Plane & Pilot
Tuesday, July 9, 2013

The Reality Of Deep Stalls

Stalls are simply stalls, except when they’re not

The Airbus A330 entered a stall at 38,000 feet and descended in a nearly flat attitude for the next three minutes. Indeed, flight recorder data later suggested it was descending nearly vertically, but wings level in a slightly nose-up attitude when it hit the ocean at a vertical speed of roughly 11,000 fpm, about 120 knots straight down.

The full story of the accident is a tragedy of errors, but the bottom line was that the copilot held the airplane's side stick full back practically all the way to impact. Although the Airbus was in an extremely high angle of attack, it wasn't in a deep stall. It was doing exactly what it had been commanded to do. If the copilot had simply released back pressure…

If there's any aircraft designer on the planet who could figure a way to harness a deep stall to his advantage, it's Burt Rutan. If you followed the development of Rutan's SpaceShipOne to capture the Ansari X-Prize, you've seen a perfect example of how to tame and control a deep stall. Rutan's innovative "shuttlecock" concept is a brilliant solution to the problems of returning from space while successfully avoiding the heat and turbulence associated with re-entry.
The good news is that many general aviation designs are relatively immune to deep stalls.
The SpaceShipOne was deliberately designed by Rutan to return to Earth in a deep stall. The aircraft features an unusual feathering system that deflects the rear half of the wing and the tail booms up, forming a shuttlecock configuration that allows SpaceShipOne to descend back to Earth in more of a gentle mush rather than burning up the sky at extreme Mach numbers. While there's some speed buildup (Mach 2.9 on pilot Mike Melvill's flight in June 2004), it occurs at very high altitude where air pressure is so low that there's little frictional heating from outside airflow.

At 57,000 feet, the pilot streamlines the wings and tail back to the original trail configuration, and the aircraft becomes a normal glider again. For the Ansari prize, Rutan launched two flights in two weeks, both of which exceeded 100 km altitude and landed back at Mojave without power, just as the Space Shuttle landed at Kennedy Space Center in NASA's glory days. Rutan claimed the X-prize of $10 million, and his sponsor, Paul Allen, spent only about $20 million in the process.

For a more comprehensive examination of deep stalls, read Chapter 25 of Barry Schiff's excellent book Flying Wisdom: Proficient Pilot III published by ASA in Newcastle, Wash. Schiff is one of the smartest people I know on all things aviation. He's also a retired TWA captain, but don't hold that against him.


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