Monday, January 23, 2012

Vans RV-7A In-Flight Break Up

Edit: On September 20, 2013 another Van's RV 7 was lost in an accident that seems to have been the result of the loss of the lower rudder in flight. The NTSB preliminary report is available here.

Prior to the inception of Safety Management System (shortened to SMS) safety was improved primarily by investigating accidents, determining a cause and enacting regulations, procedures or policies in an attempt to prevent the cause from resulting in more accidents. Safety Management Systems is designed to provide policies and procedures that help us act in ways that prevent accidents by identifying risk and acting in advance. This holds the promise of continuing to reduce preventable accidents; but SMS must be used in order to work. We also must still pay attention to the lessons of accidents past.

One very expensive lesson was learned at the cost of fourteen souls on board Continental Express 2574 September 11, 1991. Due to a maintenance error, the leading edge of the left horizontal stabilizer was not properly secured. During descent for landing the leading edge broke off the stabilizer rendering the Embraer 120RT uncontrollable. The crew lost control, the airplane entered a steep nose down attitude causing negative G loads that exceeded the structural limits of the airframe which broke apart and crashed in a farmer's field.

That such a small part could play such an important role in the control of an airplane may seem unimaginable to the those unfamiliar with aerodynamics; there are few extraneous parts on an airplane.

As all the other pilots of my generation I learned that airplanes have a designed maneuvering speed. This is the maximum flying speed at which the full deflection of controls will not over stress the airframe. While not a conscious conclusion, I'm sure I would have made the same assumption that the co-pilot of American Airlines 587 did on November 12, 2001. The flight encountered wake turbulence from a heavier airplane. During recovery from the encounter the co-pilot's cyclic rudder reversals, using rudder control in one direction followed quickly with rudder control in the other direction, over stressed the vertical stabilizer (or fin) of the flight 587 Airbus A300 resulting in separation. Without the fin and rudder the airplane could not be controlled. The cost of this lesson was 265 killed, five of them on the ground.

This brings us to January 23, 2010. Three  pilots fly their home built airplanes to a small airport near Linsay Ontario. This airport is a popular spot to fly to. A well travelled highway passes very close to the airport. So in addition to a vibrant local flying community, the airport boasts a full service restaurant popular with motorists as well as fliers. The three departed for Smiths Falls together, but on the way one broke off from the group headed to Bancroft.

The other two continued towards Smiths Falls in tandem. Along the way the lead airplane conducted a series of aerobatic manoeuvres, the tandem airplane, equipped with a video camera, was to follow and record the lead aircraft. During this phase of flight, the lead pilot lost contact with the accident airplane. A search was conducted which located the crash site. The fin and rudder were found to have separated in-flight landing 0.6 nm from the rest of the airplane.

The accident airplane was equipped with an electronic flight information system (EFIS) which recorded information from flight and engine instruments at 5 second intervals. This data, and the video recorded during the flight were recovered during the accident investigation. During the aerobatic sequence, following a rapid descent, the video recorded the onset of vibration around the longitudinal axis, followed by yaw, roll and ground impact. This is consistent with structural failure and separation of the fin. Without the fin, the pilot would be unable to control the airplane.

From the TSBC report for the accident airplane:
The maximum manoeuvring speed (Va) of 124 knots is the maximum permissible speed at which full and abrupt controls can be applied. Any speed in excess of Va with full control application could result in g-loads in excess of design limits.
The never exceed speed (Vne) of 200 knots is the maximum permissible speed under any condition. Any speed in excess of this could result in structural damage. Full control application at Vne would produce a load of approximately +15.0 g.
The EFIS record of the flight contained some spectacular numbers. Prior to the aerobatic sequence the airplane was level at 2650 feet above the ground travelling at 168 knots. The maximum recorded flight parameters were: vertical acceleration 3.5g; roll 115°; pitch up 19°; pitch down 45°;descent rate 12,000 feet per minute; airspeed 234 knots. The airplane was flying far faster than the maximum manoeuvring speed prior to entering the aerobatic sequence, and surpassed the never exceed speed during the sequence.

Much of the accident report focuses on the airplane paint scheme which was added after the first flight. Adding weight to a control surface can change the balance of the control adversely affecting protection from flutter:
Flutter is the rapid and uncontrolled oscillation of a flight control resulting from an unbalanced surface. Flutter normally leads to catastrophic failure of the structure. Due to the high frequency of oscillation, even when flutter is on the verge of becoming catastrophic, it can still be very hard to detect. Factors that can contribute to the onset of flutter include high speed, a reduction in stiffness and a change in mass distribution.
Even if the control balance did not bring on the flutter, deflection of the rudder too much when operating above manoeuvring speed, or even small amounts when above the never exceed speed could have stressed parts of the airplane to the point that structural integrity was compromised. In the end it does not matter if the fin separated because of progressive failure, resulting in vibration; or the early onset of flutter cause by imbalance caused structural failure, the results would be the same.

The cost of this lesson is, by some measures, less than the other two. The pilot and sole occupant was the only casualty. The lessons are just as valuable. Flight safety is often a mater of establishing margins between the known safe envelope, and catastrophic failure. Quite often we can cross into those margins and return safely. However, each action that narrows the margin takes us closer to catastrophe.

Fly safely.

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