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On August 9th, 2007 an Air Moorea Twin Otter crashed into the ocean. The cause of the crash was determined to be loss of pitch control after an elevator cable snapped.

In my world (Cessna 152), if an elevator control broke I would be pretty sad, but the pitch trim tabs would probably allow me to put the plane down safely somewhere.

As far as I could tell based on my research, the Twin Otter is equipped with a pitch trim tab (aircraft are required to have them, but they can come in a variety of forms). Why couldn't the pilot use pitch trim to descend safely?

--EDIT--

In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?

--EDIT #2--

In a highly hypothetical follow up question, would it have been possible, with 400' of altitude, for the pilot to go so hard nose down that he could pull out in an inverted configuration and fly the plane safely that way (the only cable that snapped was the elevator-down cable)?

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  • $\begingroup$ Something that hasn't been mentioned yet, there is a step in my post-landing checklist for C152/C172 to reset the trim. When one is landing one exerts backpressure on the yoke in the flare and roundout and some may trim that backpressure out. If this checklist item is missed, the "set trim for takeoff" will take out out this excessive trim. $\endgroup$ – Ed Griebel Feb 27 '15 at 18:53
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All quotes in this answer are from the English translation of the BEA accident report.

The elevator cable broke at "an altitude estimated at between three and four hundred feet",(p. 7) and there were eleven seconds between that time and the point of impact. If it had happened higher, the pilot would potentially have had time to figure out what happened and pull out of the dive with trim. Taken by surprise and not knowing what happened, the pilot simply did not have time to recover.

The Twin Otter would have been demonstrated to be recoverable from an elevator cable break as part of its certification (p. 17):

The CAR 3 regulation, basis of certification for the DHC6, requires a flight test demonstrating the aeroplane’s capacity to land using only the elevator trim in the case of failure of the primary longitudinal control of the aeroplane.

However, there's a big difference between "can be landed using just trim" and "can be recovered by a pilot who is surprised by the emergency". Flight tests performed during the investigation showed (p. 63, emphasis mine):

At [the height the incident aircraft was at], only immediate action on the trim located on the centre pedestal would make it possible to recover the airplane. The test also showed that from level flight it takes about three seconds for a pilot trained for and prepared for this exercise to recover the airplane.

Unfortunately, on this flight there simply was not enough time to discover the problem, figure out what went wrong, and recover (p. 63):

The highly dynamic nature of the events following the failure must be emphasized here. Eleven seconds passed between the pilot’s exclamation and the impact. This only left a short time for the pilot to analyse the situation and apply a solution that he had to improvise. In addition, the stress associated with the airplane’s attitude and the difficulty in estimating his height, in the conditions on the day, in relation to the surface of the water, certainly affected his powers of analysis. The pilot was not trained or prepared, either during his training or during type rating, as indeed most pilots aren’t, to react to a loss of pitch control. Only a reflex action could thus have allowed him to recover the airplane before the impact.


To answer the edits to the question, note that the airplane was not trimmed to "fall out of the sky". The pilot called out the trim checklist item before flight(p. 17), and so should have set it to the takeoff setting. Presumably this would require gentle back-pressure for rotation and climb. The event that initiated the failure process was retracting the flaps(p. 39). This changed the flight dynamics, and therefore the elevator control loads. The flight testing discovered that this created a "pitch-down moment"(p. 39):

To counter this effect and maintain the aeroplane on its initial trajectory, it was necessary to exert considerable pitch-up effort on the elevator control.

This is not a cause for concern. The procedure is to hold pressure on the controls and adjust trim to relieve the pressure. However, it was at this point that the elevator cable snapped, so the pilot was interrupted and didn't complete the process. If the incident had happened at a higher altitude, the airplane would have recovered itself from the upset (p. 42):

[S]tick free, the flap retraction led to a pitch-down moment varying the aeroplane trim between 20° and 30°, the rate-of-climb indicator going to stop at less than 3,000 ft/min and speed reaching 140 kt in twenty seconds, then the aeroplane recovered on its own in level flight in five seconds, the total loss in height being seven hundred feet[.]

To the last question, flying inverted would not be necessary for recovery. I do not know the maneuvering characteristics of the DH-6, but I am unsure that an outside loop could successfully be completed starting at 300-400 AGL.

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  • $\begingroup$ Thanks for the awesome answer! I edited the question because I am also curious about why we "set trim for takeoff" if that trim setting doesn't hold a level climb. $\endgroup$ – OneChillDude Jun 3 '14 at 5:22
  • $\begingroup$ @BrianWheeler: I don't know for sure, but I think the point of take off trim is to avoid the nose lifting off prematurely. And since you can't feel your C/G on the ground, it has to be somewhat down trim. You are supposed to retrim after take off when you can set the trim according the actual balance. $\endgroup$ – Jan Hudec Jun 3 '14 at 8:00
  • $\begingroup$ @BrianWheeler according to the "aircrash investigation" episode I saw the pilot just pulled in the flaps which pitched the plane down and when he pulled on the yoke to level out the cable snapped $\endgroup$ – ratchet freak Jun 3 '14 at 8:07
  • $\begingroup$ Answer edited. See if that answers your curiosity! $\endgroup$ – NathanG Jun 3 '14 at 21:15
  • $\begingroup$ @BrianWheeler it'd hold a climb maybe, but not if the other means for pitch control fail. In this case the elevator snapped to maximum descent deflection, more than compensating for the up-trim. $\endgroup$ – jwenting Aug 18 '14 at 6:43
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In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?

A major cause of accidents on take-off and landing is a stall followed by a spin. A wing stalls when it reaches the critical angle of attack, which is much easier to do at the lower speeds required on take-off and landing. If you set a nose-up trim you will need to use forward-pressure to keep the aircraft at a safe airspeed, if you then took your hands off the controls you could find yourself at stall speed in a matter of seconds.

It's better to require a bit of back-pressure to gain altitude, at least that way if you let go you will gain, rather than lose airspeed. Once you have an established climb speed and stable pitch attitude you can then trim for the climb.

Here's an example of how this can save your life. Imagine you are in your Cessna 152, 300 feet agl in a climb. Suddenly your seat rolls back all the way due to a broken seat lock (it's happened to me in a 152). It's much easier to put forward pressure on the controls in that situation than back pressure, and a bit of nose down trim would cancel out the sudden backward move of your C of G somewhat. Having nose up trim in that situation could potentially put you in a non-recoverable situation.

So take-off trim is actually designed to keep you from falling out of the sky.

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    $\begingroup$ The primary purpose of the trim tab in every airplane I've ever flown or heard of is to minimize the need to provide control input. It doesn't do anything more than aerodynamically put the elevator (or other control surface) in the position that it needs to be in for whatever flight demands there currently are. You can "fly" an airplane with trim tabs. But you're really just using the trim tabs to reposition the actual control surfaces. The Takeoff setting on a small aircraft is usually a fairly neutral position. But it changes with conditions on larger aircraft. $\endgroup$ – Shawn Jun 3 '14 at 19:10
  • $\begingroup$ Adding to this answer: "In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?" A second good reason for appropriate mechanical trim at all times is that it takes stress off the cables/actuators which will reduce the chances of breakage. $\endgroup$ – copper.hat Aug 16 '14 at 21:45

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