hydrogen at the same pressure as the surrounding air. This means that every L of hydrogen can displace 1.21 g of air, reducing the density of the aircraft. The amount of air displaced by the aircraft (and therefore the buoyancy) would not change, but the density of the aircraft in that space would decrease. This would decrease the lift needed, but let's see by how much.

The greatest amount of free volume in an aircraft (that isn't part of the pressurized section of the fuselage) is the fuel tanks. An A380 has a fuel capacity of about 323,000 L. If we consider filling half of that with hydrogen, this would result in a total reduction of about 190 kg. An A380 has a max takeoff weight of 575,000 kg. This change would help but would be very small compared to the total lift needed.

As David Richerby points out, hydrogen needed as fuel would need to be pressurized (if not liquified) to fit enough onboard. This would increase the density above that of any displaced air, which would instead add weight to the aircraft.

Perhaps the hydrogen could be produced instead of stored. Newer aircraft do have something called an OBIGGS, or onboard inert gas generating system. This system generates nitrogen which is used to fill the space in fuel tanks after the fuel is used. Being an inert gas, nitrogen can lower the amount of oxygen in the space below what would be required for ignition of the fuel vapors.

However, this system splits nitrogen out of air, considering that our atmosphere is mostly nitrogen. Hydrogen would have to come from somewhere else, usually water, which would add more weight. Another problem with hydrogen is that when added to air it can be explosive all by itself, which would make the situation even worse. Considering the minor benefit, this would not be a system worth installing on an airliner.

Regarding the feasibility of a hydrogen powered aircraft, NASA did a study and decided that this is not a good source of power for an aircraft. The volume and weight required to store even liquid hydrogen would be too much for the aircraft to be very useful.


What exactly happened in this bird strike incident? This Boeing 737 was hit by a bird and continued all the way to its final destination, from Prague to Las Palmas.

What actually happened here? Was the incident considered a bird strike or an engine stall? Can an aircraft continue to its final destination after a bird strike?

Since it hit a bird, it was a bird strike. Nothing else matters for that classification.

The puff of flame suggests there probably was a momentary compressor stall. That is the compressor was unable to maintain the pressure as the flow was disrupted by the bird, the flow through the engine slowed down for short while, some extra fuel accumulated in the engine and as the flow restored, the extra fuel burned past the jet, creating that puff of flame.

The engine did not flame out, that is, it continued to run. Older engines tended to get into a repeated cycle of stalls once they stalled as the accumulated fuel from one stall would cause the pressure to increase beyond capability of the compressor and cause another stall, but modern engine computers will automatically restrict the fuel flow to prevent that.

The aircraft continued, because the engine was running and all parameters (RPM, temperature, vibrations, pressure ratio, oil pressure) were indicating normal values.

If the engine stopped, or if some indications were out of tolerance, they would have to return, since two-engined aircraft are required to land as soon as possible when one engine fails. But it clearly did not and they continued (and may not have been aware of the bird strike at all until inspection after landing, because the stall handling is automatic and if no values were out of tolerance, they would not get any warnings).

The engines are designed and tested to continue running after ingesting  a small bird [1], so this was actually the expected outcome. The test usually involves launching dead chicken into the running engine (on the static test bench) from a chicken gun [2]. Said gun is also used for testing canopies and windscreens. Frozen chicken are occasionally used too; contrary to the urban legend not due to stupidity, but intentionally as tougher version of the test [3].


What is this "stick" on top of Cessna 152's wing, and how does it work? What is this device? What is it for? Seems that some variants have only one, while some others have two.

Added: As some have two, so what are they? The answers and comments have not explained which antennae they are, especially for the ones with two antennae. As I asked, how does it work?  I need an explanation; if the two antennas are for the same purpose, then why do they need two at the same place?

Short answer: They're antenna(s) for the onboard VHF radio(s).

Airplanes aren't actually required to have radios. But, if you want to fly into certain classes of airspace, you must have at least one VHF radio on board, to communicate with air traffic control. A lot of planes (in fact, I would say the majority of them) actually have two radios installed. Thus, you see planes with zero, one, or two antennas.

