Pitch Authority

[Mr Optimistic]

I wasn't sure where to post this thread (not too many suggestions pls) so hope it doesn't annoy here. Question is whether elevators always have sufficient authority in a/c with below wing engines to overcome pitch up tendency from any commanded increase in thrust, even ill advised ?

[BOAC]

No - look at Airbus PGF, 737 BOH, BA 747 LHR and THY AMS for starters.

[Mr Optimistic]

...that was a quick education !

[Chris Scott]

To develop the argument a bit:

In most cases, the problem arises when the all-moving horizontal stabiliser (tailplane) is delivering a pitch moment in the opposite direction to what the pilot needs for the new thrust setting. The elevator authority can then sometimes be insufficient.

One example of this, on a conventional jet transport, is a go-around from VREF at full flap. The pilot (or AP) has to push like mad, while simultaneously trimming the stabiliser from a very high pitch-up-trim setting to a more neutral one.** If the speed has been allowed to get below VREF, loss of pitch control may result. In older types, it was possible to "stall" the stabiliser motor, because of the enormous aerodynamic forces involved as the elevators oppose the stabiliser.

Fortunately, it should usually be possible to recover by reducing thrust a bit...
usually gives far more performance than necessary. Airbus (FBW) FMGCs do not reconfigure from Approach mode to Go-Around mode unless TOGA thrust is selected (i.e., pushing the throttles fully forward). However, this does not physically prevent the pilot from immediately throttling back to a lower thrust setting, e.g., Climb thrust.]


** Airbus FBW
In the non-autopilot case, this necessity for the pilot to push and trim forward does not apply in Airbus FBW types, even though they are of a similar configuration aerodynamically. Once the pilot has rotated to the go-around attitude, the appropriate amount of down elevator will be selected by the FBW system, with the side-stick neutral. At the same time, the system will auto-trim the THS (trim-able horizontal stabiliser), rather like CWS systems on some 1970s aeroplanes. The only indication of the down-elevator will be on the F/CTL page of ECAM.

[Mr Optimistic]

Thank you. Just been reading about the 737 BOH 'incident' as suggested. 44 degrees pitch up, which I assume was the sort of thing alluded to. Don't want to exhaust your patience, but in cruise, if the a/c approaches stall, is there any similar limitation or will pushing fwd always get the nose down ?

[BOAC]

Mr O - I do suggest reading the other incidents as well as your question is well covered there. You need to define 'approaching the stall' for a complete answer, as you will see.

[A37575]

but in cruise, if the a/c approaches stall, is there any similar limitation or will pushing fwd always get the nose down ?

I doubt there is a problem in this regard because the stab trim setting in cruise is close to neutral. If the autopilot is engaged and for some reason the thrust levers have closed then the autopilot will steadily apply back stab trim until the crew discover the approaching stall situation. Even then as the aircraft is clean in cruise there should be plenty of elevator available backed up by a hasty burst of forward stab trim I should imagine.

The important point to make with regard to stalling with land flap with autopilot engaged and possibly thrust lever closed (Turkish Airlines B737-400) is that the pilot must rapidly apply forward stab trim at the same time GA thrust is applied and nose down elevator takes place

If the pilot hits GA and initially tries to lower the nose by elevator alone he may quickly run out of elevator and by the time the nose has pitched up to a dangerously high attitude, the aircraft may stall before the pilot can get on to the stab trim. This is a important exercise in the simulator where the event is timed to take place below 500 feet and thus ground contact is imminent as Boeing say. The real danger is if someone calls for Flap 15 while near the stall as part of the GA (737). It is not a normal go-around procedure but recovery from a stall and that is a different technique.

[Mr Optimistic]

...to all for your answers. Hadn't appreciated the authority of trim.Will read up.

[Spendid Cruiser]

Question is whether elevators always have sufficient authority in a/c with below wing engines to overcome pitch up tendency from any commanded increase in thrust, even ill advised ?
I tried the Turkish AMS scenario in the sim the other day. To recover from stick shaker with full thrust requires full forward control column is required just to stay at a level pitch. It takes several seconds to trim out the forces. But this is enough to take it out of stick shaker almost immediately.

