PPL PoF Question

[Binners93]

Morning Pruners! :)

So some advice would be much appreciated from those who have a sound understanding of all things PoF related, as one question that came up in my flight school had me scratching my head!

The question went along the lines of;

"You are flying at Va and pull back on the controls in an full abrupt manner, what is the first thing that will happen?"

a) You will enter a zoom climb.
b) You will cause structural damage to the control surfaces.
c) A silly irrelevant answer.
d) Another silly answer to add a forth option.

My first thought was that the question has specified Va, not Vfe. At Vfe, its my understanding that ANY control surface deflections are likely to cause structural damage, let alone a full pull back. I also was thinking about the lift principle and how an increase in the angle of attack and such a fast velocity is likely to climb the aircraft quite dramatically. I selected a), turns out it was b) :ugh: .

Can any of you lend some advice with this one? I understand why structural damage will occur, but the questions asks what is the FIRST thing that will happen. I would have thought that if it was at Vfe, first thing to happen is a freefall skydive with the plane in tatters but at Va, a climb and then possible damage?

I know I'm probably missing something obvious so would greatly appreciate any assistance! :ok:

[Jan Olieslagers]

What's PoF?

[Binners93]

"What's PoF?"

Principles of Flight :)

[Level Attitude]

Although a) is possible that is not the first thing that will happen - need a change in AoA first (which would not be instantaneous) and before the abrupt control deflection causes an AoA change it would (possibly) cause damage.

b) is the correct answer by definition:
In aviation, the maneuvering speed of an aircraft is an airspeed limitation selected by the designer of the aircraft. At speeds close to, and faster than, the maneuvering speed, full deflection of any flight control surface (http://en.wikipedia.org/wiki/Flight_control_surface) should not be attempted because of the risk of damage to the aircraft structure

VA = Design maneuvering speed (http://en.wikipedia.org/wiki/Maneuvering_speed) (also called corner[ing] speed[citation needed (http://en.wikipedia.org/wiki/Wikipedia:Citation_needed)]). This is the speed above which it is unwise to make full application of any single flight control (or "pull to the stops") as it may generate a force greater than the aircraft's structural limitationsNB: VFE = Maximum flap extended speed Nothing to do with control deflections.
(were you confusing it with VNE)

Above definitions courtesy of Wikipedia, that well known authoritative aviation reference.

[dublinpilot]

I'd no idea what PoF is either.

But you shouldn't consider VA as a strict line, and one side you're perfectly 1kt below and will instantly fall apart 1kt above.

Approach it with caution. You'll probably get away a bit over, but don't be sure. Slightly over damage is likely to be minor rather than total destruction.

So I think that the answer is enter a zoom climb and risk damage.

[BackPacker]

At Va, full and abrupt control deflection should not lead to any damage, as the aircraft will stall before the forces increase beyond the design limit. So AFAIC the correct answer is "the aircraft will stall".

If you want it to enter a zoom climb, you should not use full, but only partial deflection. It's done by feel and by listening to the stall warner: You haul back on the controls just so that the stall warner sounds, but no so much that the aircraft actually stalls.

(And note that Va decreases as the mass of the aircraft decreases. So if you're flying solo, your Va will be lower than when flying 2-up for instance.)

At Vfe, its my understanding that ANY control surface deflections are likely to cause structural damage,

Nope. Vfe is just your flaps extend speed. You can't extend flaps above this speed, since they might be blown off. (Some aircraft, including the C172, have multiple flap extension speeds, where partial flaps can be extended at a higher speed than full flaps. Vfe will then be the speed where full flaps are possible.) This has nothing to do with control surface deflection - although the majority of aircraft do limit the G-forces that are sustainable with flaps extended. For instance, an aircraft might be capable of +4 and -1 G with flaps retracted, but only +2 and 0G with flaps extended.

If you're confusing Vfe with Vne, then again you're not there yet. Vne, the "never exceed" speed is typically not determined by control deflection/g forces/stall speed like Va, but is determined by flutter tests and similar. So at Vne it is still possible to use some careful measure of control deflection - otherwise you'd never be able to pull out of the Vne dive. Sure, you can't use full deflection - you're above Va after all - but some measure of control deflection is certainly possible.