Having two radios has several advantages, beyond just the obvious "backup in case one fails" advantage. Most often, one will be used to listen to the ATIS/AWOS/ASOS or other automated information broadcast while the other is used to keep in touch with ATC. You could also use the second one to contact Flight Services, or (if you're charter/commercial) keep in touch with your company (although that last one is getting more and more likely to be done with a cell phone or the internet).


How are in-air refuelling craft grounded to each other? This answer mentions that before refueling equipment is connected the F-16 and the refueling vehicle must be connected to each other. This does not seem to be the case for in-air refueling.

Why is the above-mentioned restriction not practiced for in-air refueling? Is there no chance of electrical voltage potential between two flying aircraft? Are the fuel lines purged before connecting in flight? Are fuel explosions not a big problem while the craft are flying? Are in-air safety considerations lower than on-ground safety considerations?

The metal of the tanker's boom (or drogue) contacts the metal of the receiver's receptacle (or probe) first, and that contact is maintained, so that would serve to ground the two aircraft to each other.

It's a good question, but I've never heard of any fuel explosions due to static buildup during air-to-air refueling, so evidently the systems in place alleviate the risks.  Aerial refueling is pretty routine these days, so in relative terms it would seem to be fairly safe.

Purging the fuel lines is a relative term; you could remove the liquid fuel from the lines, but you'd still have vapors, and those are more flammable than the liquid jet fuel itself.  Maybe somebody who flew (or flies) tankers can discuss how far down the boom the fuel goes BEFORE everything is connected and they start pumping; as a receiver I just remember that the fuel flowed into the manifold and then into the tanks.  We didn't do anything special to clear the manifold or fuel lines on our end.

There ARE some inherent risks when refueling between two aircraft flying in close formation that are accepted (i.e. the risk level is okay for the military mission, but nobody does it for commercial airliners), but I don't think this is one of them.


Can a low flying jet fighter lift water? Can a low flying but super fast jet spray water from the surface of the ocean?

I've seen this visual effect done many times in movies. It's often done to highlight supersonic speed just a few feet above the water.

Do the laws of physics even come close to this being plausible?

Bonus: Can you provide photograph evidence of this happening?

The visual effect in the picture you found seems to suck water out of the sea, in this real footage the f-18 air shockwave tends to push water down, and makes a trail.


How far away from a wing's surface does the air stream remain unaffected? I would like to know how far above and below a wing's surface an observer must look in order to see no change to the airflow. Not simply visually but mechanically, assuming a perfect scenario. I am not particularly adept at mathematics, so if there is a rule of thumb, or a general upper boundary, that would suffice for the figure I need.

For a visual representation, a wind tunnel with smoke trails. At some point distant from the top and bottom of the wing in the tunnel, all flow lines will be identical again, implying that wings have an area of effect. How far is this point from each respective surface?

Another way to ask would be to say how far apart must biplane wings be (in a perfect scenario) for them to have no effect on each other?

A precise answer would be: Only outside of the supersonic shock cone. In subsonic flow the change has unlimited range, but dies down with the inverse of the distance.

A mathematical description is possible with the Biot-Savart law. It was first formulated for calculating the magnetic field induced by a current, but can equally well be applied to aerodynamics.

Now it depends on how much error you still accept. If you need the interference-free distance between two biplane wings, express that distance in multiples of the wingspan - that is how it scales. A few wingspans of vertical separation should be enough to make the interference between wings negligible. In flow direction the distance must be much larger: Munk's displacement law postulates that in inviscid aerodynamics the distance in flow direction between two wings makes no difference. In reality, air traffic control requires a separation of up to 8 nautical miles between heavy airplanes for the turbulence to die down or be washed away by wind.


Symbology on standard terminal arrival (STAR) charts: T-ended vs arrow-ended lines? I was looking at the STARs at KPHX. The ARLIN FOUR arrival here has T-ended lines between the BLYTHE VOR and SCOLE, and between SCOLE and CALIE fixes. It also has arrow-ended lines between CALIE and HYDRR, and between HYDRR and PAYINT.