[Daysleeper]

I tried the Turkish AMS scenario in the sim the other day. To recover from stick shaker with full thrust requires full forward control column is required just to stay at a level pitch.

Different model of 737 (-300) but the BOH incident aircraft continued to pitch up even with full forward column.

[BOAC]

We are in danger of re-inventing the wheel here. This is NOTHING NEW - it has been a factor on the 737 (for one) and probably all low-slung jets since Pontius was a pilot. We have several threads on it. All training departments and manufacturers SHOULD be now emphasing it. All pilots who need to be should be aware of it. I'll even stick my neck out and say you can trim ANY trimmable tailplane aeroplane into a position where the elevators cannot cope, with or without low slung engines.

Let's not go round again in PPRune circles on it, please?

Easter grump over.

[Checkboard]

It's a particular problem with a movable stabiliser, as these trim systems are very powerful compared to elevator authority.

With a variable incidence tail, however, as the C.G. is moved over comparatively large distances, the incidence of the tail is altered to provide a balancing force and the elevator remains in the streamlined position. Because the tail area is much larger than the elevator area the tail can be moved through a smaller angle to produce the required balancing force; and the elevator, always being 'neutral' to the tailplane, remains available over its full range at all times. This large increase in balancing forces available from a variable incidence tailplane, together with good pitch control from and unrestricted elevator, makes a large C.G. range a practicable proposition.

...

Summary
In dealing with the consequences of having a variable incidence tailplane one basic fact must be kept in mind - it is very powerful. Because the elevator, when in trim, is always slipstreaming the rail it remains available over its full range and can be smaller than the elevator on a fixed tail aircraft. This is simply because the stabiliser can be set to handle the bulk of the demand and the elevator remains to look after the rest of the demand. On a variable incidence tailplane aeroplane, therefore, the elevator is smaller, and consequently less effective in isolation than it is of a fixed tailplane aeroplane.

D.P. Davies HANDLING THE BIG JETS, Third Edition
pages 37 & 39

First published in 1967, so this information isn't new! :hmm:

[411A]

First published in 1967, so this information isn't new!

Indeed not...and is one reason every new jet transport pilot should have it assigned as required reading, without fail....in my opinion.

[Chris Scott]

Quote from BOAC:
We are in danger of re-inventing the wheel here. This is NOTHING NEW - it has been a factor on the 737 (for one) and probably all low-slung jets since Pontius was a pilot. We have several threads on it. All training departments and manufacturers SHOULD be now emphasing it. All pilots who need to be should be aware of it.
[unquote]

BOAC,

I agree with your first two sentences, take your word for the third, agree with your fourth, and think you are being presumptious in your final (underlined) point.

You are one of the most experienced and long-time contributors to PPRuNe; a pillar of this Forum. Rather than admonishing some of us newer guys for starting/contributing to a fresh thread on something YOU (and, as it happens, I) regard as old-hat, how about giving us a few links? So far, my use of Advanced Search has been unsuccessful, so maybe others are also giving up... :ugh:

On a more serious note, there is an expression somewhere about "LESSONS (NOT ALWAYS) HANDED DOWN" that springs to mind. Not everyone on this Forum has the benefit of our experience, the best training departments, or the amount of spare time to trawl through the PPRuNe archive.

Chris

[Mr Optimistic]

..for the answers. I am not aircrew or a simmer and BOAC was merely pointing me in a better direction.

[BOAC]

Chris - using PPRune search is often fruitless since a minimum of four letters is required, and since aviation revolves largely around 'TLA's - work it out for yourself! There is a way around the 3 letter limit via Google. "ba747+lhr+pitch site:pprune.org" produces d)

As has long been know, learning is more effective when work is done by the student, and kudos to Mr O, he did indeed research for himself, and I'm sure now is better briefed on the issues.

For those who cannot or will not search, some Google suggestions which work:

a) Airbus crash Perpignan (or PGF)
b) Thomsonfly 737 Bournemouth
c) THY 737 crash Amsterdam

and from PPrune
d) http://www.pprune.org/rumours-news/124808-ba-747-100kts-stick-shake.html

The first 3 will also take you to the PPRune threads.

