Pitch Authority
Join Date: Aug 1999
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No break out? No response from the disconnect handle?
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.
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.
Elevator Jam
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
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
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.
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.
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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!!
D P Davies interviews on certificating aircraft
I have also had the pleasure of flying with him, and he was equally robust!!
D P Davies interviews on certificating aircraft
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...
,
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...
,
Last edited by gums; 10th Apr 2020 at 19:27. Reason: typo