[flying apprentice]

When did Zoom Climb enter PoF ? I did ATPLs 8 years ago and never heard of it.

[Binners93]

Apologies all, I meant Vne - not Vfe :ugh:

Thank for the clarification, that makes more sense now! :ok:

[Above The Clouds]

If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage.

[phiggsbroadband]

quote...
If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage.


That design works Ok up to Va, where the 3G loads cannot be achieved, limited by stalling.


However beyond Va, I am afraid it doesn't work like that... Some numpty is sure to do a 'Watch This!' vertical dive from 12,000ft and then wonder why the wings folded when he pulled up.

[Above The Clouds]

"You are flying at Va and pull back on the controls in an full abrupt manner, what is the first thing that will happen?"



The OP states flying "at" VA, therefore in theory the aircraft should still stall within the 3g limit because he is not exceeding VA, it is hypothetical, but only answering the OP question in the literal sense, if some numpty is daft enough to put it to a test from 12000' then good luck to them.

There is a % safety buffer built in, for the same reason why flying 1 kt over VNE the aircraft does not immediately disintegrate.

[Level Attitude]

If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage. At Va, full and abrupt control deflection should not lead to any damage, as the aircraft will stall before the forces increase beyond the design limit. So AFAIC the correct answer is "the aircraft will stall".But is Va the speed:
i) "below which the aircraft will stall, if full and abrupt control inputs are applied, before structural damage will occur." ?
or
ii) "above which structural damage will occur, if full and abrupt control inputs are applied, before the aircraft will stall." ?

Binners93 original Post seems (to me) to be asking how to answer a written theoretical question so "should" or "might" are not very helpful in this context.

In the question posed by Binners93 the aircraft is flying (exactly) at Va.
I believe the definition of Va is i) above, therefore b) would be the correct answer to the question asked.
(especially as "the aircraft will stall" is not given as a possible answer)

[Shaggy Sheep Driver]

And remember that the point that stall AoA is reached, and therefore the value of VA, varies with weight.

[BackPacker]

therefore b) would be the correct answer to the question asked.

I don't agree. At Va, you haul back at the controls. That means the aircraft stalls exactly at the moment the maximum design forces are reached. For which the aircraft is designed (duh). So there is no structural damage, so B is not the correct answer.

B is not the correct answer for another reason too. B specifies "...to the control surfaces". There is no reason why specifically the control surfaces should or should not be damaged, and the rest of the structure not. In fact, my gut feeling tells me that even well above Va, hauling back on the controls will not damage the control surfaces, but will damage the main spar.

A is also not the correct answer since entering a zoom climb requires that you do not enter a stall. So hauling back fully is not going to work to enter a zoom climb.

My gut feeling is that actually either C or D is the correct answer, and will be along the lines of "the aircraft will enter an accelerated stall".

Anyway, what happens at Va, or one knot below or one knot above, is splitting hairs. Like others said, there is some measure of additional safety factor calculated in. So if you are a few knots above Va and you haul back on the controls, you will not break the aircraft. (But that's the kind of thing you should only do if you're a test pilot.)

[Andrewgr2]

And another thing (I believe)

At Va you can apply full deflection to any single control without risk of structural damage. However full up elevator and full aileron - or rudder - might break something ...

[Binners93]

My gut feeling is that actually either C or D is the correct answer, and will be along the lines of "the aircraft will enter an accelerated stall".

Backpacker, I remember one of the answers was along the lines of 'The aircraft will climb at best rate/angle' and the other was something to do with ATC??? Hence why i dismissed them :uhoh:

[Baikonour]

Above The Clouds wrote There is a % safety buffer built in,

Backpacker wrote there is some measure of additional safety factor calculated in

Is that really so?
Can you point towards a place in e.g. CS-23 where there is a requirement for margin beyond, for instance, Va?

The reason for asking is that although we are used to most things being over-engineered, when there is no requirement to do so, we really should not rely on it actually being done...

