Blast from the past...the following I obtained from a different web site three years ago, and have since posted it on several different forums for discussion. It may apply here. Notice not only the commentary regarding the potential problem that the author notes, but the results of several aircraft entering beta and/or reverse in flight. Specifically the rapid pitching moment when the download on the horizontal stab is lost and/or the tail is blanked or starved of driving propeller airflow.
IS IT REALLY IMPOSSIBLE ?
by Lee Coffman
Aero Services Unlimited
Santa Fe, NM
Has the impossible happened to you? For over twenty years the turbine engine manufacturers and the aviation industry have said it was impossible for a PT6A engine to go to beta or reverse without the pilot initiating the command. To initiate the command the pilot has to move the power lever behind the gate toward beta and reverse. An uncommanded beta or reverse would be one in which the pilot did not move the power lever behind the gate.
I have now interviewed the pilots and operators of six (6) aircraft that went to uncommanded beta or reverse in flight. When these pilots try to talk to people in the aviation industry about what happened the word “Impossible” keeps coming up.
Facts about these occurrences
An AT-802A with a PT6A-67AG engine went to reverse when the pilot reduced the power to descend out of 10,000 feet during a delivery flight. The aircraft pitched over very steeply and the pilot discovered that pushing forward on the power lever made matters worse, causing the aircraft to pitched over even steeper with the tail stalled the aircraft went over on its back. The pilot slammed the propeller and power levers forward forcing the propeller out of reverse. The pilot recovered at 5,000 feet. The pilot flew over an airport and shut down the engine and feathered the propeller before attempting to land, dead stick. After landing, the engine was started and an attempt to taxi was made. When normal propeller operation could not be obtained the aircraft was towed to the shop. The propeller governor was changed and engine/prop operations returned to normal.
An AT-502B with a PT6A-34AG engine went to reverse when the pilot reduced the power to lower the flaps before landing. The aircraft pitched over 45 to 50 degrees. The pilot slammed the power lever forward. The aircraft was about 300 feet high when it went into reverse. The aircraft hit the ground about one half mile from the end of the runway, bending the main gear. The aircraft landed in tall soybeans and stopped in 150 feet. The engine was still at full power with the power lever full forward when the aircraft stopped. The propeller blades were all bent aft indicating the engine was making high power and the propeller blades were in reverse. The propeller blades did not hit the ground. The propeller governor was found to have heavy corrosion under the beta valve cap.
An AT-502B with a PT6A-34AG went into beta or reverse when the pilot reduced power during a turn to land at about 100 feet. The aircraft pitched over and the pilot pulled back on the stick and was leveling the wing as he hit the ground. The beta valve was found to be very stiff from corrosion under the beta valve cap. The beta valve was cleaned up, lubricated and returned to service.
An AT-502B with a PT6A-34AG went into reverse at about 15 feet as the pilot reduced the power to land. The pilot slammed the power lever forward. The aircraft hit the ground, stopped and started to back up. The pilot pulled the power lever to idle and tried to figure out what went wrong. The pilot then tried to taxi to the hangar, but the aircraft would only go backwards when the power lever was pushed forward. The beta valve was found to be very stiff and heavy corrosion was found under the beta valve cap. The propeller governor was changed and operation returned to normal.
An AT-502B with a PT6A-34AG started going into beta or reverse when the final power reduction was made just before landing. The aircraft slammed into the runway instead of making a normal touchdown. The rigging and other items were checked and when nothing wrong could be found the propeller governor was changed. The operation retuned to normal. When the beta valve was removed from the old governor, heavy corrosion was found. During the same season the operator had another AT-502B with a PT6A-34AG that slammed into the runway when the final power reduction was made. Another propeller governor was installed and operation returned to normal.
Common Facts
The aircraft propeller goes to beta or reverse when the power is reduced to the point that the air load is driving the propeller and the propeller governor goes to an under speed condition.
In the 1980s, Beta valves had a leak or seep problem until propeller governor manufacturer, Woodward, increased the “O” ring size to eliminate the problem.
All the PT6A-34AG propeller governors that were checked had corrosion under the beta valve cap and there was no evidence of any seepage or leakage. The beta valves appeared stiff or appeared to stick. When the propeller governors were bench tested they passed the test.
The PT6A-67AG governor was not checked or tested. However, Woodward has issued a service bulletin that applies to some of governors on –60 series engines to replace the beta valve packing with the old style because of a possible controllability problem. The service bulletin says this will contribute to leakage.
Air Tractor issued Service Letter # 172 dated 10/19/98 to address the possibility that a sticking beta valve could prevent the propeller from coming out of beta when power was applied.
I feel that the Air Tractor SL # 172 should be complied with at each annual. I feel that turbine oil should be applied to the spacer as a lubricant to prevent corrosion since we did not have a corrosion problem when we had seepage.
I am seeking information from any pilot or mechanic that has seen the problem or been involved with the repair of the problem. “Please contact Lee Coffman of Aero Services Unlimited” at 505-820-1476 or
[email protected]. Any input will be greatly appreciated.