An Air Force version of the V-22 tilt-rotor Osprey aircraft last week experienced a compressor stall of both engines after flying into a cloud at 18,000 feet, presumably because of icing problems, sources have told the Project On Government Oversight. The aircraft, CV-22 #6, was on a routine flight to Edwards Air Force base in California. It did not recover from the stall until it had descended to warmer air at about 10,000 feet, the sources said.
As a precaution the aircraft landed in Prescott, Arizona.
"This is very disturbing. Only last month the Pentagon approved the Marines version of V-22 for full-rate production," said POGO Senior Defense Investigator Eric Miller. "And now we find out the aircraft can't even fly into a cloud."
At the time of this release, it was not known whether the aircraft that experienced the stall had a de-icing system onboard. It's also unclear just how much, or if any, de-icing system testing has been performed on the CV-22. A report of testing issued last month by the Director of Operational Test and Evaluation on the Marines V-22 did not address the issue of icing. A 2000 DOT&E report said that icing testing on the Marines V-22 had been waived by the Navy. Sources have speculated to POGO that the V-22 cannot take on extra weight without impacting its performance, and a de-icing system would add weight.
The requirement that the aircraft be able to operate in icing conditions was waived during the first phase of operational testing in 2000. The report also predicted that there was no plan to evaluate operations in icing conditions during OPEVAL Phase II. "The operators will be restricted from flying in icing conditions until the development testing and follow-on operational testing is completed," the 2002 report to Congress said.
There is another concern raised by the dual-engine failure. Because the Pentagon and defense contractors have been saying that the loss of both engines in the V-22 is "remote, but possible," they have deleted the original requirement that the V-22 be able to autorotate like nearly all other helicopters to a soft landing in the event of engine failure. In the event of a single engine failure, V-22 flight procedures require the pilot to transition to aircraft mode and in the event of a second engine failure perform a "fixed wing glide approach to an emergency landing site," according to an April 2002 report to Congress.
In fact, had the emergency dual engine stall over Arizona been below 1,600 feet, it would "not likely" be survivable, according to the recent DOT&E report.
The Air Force plans to buy 50 CV-22's to replace its fleet of MH-53J Pave Low helicopters used to insert and extract special operations force from enemy areas. Although the CV-22 is on a different development and testing track than the Marines MV-22, it team of developers and testers work together on many common areas.
The Air Force version of the V-22, the CV-22, is a modified version of the Marines MV-22 to perform longer-range, special operations missions. The CV-22 is modified to have long-range fuel tanks, advanced radar, and more sophisticated situational awareness and radio frequency countermeasures. These modifications are designed to improve operations during night and low altitude flights in bad weather. The report to Congress also said there was no plan to evaluate operations in icing conditions during OPEVAL Phase II. "The operators will be restricted from flying in icing conditions until the development testing and follow-on operational testing is completed," the report said.