B.

[keith williams]

Many pilots would define Va as something along the lines of " The speed at which the aircraft will stall at its limiting load factor" or " The maximum speed at which full control deflection can be carried out without exceeding the limiting load factor". But neither of these definitions are correct.

The following material is taken from a FAA discussions following the loss of the A300 fin over new York.

Quote:
Code of Federal Regulations Sec. 23.1507Part 23 AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES

Subpart G--Operating Limitations and Information
Sec. 23.1507[Operating] maneuvering speed.[The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than established in Sec. 23.335(c).]Amdt. 23-45, Eff. 09/07/93 Comments Document HistoryNotice of
Proposed Rulemaking Actions:Notice of Proposed Rulemaking. Notice No. 90-18; Issued on 06/15/90.Final Rule Actions:Final Rule. Docket No. 26269; Issued on 07/28/93.



The following text is extracted from the above document.

Reference: Conference proposals 119 and 120.
No action is being taken to amend Section 23.335 Design airspeeds.

Quote:
Explanation: Conference proposal 187 recommends revision of Section 23.335(c) to increase the design load factor to account for possible overloads resulting from maximum airplane maneuvers at speeds greater than VS√n for cases where the applicant chooses a design maneuvering speed greater than V=VS√n as allowed by Section 23.335(c). In support of conference proposal 187, the submitter states that the purpose of maneuvering speed (in addition to supplying a speed for design of control surfaces in accordance with Sections 23.423, 23.441 and 23.445) is to provide an operating speed where a pilot can be assured of not exceeding the design limits during maneuvers. If a design maneuvering speed in excess of VS√n is chosen (as currently allowed by Section 23.335(c)), and if the airplane is operated at that speed during maneuver, the potential exists for a pilot to exceed the design limit load factor unless that load factor is increased accordingly.

Post conference review indicates that the design maneuvering speed criteria provided in Section 23.335 is necessary and sufficient for control surface design. As such, design maneuvering speed selections greater than VS√n are appropriate, and requiring increases in load factor above those specified in Section 23.337 are unjustified.

However, the FAA recognizes that maneuvering speed is also used by the pilot as that airspeed below which full control surface inputs can be accomplished without structural damage. Maneuvering speed may also be used as a gust penetration speed to minimize the possibility of airframe damage. If the airplane is maneuvered at its maximum weight at airspeeds less than VS√n the airplane will stall prior to exceeding the maximum design load factor. If the airplane is operated at speeds greater than VS√n in the same conditions, the maximum design load factor can be exceeded.


So a single full deflection of the pitch control at exactly Va will not damage flight controls, but it may damage the aircraft structure, if the aircraft designers have selected a value of Va that is higher than the minimum permitted.

So option b in the OP's question may or may not be correct.

More discussion of the subject can be found at the link below.

http://forums.jetcareers.com/threads/va-maneuvering-speed-whats-it-mean-to-you.94018/

[ChickenHouse]

My first reflex was to take "PoF" as the sound you make upon impact doing so ... ;-).

[Mach Jump]

This is a very badly worded question, if reproduced accurately.

The Manoeuvering Speed (Va) is conventionally understood to be the maximum speed at which you can apply full 'up' elevator without exceeding the aircraft's maximum certified 'G' (wing loading), as the wings will stall and lose lift first.

If the aircraft is flying at exactly Va for its mass, and full 'up' elevator is applied, bearing in mind that it takes a certain amount of time, however short, to move the control to full deflection, the 'first thing' that will happen, is that during the short time taken to apply the deflection, the aircraft will begin to enter a 'zoom climb' with increasing 'G'.

As the elevator reaches full deflection, the aircraft should simultaneously reach it's maximum certified 'G', and stall.

Notwithstanding Keith's reference above to full deflection above VS\/n, but below Va, the aircraft should not be damaged, as a considerable safetey margin (usually +50% for aerobatic certification) is allowed above the maximum certified 'G' before permanent distortion to the airframe is sustained.