What does the difference in symbology mean?

At first I thought it indicated a different MEA, but then I looked at the DSERT TWO arrival here, and I see the T-ended lines between SHOCK and BUSTD, and between BUSTD and YOLOW, and the MEAs on both the neighboring segments is the same (9000).

According to the FAA Chart User's Guide, the arrows are transition routes and the 'T' is "Altitude change at other than Radio Aids." So your original assumption is correct.

Regarding DSERT TWO ARRIVAL and BUSTD. You are correct that the altitude doesn't change at BUSTD when proceeding to YOLOW, but it does change (9000 to 8000) when proceeding to BAYTA. That is why there is a 'T' at BUSTD.


Can a VFR flight follow an IFR route? Can a VFR flight follow the same route of an approved IFR route (such as those found on Forefight Route Adviser) using waypoints and Victor airways?

Yes.  In fact, before GPS came along, if you were flying somewhere and wanted to use your VOR to get from A to B, that's exactly what you did a lot of the time if it was convenient; fly along the victor airways if they were more or less on your route.  IFR traffic is only separated from other IFR traffic and when VFR in non positive control airspace you can go wherever you want notwithstanding other restrictions.

In the GPS era nobody uses VORs very much any more and you can fly direct almost all the time when VFR (I haven't used the VOR in my plane in eons; mostly I just use a tablet or my phone), but there is nothing to stop you from flying along an airway if you feel like it.


What is the typical cruising altitude for a civil airplane? I'd like to know the interval of altitudes airliners usually cruise at (and its reasons)? And if there are international restrictions about this?

Also I read somewhere that recently about a restriction that airliners should not fly bellow 26000 ft over egypt. How hard that restriction would be?

I think there are already plenty of answers here to give you a clue:

How do pilots decide what their cruising altitude will be?
why are regional cruising altitudes higher than those of international much longer flights?
How can you find the best altitude for range or speed?
Why are many jet aircraft designed to cruise around FL350-370?

Normally, atitude restrictions give a top limit. A minimum altitude is hard to enforce when one engine fails - then the aircraft can only continue at a lower altitude (typically around 15,000 ft for a two-engined jet) or the cabin pressurization fails (now the aircraft needs to continue below 10,000 ft).

This is speculation, but maybe the 26,000 ft limit is caused by information that rebels in the West Sinai have gotten hold of shoulder-launched anti-aircraft missiles. Those have a fairly low upper limit to their effectiveness.


What protections and feedback does the fly-by-wire on the Boeing 777/787 provide? I know the Boeing 777 and 787 run on Fly-By-Wire. I was wondering:

Do they have the Alpha Floor or Flight Envelope protections Airbus aircraft have?
Do they have force feedback/force feel? The 737, for example, is connected using physical cables that move the flight control surfaces, so you can feel the yoke trying to move and when there is turbulence, it will actually shake a bit. Can you see or feel this in Boeing Fly-By-Wire aircraft?
When banking (without any pitch input), will the aircraft start to descend, or does the system counteract it like in an Airbus? In non Fly-By-Wire aircraft, they will start to descend a bit when banking, but Airbus Fly-By-Wire counteracts it by adjusting stabilizer trim.

1. Do they have the Alpha Floor or Flight Envelope protections Airbus aircraft have?
The Alpha Floor protection on an Airbus overwrites the thrust setting commanded with the thrust levers to TOGA. Boeing does not do this. The autothrottles (if armed) will engage to prevent a stall, but the computer never overwrites the actual thrust lever position in the cockpit:

With the autothrottle armed, the autothrottle automatically activates if not autopilot
or F/D is active or and autopilot or F/D is in VNAV XXX, ALT, V/S, or
G/S, and:

speed less than FMC calculated value for one second
thrust below reference thrust
airplane altitude above 100 feet RA on approach, or airplane barometric    altitude 400 feet above airport on takeoff

Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection.

