Seven AT-6 Engine Start Tests in Two Sorties

at-6By Dr. Lionel D. Alford,Jr.
Hawker Beechcraft Lead Test Pilot

3 and 4 Jan 2013

I accomplished 7 engine air start tests in the AT-6 in 2 sorties. 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 125 KIAS, I had to start the descent at 20,000 feet. This means we had to be on an IFR flight plan although the weather was VFR. In Wichita, Wichita Approach controls the airspace to 15,000 feet, Kansas City Low controls the airspace to FL230, and Kansas City High controls the airspace above FL230. 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, on the first flight and the first event after takeoff I made a practice flame out pattern from 3,500 feet at high key (2,000 feet AGL). I made an entry at high key at about 3,000 feet and 135 KIAS. This was just too low for the winds, so I made a go around and tried it again from 4,000 feet (2,500 feet AGL). The aircraft had plenty of energy. The T&G was great. I’ll mention again, 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.

On the ground before start, I began prebreathing 100% oxygen. The reason for this was to combat the bends during the rapid decompressions at altitude. For starting to heat the battery up and for practice, I made a PMU (Power Management Unit) OFF start. This is a tricky start that is normally an emergency procedure. We do them routinely for new T-6 aircraft. I had made 3 in the AT-6 before. These were the first PMU OFF starts in the AT-6. The trick with the start is to make certain you don’t give the aircraft too much Jet Fuel too fast. If you do, you can torch the engine. I wanted to practice a PMU OFF start because we planned to make at least two during the restart testing, and the battery gets a greater work out. After the start on the first sortie, TM reported some dropouts at the end before the backup battery was placed back to charging.

After the SFOs, I picked up my IFR clearance and headed to the north to set up at FL200 for the 125 KIAS and 15,000 foot restart. The biggest difficulty was getting ATC on line with the events. They were very helpful, but didn’t seem to fully comprehend what we were doing. This was in spite of the fact I put AT-6 Air Starts on the flight plan per their and tower’s instructions. I was cleared an unrestricted descent from FL200 to 10,000 feet. 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 even at 125 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 waited to restart the engine at 15,000 and 125 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 13,700. Now, remember this information. Except for altitudes, the engine restarts were almost exactly like that every time. The engine is started and producing power in about 1,000 feet–that is awesome. I don’t think many other engines or aircraft can give that kind of performance.

The next point was FL300 for a FL200 and 200 KIAS restart. It was a perfect relight, but the TM stream died, and I had to RTB.

The next day, I made the same steps for preparation, but it only took one SFO for a good practice. I went to FL250 immediately for a FL200 and 125 KIAS restart. After that I made a PMU OFF restart at FL200 and 200 KIAS and FL 200 and 125 KIAS. The only thing to say about the PMU OFF air starts is that they are easy. You just give the engine a little gas and it lights off great. The power is a little quicker as the prop comes out of feather, but it is easy to control.

Next, since I knew the engine started great at high altitude, I moved down to 17,000 for a 10,000 foot and 125 KIAS restart. This was done with a simulated failed start. To do a failed start, you keep the ignition from the fuel and run the engine for 5 seconds with gas flowing from the exhaust stacks. Then you motor the engine for 20 seconds to clear the fuel. Then you start it up. It did. The start was great.

Finally, the last point was the most dangerous and difficult. The start was planned for 5,000 feet AGL (6,500 feet MSL). I started at 11,500 and 125 KIAS at exactly 9.3 NM from BEC. The trick was to arrive at above 4,000 feet MSL at BEC so if the engine didn’t start I could make a flameout landing. ATC gave me a straight-in to runway 19 at BEC. Everything worked great and the aircraft arrived at high key at 4,700 feet MSL with the engine running.

With a total of 8 air starts, you can immediately see the AT-6 is an outstanding aircraft. It is easy to predict and easy to restart. I don’t recommend inflight restart as a normal procedure, but in a pinch, when the pilot needs to restart the aircraft engine in flight, it will do it every time and it is easy to control.