anti balance tab or servo tab anybody?

[Gin Slinger]

I’m casting out for informed opinions on this question.

If an aircraft is fitted with powered flying controls, which of the following would be used in the case of manual reversion?

a) Spring tab
b) Anti-balance tab
c) Balance tab
d) Servo tab

I’m thinking along the lines of (b), but (d) was the answer from one reputable source and (c) has been suggested too.

It's for a JAA ATPL PoF exam, so if anybody knows the CAA's take on this inparticular, it would be mighty darn helpful!

[Genghis the Engineer]

Can't quite see how it could be anything but (d). All the others are used to modify control forces or characteristics, the servo-tab will actually control the surface and hence the aeroplane (at-least so long as you can disconnect the hydraulics).

One British transport aircraft, I think it may have been the Bristol freighter used servo tabs entirely for control - the main controls simply drooped on the ground, ugly but technically elegant.

G

[Gin Slinger]

Thanks Gengis, I appreciate it.

[Keith.Williams.]

Gin,

This question is (probably) the result of defective feedback. (or the examiners were having a really bad day).

Balance tabs and anti-balance tabs are not under the direct control of the pilot. They are both anchored to the wing by a fixed rod. To move either of these tabs, the pilot must move the control surfaces. So balance and anti-balance tabs cannot provide emergency control facilities if the powered control system fails. More importantly such tabs are not used in powered flying control systems because the hudraulics carry the aerodynamic loads.

Spring tabs and servo tabs will still move if the main controls seize, but once again, are not used with powered flying controls.

What the examiners were (most probably) asking for, was the fact that trim tabs usually employ a separate power system so if the powered flying controls fail, the trim tabs will still give limited control.

Although trim tabs are not strictly essential with powered flying controls (they often use parallel actuators to alter the neutral setting of the entire in the control runs), trim tabs are sometimes used in order to give an emergency control facility

I think that this is a case where feedback has made the questions appear to be more difficult than they really are. (but I could of course be wrong)

Good luck with your exams.




















.

[Gin Slinger]

Thanks Keith, the ‘real’ question might or might not be like that, but what Genghis says makes sense, and re-reading my notes, they confer with the thrust of what he has written.

You learn a little bit more every day.

[comxtek]

the answer is balance tabs. The balance panels assist during manual reversion. I was going to explain it, but I forgot, I have an official explanation available. All B737s have balance tabs (along w/balance panels) on the Ailerons and Elevator to assist during Manual reversion. Here it goes:


AILERON AND AILERON TRIM CONTROL SYSTEM - BALANCE PANEL AND TAB

Purpose

The aileron balance panels and tab decrease the force necessary to move the aileron in flight.

Location

The balance panels are in the number one through four aileron bays.

Physical Description

The forward edge of the balance panel attaches to the wing structure through an idler hinge. The aft edge attaches to the aileron.

Seals connect the hinge assemblies along the two sides of the balance panel. This makes two separate chambers. The upper chamber vents to the airstream over the wing and the lower to the airstream under the wing.

Functional Description

When there is no lateral input, pressure forces occur across the balance panel and keep the aileron in the neutral position. When the aileron moves, it causes a differential pressure. This moves the balance panel in the opposite direction of the aileron movement.

As the balance tab moves, differential forces occur at the tail of the aileron. They help the power source that moves the aileron. These forces are always applied but they are most useful during manual control.

[Chryse]

I agree with Comxtex.

Aerodynamic balance tabs and balance panels assist in the operation of the primary control surfaces namely the ailerons and elevators. This system of easing pilot applied control surface displacement force is termed Control Balancing. In powered operation, the tabs on all control surfaces move opposite to the direction of surface movement to assist in positioning the control surface aerodynamically. If all hydraulic power to the aileron or elevator is lost, the system will automatically shift to manually operate the aerodynamic tab control.

Take the case of an aileron with control balancing, when the control wheel is turned, the aileron cable moves the aileron tabs and the ailerons are repositioned through aerodynamic action.
In the event of total hydraulic failure on most jets with outboard aileron lockout (during normal powered flight, and flaps retracted), the pilot controls the inboard aileron tabs, which in turn repositions the inboard aileron which subsequently transmits the movement to the outboard aileron through cable linkages. In other words during manual operation the outboard aileron tabs do not shift to a control function, but move in response to the movement of the inboard aileron.

Aerodynamic balance panels are located in the wing structure attached to the leading edge of the aileron and the aft wing spar. Differential air pressure acting on these panels assists in aileron operation. This is particularly significant during manual flight control operation. Therefore the correct answer should be Balance Tabs

Hope it helps and good luck on your ALTP.