(Boeing 777 FCOMv2 4.20.9 - Automatic Flight - System Description)
Also, unlike an Airbus, this protection can be disabled by simply disarming the autothrottle using the switch on the MCP.
The Boeing 777 and 787 do however have Flight Envelope Protections. These also work a bit differently than in a modern Airbus, where sidestick inputs are simply ignored or limited by the computers. In a Boeing, the flight envelope protection will use artificial forces on the yokes to provide the following:

Flight Envelope Protection
The flight envelope protection system reduces the possibility of
inadvertently exceeding the airplane's flight envelope. The flight
envelope protection system provides crew awareness of envelope margins
through tactile, aural, and visual cues. The protection functions do
not reduce pilot control authority. The protection functions are
described later in this section and include:

stall protection
overspeed protection
roll envelope bank angle protection.

(Boeing 777 FCOMv2 9.20.5 - Flight Controls - System Description)
It says that the pilot control authority is not reduced because the pilots can always overpower the simulated forces on the yoke and command the aircraft to go beyond the protected flight envelope.
Essentially, the philosophy for the control authority differs:

Airbus will not let you do control inputs that might get you killed.
Boeing will stop you from accidentally making control inputs that might get you killed, but you can overwrite them.

See also:

What are the main differences piloting Boeing vs. Airbus aircraft?
How does envelope protection work in Airbus vs. Boeing aircraft?

2. Do they have force feedback/force feel?
Yes, unlike an Airbus sidestick the Boeing 777 and 787 simulate the forces on the yoke and therefore provide force feedback:

The primary flight control system uses conventional control wheel, column, and
pedal inputs from the pilot to electronically command the flight control surfaces.
The system provides conventional control feel and pitch responses to speed and
trim changes.

(Boeing 777 FCOMv2 9.20.1 - Flight Controls - System Description)
You won't see the yoke shake in turbulence like on a 737, but you will see or feel the autopilot inputs as the yoke moves.
See also: How does the Boeing 777's yoke of both the captain and the first officer have synchronized movement?
3. When banking (without any pitch input), will the aircraft start to descend, or does the system counteract it like in an Airbus?
It will counteract like an Airbus, up to 30° bank angle, so you can fly a turn by rotating the control wheel only:

The PFCs also provide compensation for flap and speedbrake configuration
changes, and turns up to 30° of bank. The PFCs automatically control pitch to
maintain a relatively constant flight path. This eliminates the need for the pilot to
make control column inputs to compensate for these factor. For turns up to 30°
of bank, the pilot does not need to add additional column back pressure to
maintain altitude. For turns of more than 30° of bank, the pilot does need to add
column back pressure.

(Boeing 777 FCOMv2 9.20.10 - Flight Controls - System Description)
The PFCs here are the three Primary Flight Computers.


Is it common for automated control systems to use non-redundant sensors? While reading an article about the Lion Air Crash with a Boing 737 Max 8, I was very surprised by the following paragraph:

  In designing the 737 Max, Boeing decided to feed M.C.A.S. with data
  from only one of the two angle of attack sensors at a time, depending
  on which of two, redundant flight control computers — one on the
  captain’s side, one on the first officer’s side — happened to be
  active on that flight.

My naive assumption was that any system that is capable of pointing an airplane towards the ground would certainly have redundant sensor input, and switch off if there is serious disagreement between the sensors.

Is it common to have non-redundant sensors for this kind of purpose in commercial airplanes?

In general, yes. Except in certain configurations, namely autoland, autoflight functions are usually fed with one set of sensors.

This matches the fact that pilots flight instruments are fed from a single source as well. Therefore, the autopilot behavior and instruments should have a coherent and predictable response.

The main reason is fault isolation. If the flight crew identifies a fault with the captain's instruments, then disabling instrumentation and autoflight functions on the captain's side will guarantee the faulty data will not influence the copilot's instrumentation and controls.

If you have the system try to "intelligently" vote, you run the risk of not being able to isolate a faulty sensor, especially in 2 faulty of 3 sensor situations. Another classic problem is how to prevent the voting system from becoming a single point of failure.

