21 Dec 2012
By Test Pilot Dr. Lionel D. Alford,Jr.
I accomplished an engine air start test in the AT-6. This was for military certification and is likely the most dangerous type of testing we accomplish with a single engine aircraft. The point is to shut down and restart the engine at specific points in the air start envelope. Since the T-6 was fully certified with the 1100 shaft horsepower engine, we needed to certify the AT-6 at the limits of the air start envelope with the 1600 shaft horse power engine.
The manufacturer wanted us to allow the engine to idle for two minutes before shut down. This was to ensure the engine didn’t experience any unwanted effects due to immediate heating or cooling. This also meant that for the first point at 15,000 feet and 200 KIAS, I had to start the descent at 25,000 feet. This means we had to be on an IFR flight plan although the weather was VFR. I also planned to begin the descent at 10 NM from Beech Field (BEC) so the shutdown was over the field to allow time to turn for another restart attempt (if the first didn’t work) and to place the aircraft in a safe position for an flame out pattern if the engine wouldn’t restart. For safety and proficiency, the first event after takeoff was a practice flame out pattern from 3,500 feet at high key. I made an entry at high key at about 3,200 feet and 135 KIAS. I made it by waiting until the last minute to put down the gear and the flaps. The T&G was great. I also should mention, I was alone in the aircraft with telemonitoring (TM) watching the aircraft and recording the events. The backseat was filled with an oxygen bottle and backup batteries for the data acquisition system (DAS). The oxygen bottle was necessary to provide oxygen when the engine was shut down (our OBOGS generates oxygen, but requires engine bleed air). The battery was required because we didn’t want to lose data when the engine quit providing generator power. For a real air start, the pilot has backup oxygen in the ejection seat, but that is a single shot.
So with the great help of air traffic control and our tower, I started 10 NM to the north and at 25,000 feet. I started down and reached 2 minutes and 20 seconds at 16,000 feet and right over the field–the shut down point. The engine shut down perfectly at 16,000 feet. I thought everything would get quiet. It didn’t the engine stopped, but the cabin depressurized (making a lot of noise), there was significant air rush at 200 KIAS, and I could hear my breathing over the intercom. I was told that when the prop feathered the aircraft would feel like it was on ice because of the drag reduction. I couldn’t appreciate it because the loss of torque made the aircraft yaw and all the expected red warnings and cautions kind of took my attention.
I ran through the checklist and restarted the engine at 15,300 and 192 KIAS. Pretty good test point management. The engine lit off immediately and the prop started to turn with increasing speed. It seemed to take a long time, but it was only about the expected 40 seconds. I was very deliberate and slow about the restart, and we were back in action before 13,000. ATC had cleared me to 10,000 feet–I didn’t have the heart to tell them I might need a lot lower if I couldn’t get a restart. As soon as the engine was back at power, TM told me to RTB. We had some details we needed to work out before we tried another restart.
All in all, I was very pleased with the AT-6 and engine restarts. We’ll be accomplishing more for certification in the new year. By the way, the landing was great. I made a high energy descent from 13,000 feet into the pattern for an extended straight-in to runway 19. Thanks to all our engineering experts and tower, we had a successful flight and certification test.
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