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Allegiant MD-83 Elevator Failure

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Allegiant MD-83 Elevator Failure

Old 31st Aug 2015, 16:27
  #21 (permalink)  
 
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Elevator jam or elevator trim tab jam?

Thanks for the education about MD-83 elevator control.

Given that the MD-83 elevators are tab driven, is it known whether the problem for this event was with the elevator itself or with the elevator tab?

Regardless of where the local breeze positions the elevators themselves when at the gate, the tabs should be positioned per the flight deck controls. Does the pre-flight walk-around include observing elevator tab position to confirm that they are consistent?

I fully agree with the comments that this crew saved the day by recognizing improper response to their control inputs and electing to abort the takeoff. My interest here is from a lessons learned perspective with an eye toward recognizing such a problem in the future before leaving the gate if at all possible. The next crew might not be so sharp and might elect to proceed with the takeoff. Sitting at the gate, would there have been any visible clue (either observing the tail from outside or observing the cockpit instrumentation) that this problem existed?
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Old 31st Aug 2015, 17:12
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Assuming the design of the servo tab - elevator system enables the elevator to be ‘picked-up’ at the extremes of the tab movement, then with a jammed elevator there could be some force feedback to the crew at the end positons of the full and free check. This might only be a small change in the stick force and this could be masked by an adverse wind.
IIRC the MD80 pre takeoff checks require a final control check on entering the runway – into wind; this requirement arose after an elevator jammed due to a stone being blown up by a preceding jet entering the runway.

A jammed tab should be identifiable via a stick restriction or jam. It is unlikely that a visual check could be relied on as a safety check, even if the tabs could be seen (dark wet night).
If a tab is disconnected then it and the elevator should assume a free air trailing position, but this could change the overall elevator effectiveness such that the takeoff rotation trimmed condition is biased nose up.

The design requirements for the primary control systems require dual paths and an ability to separate them in the event of a jam. Thus a worst-case pitch-up elevator jam during rotation should be controllable via the other elevator; demonstration of requirements (CS 25-671) should enable the takeoff to be continued after V1 and that the aircraft can be safely manoeuvred for landing.
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Old 31st Aug 2015, 18:14
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@safetypee

I don't think there's a force feedback based on the elevator position per se. The feedback is based on trim position, to give (an artificially) heavier feedback when the aircraft is trimmed for high speed.

A pilot cannot "feel" a jammed elevator (vs a jammed control tab). (See incident below).

@FCeng84

In general (on any aircraft) control surfaces including tabs are checked during pre-flight. But the level of checks will realistically vary. Most visual checks might only detect obvious damage, or when things are seriously out of alignment.

Plus checks for the "first flight" of the day might be more thorough than checks during a quick turnaround, especially if there were no issues noted during the previous flight. Yet a maintenance or other mechanical issue can manifest itself at any time.

Also on the MD-80 series there are actually three different elevator tabs on each elevator. There is the main "control" tab, a second tab geared to the control tab, and a third "anti-float" tab geared to the entire horizontal stabilizer. No one will check all six tabs for correct operation during pre-flight.

(All this is assuming the problem is with one of the tabs -- but the jam is equally likely to be with the elevator itself. Because the elevators are free-floating, they are prone to damage if there are strong winds / gusts on the ground.)

In fact there was a previous incident where an MD-80 had a high-speed RTO because one of the elevators was jammed in the nose-down position. So in this case the plane could not rotate at Vr. Apparently the elevators were exposed strong tailwinds on the ground (100 mph gusts) causing one of them to jam past the nose-down limit.

In that case, both the maintenance crew and the pilots had checked the elevators prior to the next flight by exercising them from the cockpit. Unfortunately, as noted above a jammed elevator feels the same as a normal elevator so they could not detect the problem until the rejected take-off.
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