Instruments are accompanied by a comparator, which detects significant differences between redundant readings and causes an appropriate warning. Most systems treat the warning as binary: the flight crew manually compares all readings and using human logic, decides which are discrepant.


Is high speed buffet possible at supersonic speed? Since high speed buffet is caused by boundary layer separation due to shock wave and at supersonic flight there is no shock wave directly onto the airfoil (it is attached to the trailing edge), is high speed buffet then possible?

All pilots who transited into supersonic speed on older designs which were not perfected for transsonic flight reported that suddenly all that shaking and vibrating stopped and flight became smooth again as soon as they went over Mach 1.

So the answer is no. With supersonic speed the buffeting stops.


What does the acronym PDA stand for in the aviation domain While reading an article about Hang Gliding reserve parachutes, I came across the acronym "PDA".  The article seemed to imply that this is a type of parachute and a further google search seems to confirm that.

What I haven't been able to find is a source explaining what this acronym stands for.  Wikipedia disambiguation offers no help and neither do several aviation glossaries I a familiar with.  Lastly, a search for "PDA" in this forum provides no results.

Pull-down apex. It's an additional attachment point in the center of the parachute. Putting force on it makes the parachute open up more like a half-donut, rather than a half-sphere. That provides more effective area for an annular parachute. That hole in the middle is needed for stability.

Link: one product of this type with a bit of explanation.


Has the typical cabin pressure in airliners changed in recent decades? Two or three decades ago, I would experience quite uncomfortable blocking of the eustachian tubes, which would noisily unblock and become blocked once more as the plane descended.

I haven't experienced that in recent years, in which I have been flying on considerably newer aircraft.

Is this likely to be a change in the cabin environment or my own physiology - that is, have cabin pressures generally increased so that changes during descent are less dramatic, or have I just become older and less susceptible to blocked ears?

Recent airliners such as the 787 and the A350 have much-touted higher cabin pressure levels; has there also been a general increase in standard cabin pressures over the years?

Let's take the Boeing 737 as an example: an airliner that first flew in 1967 and is still being produced, its variants—old to new—indeed show a trend of increasing pressurization capability.

Note that even the 1967 variant had an automatic controller for the cabin pressurization, and the 200Adv and the Classic shared the same max ceiling of FL370.

The better the engines and the aerodynamics are, the higher they'll fly, and the stronger the aircraft will be made to offer better cabin altitude. However stronger is now easier than in the past thanks to the materials engineering advancements.

  1/200 (7.5 psi) → 200Adv (7.8 psi) → Classic (8.65 psi) → NG (9.1 psi)

The higher those values, the lower (denser) the cabin altitude is, for a plane flying at the same altitude.

Those values are the limits, each variant will use lower values in the day-to-day operations to reduce the stresses.

Source of the figures: b737.org.uk


Do any airplane designs exist that don't involve a flight surface that provides downforce? Most aircraft maintain longitudinal stability by balancing three forces:

The down force acting through the center of gravity (CG)
The lifting force acting through the center of lift
The down force acting through the center of pressure on the horizontal stabilizer.

My question is, what are some aircraft designs that deviate from this basic principle? Are there aircraft we see all the time that don't work this way? As a side note, how would weight and balance be affected if this principle isn't used?

There are multiple configurations which are possible with tail or canard, which based on their locations and whether they produce lift or down-force, results in a stable or unstable aircraft (taking the aircraft center of gravity into account). The figures below show some of the possible configurations.

Source: f-16.net

In the most general case, there is no need for even a canard or a horizontal tail as long as a (down or up) force is produced in some way. A good example is the Boeing Bird of Prey, where the chines produced (vortex) lift and thus a stabilizing force. The forces acting on the longitudinal axis of the aircraft look something like this, leading to a statically stable aircraft without any lifting surface except the wing.

Profile from bj-o23.deviantart.com, others own work.


