Recent British Recommendation May Prove Useful

In the aftermath of the crash of an Avid Speedwing a year ago (May 11), the Air Accidents Investigation Branch has had another look at what could be a problem with certain Avid Flyer Speedwings (and perhaps other, similar models).
The pilot and his passenger were badly injured in the accident, which the AAIB seems to believe was really the result of low airspeed. However, this design has what the AAIB considers a weak link. It would behoove owners to check. Here's what the British inspectors say:

Vulnerability of pitch control linkage

Although the fracture of the pitch control attachment fitting to the control column torque tube was assessed as post-accident, it was not possible to discount some measure of pre-impact bending of the threaded rod. Bending could have occurred because there was no elevator travel stop in the aircraft nose-down sense. After the accident the elevator was free to move to about 80° trailing edge down but it should not have done so before the accident because of the geometry of the pitch control mechanism which is illustrated below in a diagram supplied by a PFA inspector.
This mechanism would have limited the elevator travel to less than 80°. The diagram shows that the limit of elevator down travel is reached when the push-pull tube hits the control column torque tube. Any forcing of the stick or the jolting of the push-pull tube (due to forces generated at the elevator) tends to bend the threaded rod. Opportunities to bend the rod might occur through careless stick positioning or forcing whilst entering or leaving the cockpit, or when the aircraft was being towed, particularly over rough terrain, although G-BUSZ had not been towed since its annual inspection.
Over-travel of the pitch control mechanism is a known problem with the Avid Speedwing. There have been at least three instances of failure or significant bending damage of this threaded rod end. Fortunately all three instances were detected on the ground, although, apparently, none were reported to the PFA or the CAA. A recent occurrence is illustrated in the photograph below which was provided by the PFA recognised 'type expert'.

Recommendation

Design deficiencies in the Speedwing's flight control systems have been identified and simple modifications developed to alleviate these deficiencies. For instance, Avid Directive AD004 concerns the aileron control linkage at the port end of the control column torque tube and MOD/189/002 provides enhanced protection against a control jam arising from collapse of the seats onto the control runs beneath. However, at the time of the accident there was no formal modification that addressed the vulnerability of the pitch control linkage.
According to the PFA, the kit manufacturer developed an elevator stop assembly in the early 1990s which they issued to constructors. Some UK Speedwings have the stops fitted but the down stop assembly did not reach all the constructors in time to be embodied. This explains why G-BUSZ and some other UK examples were built without an elevator down stop.

Recommendation 2001- 95

In view of the potential for serious damage to the pitch control linkage within Avid Speedwing aircraft that lack an elevator travel stop in the elevator trailing edge downwards direction, on 29 November 2001 the AAIB wrote to the Popular Flying Association recommending that the Association should:
a. Identify a modification that introduces an effective and durable pitch control stop in the elevator trailing edge downwards direction.
b. Classify the modification as a Mandatory Permit Directive.

Response to recommendation 2001-95

As a result of this accident, the PFA have now issued MOD/189/006 to all Avid owners. This modification requires owners either to fit the standard Avid elevator stop assembly or an equivalent stop (made to a drawing and fitted in accordance with instructions provided) within the next 5 flying hours. The PFA stated that an approved stop is effective, durable and operates in the elevator down direction.

FMI: http://www.aaib.dtlr.gov.uk/bulletin/mar02/gbusz.htm