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Wikipedia says that the control surfaces were locked.

What does this mean, and if the control surfaces cannot be moved, how did the aircraft get into the air in the first place?

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The prototype aircraft had small "blocks" (more like wedges) which were used to prevent the control surfaces moving in the wind and potentially damaging the control surfaces, or even moving the aircraft on the ground. A bit like putting chocks under the wheels to stop the aircraft rolling.

These were not removed when the aircraft was taken on a test flight, so the control surfaces could not move. Essentially, they were "fixed" in an approximately neutral position. Imagine if you couldn't move your steering wheel in your car because someone had put wedges next to your front wheels to prevent them turning: that's basically the situation the pilots were dealing with, but unlike you they can't just hit the brakes and stop once the aircraft is in the air.

The Boeing 299 / B-17 are "taildragger" aircraft.

As you can see in this photograph which I won't link directly due to the size, the aircraft's nose is pointing quite sharply "up" when on the ground.

Here's a B-17, which isn't identical to the Type 299 but is close enough that it makes no difference. Hopefully this gives you the visual to see why the aircraft will lift off at a high enough speed.

B-17

When accelerating the nose being pointed slightly up gives a positive angle of attack for the wings. This means that once the aircraft gets above stall speed, it will naturally lift off into the air. Essentially, the aircraft is already pitched slightly up before any control surfaces are moved.

That also means that when the aircraft takes off, it will climb - at least initially. With the locked elevators, there is no way to change the pitch angle of the aircraft, and the aircraft will either pitch up or down naturally - there is almost no chance that the aircraft would be perfectly trimmed by accident.

In this case, the position the elevators were locked caused the aircraft to continue pitching up until it stalled. The crew had no way to correct this and the aircraft stalled out, rolled and crashed into the ground.

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    $\begingroup$ This has absolutely nothing to do with being a tail-dragger! As the aircraft picks speed and the wings start to generate lift, the aircraft will pitch down, since the centre of lift is behind the main wheels. If it is trimmed for low speed (nose up), it will remain nose up and lift off, but if it is trimmed for high speed (nose down), it will happily continue rolling on the main wheels with tail raised. $\endgroup$ – Jan Hudec Apr 11 '16 at 17:45
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    $\begingroup$ Yes, low mounted engines cause a pitch-up moment when running. But while that makes the problem worse, it does not create it. If the trim speed is above stall (in ground effect) and within what the plane can reach on the ground, it will lift off without control input and whether it has tricycle or conventional gear does not matter. $\endgroup$ – Jan Hudec Apr 11 '16 at 18:08
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    $\begingroup$ If B-17 lifts off in almost 3-point attitude, it is because it corresponds to the appropriate angle of attack. But if you trimmed it nose-down, it would still raise the tail and continue rolling on the ground and not lift off. Whether it lifts off depends on the trimmed speed, not type of gear. $\endgroup$ – Jan Hudec Apr 11 '16 at 18:12
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    $\begingroup$ I admit there is one difference, but it is in when the crew could notice something is wrong. In tail-dragger the crew is waiting for the tail to raise before doing anything, so they won't realize it is going to lift off early before the wheels are off the ground. In a tricycle, the crew expects the aircraft to only start rotating on positive control input, so when it starts raising the nose early, they will try to keep it on the ground and thus get a chance to cut the engines before they gain altitude. Still, it does not make the situation exclusive to tail-draggers, merely more dangerous. $\endgroup$ – Jan Hudec Apr 11 '16 at 18:19
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    $\begingroup$ @reirab this accident was one of the main catalysts for the introduction (or at least increased use) of the pre-takeoff checklist. $\endgroup$ – Jon Story Apr 11 '16 at 18:25
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If you follow through the links to the one on Ployer Peter Hill the B-17 (Model 299) test pilot, it says this about his death:

On October 30, 1935, Ployer Peter Hill died as a result of injuries received from the crash of the Boeing experimental aircraft Model 299 at Wright Field. The crash occurred because the crew neglected to remove the devices intended to keep the control surfaces from moving when the plane was on the ground. This aircraft was the prototype of what would later become the famous B-17 Flying Fortress of World War II. Major Hill was buried in Newburyport, Massachusetts, on November 3, 1935.

So basically they forgot to remove the gust-lock devices. Yet another case for doing a thorough pre-flight check and control check before taking off.

I can't find any evidence or anything to suggest the aircraft ever left the ground, however its possible for the aircraft to become airborne just through the amount of lift generated at higher speeds. Many airplanes will fly even with slight nose-down attitudes. Its also possible that the gust locks still allowed enough elevator movement to get off the ground. The link that Federico provided seems to suggest that the elevator was in the up position and allowed the aircraft to climb into a stall, but still not completely clear since most tail-draggers require some down-elevator to get the tail up, then up elevator to climb.

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    $\begingroup$ also: Yet another case for doing a thorough pre-flight check and control check before taking off. pre-flight checks were not a thing at the time. and, to say it all, they became a thing because of this accident: thisdayinaviation.com/30-october-1935 As a direct result of this accident, the “check list” was developed, now required in all aircraft. $\endgroup$ – Federico Apr 11 '16 at 15:03
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    $\begingroup$ Worth noting that a tail-dragger aircraft like the B-17 will generally lift off naturally when accelerating due to the nose-up attitude. This source suggests that the aircraft did indeed lift off before stalling: code7700.com/mishap_boeing_model_299.html $\endgroup$ – Jon Story Apr 11 '16 at 15:32
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    $\begingroup$ Please don't click on this link unless you understand that it shows an actual aircraft crash in which people died. This video is a particularly horrific example of what happens when the gust locks are not removed. $\endgroup$ – Simon Apr 11 '16 at 17:29
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    $\begingroup$ @JonStory, tail-draggers will often be trimmed to lift off naturally, because they are trimmed to keep the tail on the ground until they pick up enough speed for the rudder to be efficient. But the nose-up attitude is only reason indirectly by motivating that default trim setting. Any aircraft will assume the trimmed attitude before lifting off. $\endgroup$ – Jan Hudec Apr 11 '16 at 18:38
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    $\begingroup$ To be more precise, it was this accident that caused WRITTEN checklists to become universal. Pilots did preflight checks before this accident, but they relied on memory to tell them what to check, and this particular pilot had a brainfade on that particular day, with bad results. $\endgroup$ – John R. Strohm Apr 11 '16 at 20:38

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