What is the difference between the V-shape tail and the straight tail on the Beechcraft Bonanza? In one of the answers to the question which aircraft is the oldest in production, the
Beechcraft Bonanza is mentioned. While reading the Wikipedia entry on this interesting plane, I noticed that there are some models with a V-shape tail and some with a more conventional tail.

What is the difference between the two designs?

 Source: Wikipedia

and

 Source: Wikipedia

V-tails have been fashionable in the Fourties and Fifties, and the claim was that they would cause less drag than an equally effective conventional tail. This has two reasons:

Average chord length is longer, so the Reynolds number of the airflow is higher, causing relatively less friction drag.
Instead of three surface-fuselage-joints, the V-tail has only two, so less interference drag is created.

Flight testing of V-tails showed only marginal advantages, and in damping they are less effective than a conventional tail which is dimensioned for the same control effectiveness. The control effectiveness of a control surface is proportional to the cosine of the V-angle, but the damping characteristics go down with the square of the cosine.

@nimbusgb mentions the tendency of the V-tailed Bonanza to fishtail: That is a consequence of too little lateral damping.

Also, when a combined elevator-rudder input is commanded, the V-tail will produce a very high deflection on one side, while the two commands will cancel out on the other side. This will reduce overall control power in cases where a combined elevator-rudder input is commanded over what is possible with a conventional tail.

Even the glider designers, who had enthusiastically tried out V-tails, have now returned to conventional tails (more precisely, T-tails) because the expected benefits did not materialize. And glider designers will try anything to improve the aerodynamic quality of their designs.


What is the difference between an engine "shut down", "cut-off" and "cut down"? Explain please for non-native speakers the difference between these engine (jet or piston) terms:

shut down
cut-off
cut down

I'm studying at a civil aviation university to become a pilot and a teacher told me "shut down" is used when you switch the engine off, "cut-off" is used when the engine electrical supply fails, and "cut down" is when you reduce throttle setting. But I'm not sure it's correct.

To be specific:

Shutdown: To take a step or series a steps to stop the operation of a machine and put it into its normal inoperative state. A shut-down procedure may include cutting off fuel, closing valves, and turning off pumps, or may just be flipping a switch to OFF.

Cut-off: To interrupt (or a noun describing something that interrupts) the supply of something to a machine; fuel, water, electricity.  Generally refers to closing of a valve or opening an electrical contactor or switch.  On a piston engine, you have a mixture setting called "idle cut-off" that blocks flow of fuel at the carburetor. You might see a switch that opens a contactor to an electrical bus, for use in emergencies, labelled "Bus XXXX Cutoff".

Cut-down: I've been working in aviation in N America since 1975 and have never heard of this term as relating to operation of an engine (maybe in UK?).  Normally if I see the word cut-down it's an adjective that describes something that has been physically altered to make it shorter or smaller.  Like a cut-down vertical fin, or a cut-down antenna, or cut-down propeller blades.  I'd be surprised if anybody on here has ever heard of the phrase "cut down power" as an instruction to reduce power, or thrust.  You just say "reduce".

You DO sometimes hear the word "Cut" by itself as an instruction to go to idle or to remove power completely on an engine.  A skydiving pilot may get an instruction to "Cut" when it's time for jumpers to exit.  Some day when you are on a jet type course, one of the simulator sessions will include "V1 Cuts", where one engine is failed just as you are passing V1 (whereupon hilarity ensues the first time you do it).  So in those cases, power is cut, but not "cut-down".


What happens to cabin pressurisation when the engines fail? I was reading about the cabin pressurisation mechanism from Wikipedia and another article here. According to both of them, the aircraft's engine plays a vital role in the  pressurisation process.

My question is:

I have come across many air crash investigation articles mentioning that all the engines of an aircraft had failed, but the pilots were able to glide the aircraft and safely land it with no casualties. How is it possible for everyone onboard the aircraft to survive without cabin pressurisation (with all engines out) ?