Bye for now.

Chryse

[Keith.Williams.]

I must confess that in my previous post I made the mistake of answering the question I wanted to be asked rather than the one actually asked by Gin Slinger. A clear failure to RTQ!!

My post was based on the assumption that the controls had jammed, in which case only trim tabs, spoilers or variable incidence tailplanes would do the job.

The question actually asks about the manual reversion condition. This is rarely used in modern aircraft, because they tend to have multiple redundant hydraulic systems.

In those aircraft which employ manual reversion, the use of spring tabs, balance tabs or servo tabs will be capable of providing assistance in this condition. This obviously poses a problem in selecting the best answer to this question.

One problem with balance tabs and servo tabs is that they move in the opposite direction to the main control surfaces, so they reduce the overall effectiveness of the controls. They are therefore unlikely to be used in multiple redundant power operated systems.

A further problem with servo tabs is that control effectiveness is very poor until a reasonably high airspeed has been attained.

The descriptions of the B737 system provided above appears to refer to an internal aerodynamic balance system. This is not one of the options in the original question. If however the B737 actually uses both internal aerodynamic balance and balance tabs to provide manual reversion, then this is probably what the examiners are looking for.

A number of questions in feedback lists appear to indicate that the examinaers look through the manuals of their reference aircraft (B737 and A310) and pick out interesting little curiosities to ask questions about! Another example is "where would you use Krueger flaps"? The answer they want is at the wing roots. But this ignores the fact that some older aircraft still in operation have full span Kruegers.

[411A]

Comxtek

The use of balance panels (on jet transports) was pioneered by Boeing on the 707...nifty kit, especially when you consider that there is no hydraulic power assist on ailerons and elevators on the old machine.

[Gin Slinger]

Balance tabs it is then. Tick! (or perhaps I should say line....)

[Gin Slinger]

After all that, the darn question didn't come up...

[comxtek]

Never hurts to learn something though....

[Gin Slinger]

Quite.

[Keith.Williams.]

Gin,

Was there anything new in the exam?

[Gin Slinger]

Keith,

No, I didn't notice anything.

There was a duff question in Perf relating to gliding with reduced mass which had no correct answer. Alex Whittingham appealed this question from Sept, and I've made him aware that it reappeared.

[FDXmech]

Interesting post.

The correct answer, however, would be (d) servo tabs.
-----------------------------------------------------------------------------------

Comxtek wrote >>>AILERON AND AILERON TRIM CONTROL SYSTEM - BALANCE PANEL AND TAB

Purpose

The aileron balance panels and tab decrease the force necessary to move the aileron in flight.<<<

Yes, they do indeed reduce the force required to position the control surface, but they don't actually displace (move) the surface during manual reversion.

Servo tabs or in Boeing speak, control tabs, are dual purpose tabs. In powered operation they reduce the force required to position the control surface. During manual reversion, the control (servo) tab unlocks and is directly controlled by cockpit control inputs to aerodynamically move the control surface.

[fruitloop]

I would also have picked 'd' as per FDX mech's explanation !!:D

[Keith.Williams.]

FDXMECH and COMXTECH,

It is not entirely clear what type of systems you are both describing. Nor indeed whether you are describing the same aircraft type.

COMXTECH, You appear to be talking about what in JAR speak is internal balance, but you also refer to balance tabs.

FDXMECH, You appear to be talking about a type of servo tab system which becomes operational only in manual control mode. Most JAR ATPL training manuals discuss only the type of servo tab that is used in un-powered control systems.

Which aircraft are you decsribing and what exactly do they employ?

This is a matter of some significance to JAR ATPL students in that it may give a better insight into what exactly the examiners are looking for in this question.

[FDXmech]

On the other hand,

Keith Williams said: >>>It is not entirely clear what type of systems you are both describing. Nor indeed whether you are describing the same aircraft type.<<<

I was describing the 727 in which the inboard ailerons are equipped with control tabs and the outboard with balance tabs.

The control tabs (inbd. ailerons) utilize direct inputs from the yoke in manual reversion.

The outbd ailerons are driven by the inboards (flaps extended) and are equipped with balance tabs to reduce the required force during actual displacement (as Chryse said).

Keith Williams said: >>>FDXMECH, You appear to be talking about a type of servo tab system which becomes operational only in manual control mode. Most JAR ATPL training manuals discuss only the type of servo tab that is used in un-powered control systems.<<<

This might be the rub. My assumption, perhaps incorrect was that servo tab and control tab were interchangable nomenclatures.
If, "servo tab", is applicable only with normally unpowered control surfaces (eg, DC-9, MD80), then "servo tab" is wrong and balance tab is correct.

