Friday, September 16, 2011

Reno Crash - Trim Tab to Blame?

It is, of course, very early to speculate on the cause of the tragic crash at Reno. A disciplined examination of photos, however, reveals a possible clue.


This photo was shot by Tim O'Brien of the AP and Grass Valley Union just a moment before the crash. A sharp-eyed Russell Farris spotted an elevator trim tab that is either missing or severely deflected. If the trim tab was deflected downward at such an angle, the elevator would have been forced upward with a great deal of force. This would severely pitch the nose upward, likely with enough force to overpower even the strongest pilot.

In such a scenario, the pilot would push forward on the control stick with all his might to arrest the pitch rate. If he was unable to do so, the nose would continue to pitch further and further upward until airframe failure or ground contact occurs.

It's important to bear in mind that at the speed he was flying today, a relatively small control surface deflection would have resulted in an abrupt and extreme load factor ("G-force") on the pilot, making recovery that much more difficult.

Here's hoping the reaction of the public and media doesn't put an end to the Reno Air Races. You can bet that none of today's victims would have wanted that.

10 comments:

  1. While I could see a deflected trim tab causing problems, how would a missing one cause a crash?

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  2. A missing one...particularly at that speed...could cause flutter. And flutter can be deadly.

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  3. Steve (not the one above)September 17, 2011 at 3:27 AM

    Jason I do not believe you are fully aware of what you are talking about.

    Are you saying a tiny little trim tab could force a massive piece of elevator into a 400mph airflow that would actually help the pilot not hinder him??

    Could it be that a control jam or pilot input to excess above Va might have caused a tiny little trim tab to come off?

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  4. There was a pilot a few years back that had a runaway trim tab which put 12 g's on the aircraft and blacked him out, he recovered and landed the aircraft. I would bet that tab was deflected causing the problem.

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  5. Steve (not the one above) - I'll be the very first to admit that I'm no professional, and am indeed simply an "armchair investigator".

    That said - regarding your second sentence...yes, that's precisely what I'm saying.

    Did you know the DC-9/MD-80 series of airliners is designed in such a way that the cockpit controls aren't even connected to the elevator and ailerons?

    The pilots can ONLY control smaller "control tabs". These smaller control tabs deflect the larger elevator and ailerons.

    In fact, because the elevators freely float and because they are positioned by airflow over the control tabs, you can often see one elevator positioned differently than the other. This occurs when the aircraft is parked or taxiing, with no relative wind over the control surfaces.

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  6. From McDonnell-Douglas:

    ELEVATOR AND TAB - DESCRIPTION AND OPERATION

    3. Operation
    A. Elevator Control System (Ref. Figures 1 and 2)
    (1) Forward and aft movements of the control column are transmitted through cable systems to the elevator control tab drive sectors and torque tubes, which in turn move control tab pushrods and elevator power control followup mechanism linkage. Movement of the tab pushrods changes the position of tabs. Aerodynamic force acting on the tabs move the elevators. Changes of elevator position relative to the horizontal stabilizer causes the geared tab linkage to move the geared tab in the same direction as the control tab.
    This action provides additional aerodynamic assistance to move the elevator in controlling the airplane about its pitch axis. When the horizontal stabilizer is positioned greater than 10 degrees (0.175 rad) aircraft noseup, linkage from the horizontal stabilizer to the elevator anti-float tabs drives the tabs, trailing edge, down thus preventing elevator down-float.
    (2) Forward movement of the control column of sufficient travel to move the control tabs 10 degrees (0.175 rad) or more, will cause the elevator power control mechanism linkage to actuate the hydraulic control valves to the tab up position.
    Pressure is applied to the boost cylinders, driving the elevators downward. Movement of the elevator is transmitted through the followup linkage which returns the control valve to neutral position, when the elevator reaches the desired position.

    (3) Since left and right elevator surfaces are not interconnected, an asymmetric condition of the surfaces is possible during airplane ground operations. Opposite deflection of elevator surfaces is sometimes caused by gusting tail or cross winds. This condition may result in blocked operation of the elevator control column. Elevator surfaces can be freed by: (1) Heading aircraft into wind or taxiing aircraft and simultaneously applying force to the control column; or (2) manually moving elevator control surfaces to the faired position.

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  7. I noticed also on a subsequent photo of the aircraft from the same AP photographer that the tail wheel was extended, and the pilot seem to pushed down on his seat. The photo is from yahoo photo gallery: http://tinyurl.com/3le3we4

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  8. Notice how apparent the pilot's helmet is normally, in this photo of the same aircraft during taxi for take-off prior to the crash, as seen in this yahoo photo gallery: http://tinyurl.com/3lzlw7l

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  9. If the trim tab has detached at high speed, it is most likely the control column will have moved violently rearwards pitching the aircraft upwards with high G - possibly blacking the pilot out. Even if he'd been concious, unless he was able to reduce speed, the control forces would have been overpowering.

    I've had an elevator trim runaway (not detachment) an exercise in the simulator - two of us with feet on the column mde the aircraft barely controllable.

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  10. Bob Hannah, an interview about a similar situation, said that flutter took the trim tab off his Voodoo and the lack of trim cause an abrupt pitch up and he blacked out. He said he had his belts loosened a bit to be able to better reach the instrument panel. That let his upper body bend forward and the G force pinned his upper body to his lap, but fortunately the planes attitude had it in a climbing spiral. He came to a little bit and could not sit back up. He managed to get his hand up to the throttle and pulled the power back and freed himself up. Still a little groggy he thought he could get back in the race, but soon realized he had a real problem and maydayed out. He retired from air racing shortly after that.

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