UPDATE: What if a safe landing area is a bit too far? After losing altitude, the aircraft can't obviously gain altitude. Won't the plane crash due to low altitude (the descent is fast) gliding?So, isn't it better to maintain (sufficient) altitude to reach an emergency destination? I hope the altitude can be maintained provided cabin pressurisation is intact. But, how is this achieved in commercial aircrafts?

Closing the outflow valve would seal up the plane, so it wouldn't depressurize immediately. Once the pressure falls below a safe threshold the oxygen masks would drop. The masks are good for about 15 minutes.

Air Transat flight 236 which ran out of fuel over the Atlantic holds the record for the longest unpowered glide in an airliner. It lasted about 19 minutes.

The APU can provide pressurization, but if you're out of fuel that won't work either. Air Transat was running off the RAT and I doubt that can provide pressurization. BUT if you factor in the outflow valves being closed and the last part of the glide being below 10,000 feet the oxygen masks will provide ample time for the passengers to stay safe.


How do airliners prevent raising the gear on the ground? GA aircraft use a squat switch to prevent accidental retraction on the ground. Do airliners use a similar system? I've heard some reference to a manual pin, I'd appreciate clarification on that.

Most airliners have a protection system for this which is based on Weight-on-Wheels (WoW): sensors which detect load on the wheels (which implies that the aircraft is in contact with ground). For safety reasons this can be bypassed and there's usually a small lever in connection to the gear lever to do this. This image illustrates the 737 gear lever where you can clearly see a "lock override" which fills this functionality (red part)


How much electrical power does one generator produce on a large turbofan? From my related question, now I want to know just how much power does the generator produce? In other words, what is the max rating of the generator? It looks so small and that surprised me, but since it spins very fast it could get high power in a small package.

The generator from any large turbofan is fine. The answer will also help us figure how much electrical power a large commercial airliner could need at any given moment.

This is one of the four Variable Frequency Generator (VFG) of the Airbus A380. It's apparent power is 150 kVA.

Rotor, source: Safran

The total power available from the engines is 600 kVA.

Seen from the other side:

Stator, source: Thales

The A380 is a one of these "more electrical" planes, where hydraulics tends to be replaced by electric devices, and the electric circuits have been optimized by electronics and variable frequency. There are secondary sources for electricity: APU (auxiliary power unit) and RAT (ram air turbine). Overall view from the book Civil Avionics Systems (Moir, Seabridge, Jukes):

Source: Civil Avionics Systems

In case of total failure, the ultimate RAT backup is totally electric-based contrary to conventional aircraft where some hydraulics is also provided by the RAT to move the control surfaces.


Why do jet engines use kerosene rather than gasoline? Could you run a jet with gasoline? Why do all jet engines use kerosene?

You can persuade a turbine engine to run on just about anything that can burn. So the decision of which fuel to actually use depends on the side factors including, but not limited to:

availability
cost
emissions
hot section temperature
chemical reactions with engine parts

Specific examples:

Coal dust is rather difficult to pump around, and the rampies don't like shovelling
liquid hydrogen (used in the Space Shuttle) requires a lot of storage and has the nasty habit of freezing anything it touches, like rampies.
ethylacetylenedecaborane is unpleasantly toxic (rampies union again) and the combustion byproducts were rather abrasive to the engine's innards
trimethylaluminum would reduce the engine complexity (no igniters needed) because it has the nasty habit of igniting instantly upon contact with air, so leaks are rather dangerous.
natural gas is commonly used as a turbine fuel in pumping stations: it's already there and thus is "free". The required pressure vessels make it impractical to use as an aircraft fuel.

So kerosene basically became the standard turbine fuel because it's:

cheap: kerosene makes up a rather large fraction of crude oil. When you measure your fuel load in tons a few cents per litre makes a difference.
safe to handle: relatively non-toxic, doesn't ignite all that easily
storable and transportable in common structural metals
doesn't clog up the engine


How can I tune a frequency like 121.62 on a comm radio? On my radio, an ICOM A210, I can't tune 121.62 (the clearance delivery frequency at KMSN). How can I tune a frequency ending in 2?