MJ:ok:

[Genghis the Engineer]

(a) Zoom climb.


Not the best term ever, but clear enough. Pull back hard on the stick at a highish speed, and the first things that'll happen is the aeroplane pitches up, and as a result will climb.

Keep pulling, and the aeroplane will probably stall, but it'll be decelerating at the same time as approaching the g limit, so will meet the O-A curve at a speed lower than you started, and a g loading below the g limit. So, structural damage is pretty unlikely.

Whether thats the right answer in the reckoning of the aerodynamic ignoramises who set PofF exams at both PPL and CPL/ATPL level, who knows. But it's what will actually happen.

G

[BackPacker]

I don't fully agree. The question is what happens if you apply the controls in a full and abrupt manner. I agree that during the application the aircraft will pitch up, but once you reach full application of the controls, the aircraft will stall and there will be no further pitch up forces couple.

Now how long does it take for the full application? Half a second? Even at 4G, half a second is not enough to overcome the inertia of the airframe and to pitch it up sufficiently for a zoom climb. Sure, it will pitch up a bit, but not sufficient for a zoom climb.

At least, if you define a zoom climb as something along the lines of "a climb entered at high velocity, where the aim is to convert this velocity (kinetic energy) into as much height as possible (potential energy)." A zoom climb to me happens with a seriously high pitch up attitude, and is not sustainable. Anything less than that is not a "zoom climb" in my vocabulary, but a regular climb.

I don't think you will reach that zoom climb attitude before the aircraft will stall, if you apply an abrupt and full control input. In fact, if you look at how an aerobatics aircraft performs a snap roll, hauling back abruptly and fully is exactly what you do. You then put in a bootful of rudder for the roll to happen. And snap rolls are performed without any significant altitude gain, and with just a minor pitch change. (The pitch has to change because the wings need to reach the critical AoA, obviously.)

[keith williams]

Prior to the A300 fin loss accident, conventional wisdom within JAR ATPL/CPL teaching was that Va was the maximum speed at which abrupt full scale deflection would cause the aircraft to stall at the limiting load factor, so no permanent damage to the aircraft structure would occur.

After the A300 accident, the CAA made efforts to emphasize that although a single full scale deflection was OK, repeated rapid full scale deflections in opposite directions would cause overload damage.

All of the above overlooked the fact that Va is actually concerned only with the design strength of the flight control systems. It is not concerned with protection of the aircraft structure. After some discussion the JAA introduced the Operating Manoeuvre Speed Vo, to prevent overload of the structure.

I do not know whether or not this is a real EASA ATPL/CPL theory question, but whatever its origin, it looks to me like it is based on a misinterpretation risks associated with repeated full scale deflections. Of the two options listed, option a is the most correct. But as another poster has already said, I suspect that the correct answers is one of the missing ones (c or d).

[Genghis the Engineer]

I agree that during the application the aircraft will pitch up, but once you reach full application of the controls, the aircraft will stall and there will be no further pitch up forces couple.

And the question was "what happens first".

So pitch up, as you say would be a better answer, but pitch up, and the next thing that happens is a conversion of speed into height. A stall (or structural damage in some circumstances) would occur about third.

G

[keith williams]

Well if we are going to get picky then the first thing that happens is that the elevators deflect upwards. But that is not one of the options provided, and when taking multi-choice exams our choices are limited to the options available.

Of the two options listed in the OP, option a "enter a zoom climb" will always happen. But option b, "damage to the controls" will happen only if the designer has chosen a value for Va that is significantly greater than the legal minimum of Vs x root n.

So of the options available in the OP, option a is the most correct.

But as I said in my previous post, I suspect that the real answer is one of the missing options.

[Level Attitude]

All of the above overlooked the fact that Va is actually concerned only with the design strength of the flight control systems.I suspect that the correct answers is one of the missing ones (c or d)KW, Given your first statement I was just wondering why you don't agree that the correct answer is b)?


Although interesting I do not think a detailed discussion of aerodynamics is required to answer the OP's question.