Regarding ".All pilots who need to be should be aware of it." - NB 'should' - I'd be very surprised if there was a 737 qualified pilot who was not?

[Chris Scott]

BOAC,

Thanks for the tips re Google. The other problem I've had with the PPRuNe Search is that it can fail due to the presence or absence of a space or hyphen (as in DC10/DC 10/DC-10). Google seems to be more lenient (not always an advantage).

Think you have shown that it isn't an easy task to resurrect - maybe to add to - these old threads, much as one would like to for the sake of continuity (as well as minimising the number of references).

This stuff is particularly relevant to copilots who have started their RHS careers on Airbus FBW types, and may go on to get their first commands on B737s. For those who have yet to fly jets for real, it may help to hear it fresh from the horses' mouths.

[BarbiesBoyfriend]

Two weeks ago I flew an RJ-85. After seeing some strange behaviour from the AP, I switched it orf.

Imagine my surprise in discovering that there was no way to move the yoke fwd or aft.

The pedals worked and the ailerons worked. In pitch it was welded.

Flew it with trimmer.

[john_tullamarine]

One of the problems not yet mentioned .. and it applies to either prop or jet ... relates to the normal pitching force (ie a vertical force perpendicular to the local airflow direction) associated with the airflow deflection through the prop or nacelle inlet plane.

Problem can be a handful

(a) at high pitch angles (read missed approach especially with poor speed control)

(b) at high thrust settings

(c) for piston to turboprop conversions where the prop is pushed out forward to make the CG aspects a bit easier

when the normal force provides a very destabilising nose up pitching moment.

For the conversions, one sometimes sees the need for a SAS system to keep the stability under some semblance of control .. the expected pull force to keep below the trim speed can end up becoming an unacceptable push force ..

In the field, it probably is a good idea to transition to landing configuration missed approaches (ie lowest speed and potentially highest pitch angle) in a steady, measured manner to facilitate re-trimming .. in particular, without a dramatic slam acceleration of the engines to high thrust settings which can result in the pilot's becoming a one-armed paper hanger ...

[p51guy]

I always controlled power input in coordination with trim to not let power cause excessive pitch up in a go around situation. It just takes a bit of monitoring for a few seconds. Letting go around power cause control pitch problems is very poor airmanship.

[Capt Pit Bull]

BB

Two weeks ago I flew an RJ-85. After seeing some strange behaviour from the AP, I switched it orf.

Imagine my surprise in discovering that there was no way to move the yoke fwd or aft.

The pedals worked and the ailerons worked. In pitch it was welded.

No break out? No response from the disconnect handle?

[Chris Scott]

BarbiesBoyfriend,

You have only told us the beginning of the story, but - taken on face value - the following springs to mind:

(a) The incident is so serious that you must have filed an MOR, or equivalent.

(b) Assuming you are prepared to discuss it further on this forum, IMHO it would certainly merit a thread of its own. This one is mainly concerned with serviceable aircraft, and trim changes caused by changes of thrust.

(c) Am I right in saying that the RJ-85 has a fixed tailplane (horizontal stabiliser), and that pitch trim is by elevator trim-tabs? While using trim to control pitch, were you able to move the trim wheel in the correct sense, or did you have to use it in the reverse sense?

(d) Were you in freezing conditions, and had you taken off with a rain-saturated tailplane?

(e) How long did the problem last?

Regards, Chris

[safetypee]

In the early days of the Avro RJ, the autoflight system was modified after an interesting (serious) event involving a 3000ft+ pitch up and a couple of impressive wingovers. This was due to an inadvertent autopilot engagement and subsequent overpower – back driving the autotrim nose-up to a point where full forward stick (position and force), could not maintain control. Manual trim did not ‘appear’ to work because unbeknown the autopilot was engaged.