As a complete guess, you're trying to tune 121.620, but you should just be tuning 121.625. From AOPA (emphasis mine):

  All aviation communication, VOR, and ILS frequencies are in the 100
  MHz band, so the first number is always "1." The hundredths digit can
  be only 0, 2, 5, or 7; 1xx.x2 implies 1xx.x25; and 1xx.x7 implies
  1xx.x75--when a frequency is assigned, the ending "5" is often
  omitted. Trailing zeros in the hundreds and thousandths positions are
  always omitted in voice transmissions.

The standard frequency spacing in the US is 25kHz.

Omitting the last digit is also described in the FAA's ATC instructions:

  The separate digits of the frequency, inserting the word “point” where the decimal point occurs.

  (a) Omit digits after the second digit to the right of the decimal
  point.

They include this example:

  135.275 MHz “One three five point two seven.”

And although this probably isn't your issue, it's worth noting that some radios tune in 50kHz increments by default, unless you pull out (usually) the tuning knob to enable 25kHz tuning. I've been momentarily confused by that several times in unfamiliar aircraft. I assume this isn't the issue you have, though, since the manual for the A210 says it uses 25kHz spacing.


Why aren't cockpits airtight? There are accidents where the pilots couldn't see the instruments and literally flew the aircraft blindly due to the excessive smoke in the cockpit, ranging from UPS Flight 6 to other accidents.

How come no company has made an airtight cockpit to prevent smoke from entering the cockpit? Even though it can be said it is complex I think it doesn't make an multimillion airliner a lot more expensive?

Cockpit smoke will come from either of two sources:

The air conditioning pack supplying it, such as the air cycle machine gone bad (more or less a turbo charger like device) with bearings and seals failed.  This usually results in smells more than smoke.  Another one is smoke from the engine compressor - unusual because are aren't really major sources upstream of the bleed ports unless something is introduced from outside.  Cooked deicing fluid is the biggest source.  Fluid gets in the compressor, and the temperature at the bleed discharge ports can be 500-800F at takeoff thrust.  If the bleeds are started right after deicing without a clean-out engine run first to purge the compressor, you'll get stinky haze in the cabin/cockpit from cooked fluid.  This has caused many a turn-back.  Another one would be smoke from an APU that blows up while supplying bleed.
You can also get smoke from burning out fan motors in supply ducting, but cockpit smoke so intense that it causes a control crisis is almost always from burning wiring in the cockpit itself, such as behind control panels or interior panels (see Swissair flight 111).

So the main thing is, smoke coming from the air conditioning system can be reduced by turning the system off, but that isn't the problem.   It's smoke originating from within the cockpit itself, mostly from wiring.  Considering how rare those events are, it's not really practical to create a hermetic seal between the space behind interior panels and the interior space and still have a maintainable and reasonably light airplane.


Is there much turbulence in the Doldrums? A place over the equator called the Doldrums are know for the severe lack of sailing winds. In the atmosphere above it is there any presence of turbulence anywhere? Where would the least year round turbulence be found?

The intertropical convergence zone (ITCZ) (aka. the doldrums) is an area around the Equator which is notorious for heavy convective activity often leading to tall cumulonimbus clouds and associated heavy turbulence.

The area around the Equator receives the largest amount of energy from the sun because of the perpendicular angle to the sun. This causes relative large heating of the surface, resulting in heating of the air above the surface and ultimatively convective activity including turbulence. This also causes a large green band around the earth near the Equator with rich vegetation, because the convective activity causes large amount of percipitation.

The ITCZ is an important part of the global weather system, which is based on large cells of air generally moving in predictable patterns. The Hadley cell is a cell extending from the Equator to approximately 30 degrees north/south. Hot air rises at the Equator because of the heating caused by the perpendicular angle to the sun. This air travels at high levels as it cools down, and creates a downward flow of air around the 30 degree latitudes. As a result, the areas around 30 degrees latitude get almost no percipitation, which causes large areas of desert such as the Sahara.

The pattern repeats itself with Ferrel cells between 30 and 60 degrees latitude and a polar cell from 60 degrees latitude to the poles.