[fruitloop]

Keith Williams

In reply to the answer I can see your dilema unfolding to a very complex question.
My reasoning for selecting 'd' was in regard to the B737 (which has an indirectly powered controlled aileron system,that is yoke movement,captains to primary flight control,co-pilot to secondry,with-out bringing in loss motion devices for possible failure of either,then into a mixer box from either,onto hydraulic input selectors to the aileron output quadrants wich feed cable movement to a servo tab on the aileron.
In this circumstance would you consider it a powered flight control ??
Cheers

[FDXmech]

A powered flight control system is normally operated using hydraulic actuators to move the control surface.

A non-powered flight control system, such as on the MD80 utilize full time servo tabs (controlled directly from the yoke) as the primary means of moving the control surface.

The 737 has a "powered control system". Some aircraft have the actuators directly attached to the control surface. Some aircraft such as the 737 and 727 aileron system have centrally located actuators, in this case the wheelwell. The actuators because they are remotely located from the actual surface use cables/linkage to link up to and drive the surfaces. But they are powered.

[Keith.Williams.]

Fruitloop,

I am having a little bit of difficluty (cannot do it) in deciding how the system you describe enables the hydraulic selector valves to be moved without first moving the controls. Whatever the method, it is certianly not what is conventionally taken to be meant by a powered flying control system.

But the fact that it uses hydraulics means that it must be either powered or power assisted. At the end of the day this comes down to whether the hydraulics carry all of the flight loads or only part of them.

Getting back to the original question, the options include both balance tabs and servo tabs. From the descriptions above it appears that at least one aircraft (the 727) uses balance tabs and another (the 737) uses servo tabs. So both of these options can be said to be true.

I suspect that the examiner would say "ah but the syllabus is based on the 737 and the A310, so servo tab is correct because the 737 uses them". The best option is probably to tick servo tabs and lodge an objection on the basis that balance tabs are used for this purpose in at last one aircraft (it would probaly also be wiser to name it).

[Genghis the Engineer]

Given how many very knowlegeable people have given quite contradictory answers, not to mention a few good-natured disagreements about what the question really meant; I am forced to one conclusion.

A question worded like that has no place in a multiple choice exam.

G

[northwing]

Certainly it is a silly question if you don't define the aircraft since different manufacturers use different names for the same thing. Few modern hydraulically powered surfaces have any tabs at all, relying on redundancy in the hydraulic supplies and multiple control surfaces to ensure continued control after a failure.

Quite large aircraft can be successfully controlled with servo tabs alone. The Britannia had them and they worked OK, even if the ailerons did some pretty ridiculous things when revese thrust was applied. The Argosy also used them, hence its affectionate nickname - the Allsoggy. They tried them on the BAC 111 but came unstuck in the deep stall when the upwardly deflected elevator servo tab became blanked by the wing & tail at high incidence and failed to push the elevator down to effect recovery - hence the production hydraulically powered version.

[fruitloop]

I agree with Genghis's reply due to it being a "too type specific"answer.
Cheers

P.S. if you get it in a multi-choice exam,write on the front page your reasoning (CASA in Aus will respond with a possible senario of deleting the question from the total exam,that is no-one gets it wrong.)

[Keith.Williams.]

Genghis and Fruitloop,

You are both of course correct.

But students shouldn't run away with the idea that the examiners deliberately use misleading questions. I suspect it is more a question of competence!

The system by which students and schools can challenge faulty questions is usually effective. But it is based on the assumption that students can either identify such questions, or remember them accurately enough to enable their schools to identify the problem and lodge an objection. If it has achieved nothing else this string has made readers (myself included) more aware of this particular problem. It could of course be the case that it has already been corrected or deleted from the question bank.

[Genghis the Engineer]

I think this is indicative of a particular trend in CAA/JAA exams; that is, reliance upon understanding of terminology.

To coin another example, I got a question wrong myself a couple of years ago on a JAA license exam. The question was something along the lines of "is the gyro in an AI an earth gyro or a space gyro". Now, I have flight tested several such systems, wrote the apporoval reports for the AI in the Jaguar, did an extensive course on avionics and their assessment at ETPS, on a good day could design one, and for the life of me have no idea what this particular terminology means. No doubt some Janet-and-John textbook somewhere puts great store by the importance of this terminology but the fact is that it has no relevance to the purpose of the exam - that is determining whether somebody is safe in charge of an aeroplane.

In this case, it would appear that servo tabs are used differently on different aircraft, and different designers use the term in different ways. So basically it comes down to "preferred" rather than "correct" or even "important" use of terminology. Best thing would have been a system diagram and ask how the thing in the diagram works.

G