IIRC the autopilot automatic disengage logic now contains displacement/rate/duration logic (trim and servo torque) to prevent hazardous overpowering, but this is not intended to be an emergency disengage function. Disengagement is via one of two stick top buttons (dual paths), or an emergency switch on the control panel.
Again during the aircraft’s history the AP disengage buttons were modified to reduce inadvertent disengagement (white knuckle syndrome), by fitting a higher guard or lower button, not sure which. However, for some people this made disengagement more difficult (fat thumb syndrome) and thus the necessity to remind pilots to look (and hear) confirming indications of AP disengagement.

AFAIK the failure of a AP servo to disengage would be annunciated with a permanent red AP light or ‘normal’ indications that the AP was still engaged, in either case the panel switch should be used.

If the autos have disengaged, then a ‘conventional’ control-system jam might be suspected. The pitch-control system can be split by overpowering with the other stick or using one of two emergency disconnect handles.
The elevators are servo-tab controlled and are free floating about the fixed horizontal tail; thus, a stick jam requires a restriction in the stick/cable run (autopilot servos-motors are in one cable run).
If both servo-tabs had jammed, the stick / elevator should still move, but with very high stick forces - normal aircraft motion. An autopilot servo-motor ‘jam’ would be like a single servo-tab jam as the AP only drives one elevator servo-tab; limited normal aircraft motion (and roll from the single elevator).

For both of the (independent) elevators to freeze requires a significant icing event (or rags around the elevator hinge – it happened). In this circumstance, the stick should still move and displace the servo tab, and even in the extreme of both servo-tabs freezing as well, then the stick should have some movement against the servo springs, but without aircraft motion.

There is an independent elevator trim system (trim tab) directly to the elevators.
Stick jam – elevator free, then the trim moves the aircraft in a conventional sense. Stick jam and both elevators jam, then trim has a small effect on aircraft motion but in a reversed sense.
(It’s late in the day and with tiring mind, I reserve the right to revise/reverse the above).

The RJ has a highly redundant manual control system; failures require a bit of force to ‘break-out’ and split the system, but thereafter fly whatever remains active.

[ced0802]

It's a particular problem with a movable stabiliser, as these trim systems are very powerful compared to elevator authority.

What do we call "elevator authoruty"?

[Bergerie1]

I totally agree with 411A's post regarding D P Davies' book 'Handling the Big Jets', it is one of the best compendiums of aviation wisdom I have had the pleasure to read - accurate information, forthrightly expressed. May I also recommend this link here, where he discusses some of the very interesting issues he encountered when certificating aircraft.

I have also had the pleasure of flying with him, and he was equally robust!!

https://www.pprune.org/tech-log/602953-d-p-davies-interviews-certificating-aircraft.html

[gums]

Salute!

First, I gotta agree with Chris that "Barbies' friend" needs to make some noise. Just imagine if the Lion Air crew that fought MCAS for miniutes until turning off all electric trim would have crowed loudly, and soon. Even let the next crew know.

Second, I also agree with Chris that Optimistic's topic needs to be brought up every now and then, and ditto for other fundamentals of the aerodynamic characteristics of the planes that most here fly ( versus the fighters that I flew, and that others here flew before moving to the heavies).

Mostly I wish to agree with BOAC that small elevators and big stabs can easily create problems whereby you run outta pitch authority in short order. Sometimes too short. For this characteristic of a plane, I go to the designers and then the test folks to define the limits and either you never run outta authority or you can only run outta authority in exremely adverse conditions. And let us all know what those conditions are and how to avoid them.

For the newbies that did not participate in the 447 threads, if you see my vita on the profile ( one of few with actual experience and such, I might add), I only flew two planes after training that had fixed stabs and real elevators. The other four had all-moving stabs ( elevons in the Deuce) and were hydraulically actuated with zero mechanical feedback. Zero. As with the other two planes, I used the trim system, and they used electric motors and switches, not cables or pushrods. And as BOAC, Chris, JT and others here of lore, I trimmed for an attitude or AoA. And as others here have said, trimmed as needed when going around, or yanking throttle back, resulting in healthy changes in pitch moments. But I never had a huge stabilizer with many times the pitch authority of the elevator - I didn't have a steeekeeng elevator! Rant ends...

Gums sends...
,

[Pugilistic Animus]

DP Davies- Handling the Big Jets it's all there