Narrative:

While conducting a maintenance test flight a landing confign stall was being performed in accordance with company maintenance test flight procedures. As stick shaker speed was approached, I noted that #1 engine RPM had decreased to 70%, #2 engine RPM was not noted, #3 and #4 engines RPM was 80%. I advised the PF to increase power on engine #1. As the PF began to increase power on the #1 engine, the aircraft entered stick shaker and very soon thereafter stall buffet. The PF then advanced the throttles on all engines and called for maximum power. Engines #3 and #4 accelerated to maximum power, engines #1 and #2 began to compressor stall and did not accelerate. The PF began to add right rudder to compensate for the yaw being produced by the asymmetric power. The aircraft then entered stall break and entered a spin. I took control of the aircraft, reversed the inboard engines, centered the control wheel and applied left rudder. The spin stopped and the dive was recovered. 7000' of altitude was lost during the recovery. Upon recovery, ATC was advised of altitude loss. ATC questioned whether assistance was required. None was. ATC then reclred flight to 13000'. Although flap and gear speeds were exceeded and engines were reversed with flaps extended (prohibited by afm) during the recovery, all aircraft flight characteristics were determined to be normal. All crew members concurred that the test flight could be continued. In retrospect, this decision was probably not the most prudent one. Conclusion: I believe what led to the loss of control was the fact that during the stall recovery, a minimum altitude loss recovery was being attempted. This is the way we train in the simulator and is valid if encountering a low altitude stall. However, on a test flight, altitude loss does not have to be a concern. A block altitude clearance can be obtained and the aircraft nose can be lowered during recovery. In the incident described, this was not done. The aircraft could not fly out of the buffet on 2 engines and in fact went into stall break. Another contributing factor was lack of awareness of engine RPM decrease on engines #1 and #2. Supplemental information from acn 178694: the solution is to carry more power into the stall to keep engines spooled.

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Original NASA ASRS Text

Title: HVT TYPE ACFT ON TEST FLT ENTERED STALL AND SPUN. RECOVERED BY REVERSING 2 INBOARD AND CENTERING CTLRS. LOST 7000' IN RECOVERY. REPORTER STATES STALL IS STANDARD OPERATING PROC ON TEST FLTS.

Narrative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

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.