A written exam question would be looking for a basic understanding of a topic and the key (exam technique) to getting the correct answer is understanding what they are asking about and then selecting the 'most' correct answer.

This question is about Va.
The candidate would be expected to know that at speeds below Va full control deflection should stall the aircraft before overstressing it and that at speeds above Va there is a risk of overstress.
The question setters have removed the ambiguity of what would happen at exactly Va by not giving "the aircraft will stall" as an option.
So the question should be read as what would happen first if flying above Va.

Ignoring the fact that some would, in real life, overlap the order of events would be:
1) Full & Abrupt Elevator Deflection with risk of overstressing controls/control surfaces
2) Rapid Pitch Change with risk of overstressing aircraft
3) Aircraft will climb (but it will not be a very efficient climb - certainly not a "zoom")

The first event would be a risk of overstressing the controls and, although it says "will" rather than "may", answer b) matches this and is therefore the correct answer.

[keith williams]

Option b actually states "You will cause structural damage to the control surfaces".

If we ignore for a moment the possibility that Va may be greater than the limiting load factor stall speed, there is still a problem with setting the question at exactly Va. It is a bit like asking "at the time of your death are you dead or alive?" Before this time you are clearly alive and after this time you are clearly dead. But the actual time of your death is a transition point between being alive and being dead.

If we assume that Va is Vs x root n, then the aircraft will stall at its limiting load factor (n). The controls are designed to sustain loads up to the limiting load factor without permanent damage, but they will be damaged at any higher load factor. So the limiting load factor marks the transition from being damaged and not being damaged. We cannot therefor say that full control deflection at Va will damage the controls.

It does not help that many sources such as Wikipedia give definitions along the lines of "at speeds below Va full control deflection will not cause damage" or "at speeds close to or above Va full control deflection will cause damage".

The text below which was extracted from FAR 23 is more valid.

§23.423 Maneuvering loads.
Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:

(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.

The question would be far less contentious had it said "at a speed slightly greater than Va". If the author of the question had done this then I am pretty sure that this thread would not have arisen.

Aircraft will climb (but it will not be a very efficient climb - certainly not a "zoom"

I'm not sure that the term "zoom climb" necessarily implies any particular level of efficiency. My interpretation would be something along the lines of " a climb in which kinetic energy in the form of decreasing airspeed is converted into potential energy in the form of increased height." This would certainly happen if we were to simply pull the stick fully aft without increasing power setting.

[Binners93]

Well this certainly adds fire to the argument :bored:

[Jan Olieslagers]

Principles of Flight

Why didn't you say that from the beginning?

[9 lives]

There's a lot of talk about climbing here. Any kind of climb is not assured from a full nose up application of pitch controls. A pitch up is likely, but if you were in a dive, doubtful you're going to climb at all, you may just dive less, or you may pitch up, stall, and continue in a descent. I did this many times at C 150 speeds in a C 150 Aerobat, you can get a heck of a high speed stall.

Think of Va as the speed given to the pilot as a simple tool to assure that pitch maneuvers can be carried out without wrinkling the aircraft. In a certified aircraft, full pitch up at a speed faster than Va may leave some expensive wrinkles in the wings, but they are very unlikely to fail such that you cannot safely land the plane. The dangerous part for you will be turning it back in on the ramp with stress wrinkles all over it.

If aircraft were all equipped with G meters, this would not be an issue. Acceleration is one of the few stated limitations for every aircraft, which you, the pilot have no real way of measuring in flight. So provide Va, and it should keep you safe.

If I were to be in a stupid fast dive, and I could not determine G loading on the plane, In smooth air, I would rather overspeed the plane, that risk overstressing it. Your speed you do know!

Va does not count for ailerons and rudder, just elevator. You are not protected against yaw or roll caused airframe damage by Va. For example, a snap roll is entered at slower than Va, but is risky for airframe damage.

My main interest in Va, is be slower than, if I enter severe turbulence. You are not pitching the plane up, you just instantly got a whole bunch more lift than you planned on. In that case, as undesirable as a stall may be, it is more desirable than wing damage. Happily, many light GA singles barely cruise at GA anyway.