The Pawnee Factor
It didn’t feel right. The plane was accelerating down the runway, but not fast enough. I was passing the halfway point of the 2300 foot grass strip and not yet at flying speed.
Several thoughts ran through my mind. I had taken off in our Cessna 150M, with its tail wheel conversion, from this field before with pretty much the same conditions of temperature, atmospheric pressure, and gross weight. We had cleared the trees at the far end without too much of a “pucker factor.” But this time the headline went through my mind: “Airplane hits trees at end of runway” and the inevitable NTSB conclusion of “pilot error.” Perhaps my thinking was influenced by a friend’s description of another two-seat trainer departing the field a week earlier which had “barely cleared the trees.” Perhaps I was tired after a long day of towing gliders in the soaring club‘s Pawnee. None of this was as important as the trees at the other end of the field moving closer and the Cessna not lifting off the ground yet. I pulled the throttle to idle and braked to a stop. After taxiing back to the tie-down area, I shut down and thought about what to do.
I was alone at the airport. Everyone else had put away the gliders and left while I was gassing up the Pawnee at a nearby airport. I had expected to make the 40-minute commute home in the Cessna before sunset. But now I wasn’t so sure. I decided to maximize my conditions for takeoff and try once more. The sun was lower on the horizon, moving behind a large cloudbank to the west. This would lower the ambient temperature and help with density altitude. It was in the low 80’s during the day and now in the upper 70’s already and cooling as indicated on the large thermometer posted on the shack at the field. I checked the Cessna’s tire pressure, which was barely low, and added the 2-PSI to each tire (thinking every little bit might help but not really believing that factor would make much difference). I thought of offloading some fuel to reduce gross weight. But given the delay and the possibility of getting home after nightfall, I decided not to reduce my endurance safety margin. There wasn’t much else I could do. The temperature was down a bit more so I thought I would give it another try.
This time I did a run up to full power and leaned the mixture slightly to maximize RPM at the current density altitude. I was in position at the very end of the field. I am always prepared to abort a takeoff. But this time I really expected it. I added full power, released the brakes, and moved forward at increasing speed but still with pretty anemic acceleration. By midfield, still not flying or accelerating as much as expected, I again pulled the throttle to idle, slowed and turned back to the tie-down. By this time it had been a pretty long day of flying and my “little voice” was telling me: “enough.” I tied the plane down and called a friend (also a pilot) to see if I could spend the night. I figured that getting off early the next morning, just after sunrise, the temperature would be even lower with better density altitude helping with both engine performance and wing lift. It was Sunday evening. If I left early enough Monday morning, I could still make it to work on time.
At my friend’s house that evening, we talked about all the factors effecting performance of the Cessna 150. Density altitude was high due to temperature and field elevation. But we also discussed other issues. Was the grass mowed short enough? (Yes, it had been mowed recently.) Humidity was a small factor that day in the air and the grass was not wet. Wind was calm so I didn’t have the benefit of any headwind component. We talked about pilot technique. The usual short field technique is no flaps, full power, release brakes from farthest end of runway. The manual says flaps help with lifting off the ground but that any benefit of using partial flaps is lost due to decreased performance climbing over obstacles. There were trees to clear. If there were just open area beyond a short runway, flaps would be indicated.
But the manual’s admonition not to use flaps for short field takeoffs over an obstacle does not take into account a short field that is also soft. Ten degrees of flaps to maximize lift would minimize the takeoff run on a soft grass strip even more than on pavement. I have never been a fan of changing flap settings while close to the ground. But thinking about maximum performance in a 100 HP, 1600 lb gross weight, 30 year old, tail wheel converted Cessna 150M, I began to think of the best technique for high density altitude takeoff from a field that was both short AND soft. I called my partner in the Cessna. After discussions with him and my overnight host, we resolved that the best technique would indeed be to start the takeoff run with 10 degrees of flaps, lift off, stay in ground effect while accelerating, and slowly raise the flaps before climbing. This seemed reasonable, but I would “sleep on it.” In the morning with its lower temperature, the margin of safety would be higher anyway.
Well, morning came and my friend drove me to the field. My hopes of an early morning departure were dashed by ground fog at both ends of the runway. I could wait for the fog to lift (and the dew to evaporate from the wet grass which would also effect takeoff performance). But I would be late for work. I made alternate transportation plans by car and got to work with only minutes to spare.
The next day a cold front passed through the region. Ambient temperatures were low even in the afternoon and a strong headwind prevailed at the field. My partner in the Cessna and his wife were kind enough to give me a ride back up to the airport.
I did an engine run up and leaned to peak RPM at full power. With lower density altitude I probably didn’t need to, but I did put in the 10 degrees of flaps, added full power, released the brakes and moved forward. This time the acceleration was better, more like I had remembered from last time I took off from grass in the Cessna. The tail came up with forward yoke pressure and the plane lifted into ground effect about halfway down the runway. I leveled off, raised the flaps and accelerated to best climb speed or even a bit faster. I pulled back on the yoke and easily cleared the trees without even a hint of “pucker” this time.
On my flight home I thought about what had transpired. I learned a lot from this experience, most of which is described above with regard to short/soft field takeoff and density altitude. I also thought about what I later termed, “The Pawnee Factor.” My first takeoff attempt that “didn’t feel right” was at the end of a full day towing gliders in a 260 HP Piper Pawnee, a true workhorse. The last flight of the day was to get gas and did not even include towing a glider. The takeoff performance of the Pawnee is incredible. Without a glider on tow, it climbs like the proverbial “bat out of hell” and so MUCH better than the 100 HP Cessna 150 which I had not flown off of grass for months. My sensation of lack of acceleration in the Cessna was certainly accentuated by high expectations having just flown the Pawnee. This Pawnee Factor may have made me err on the side of an early abort while taking off in the Cessna. Who knows, I MIGHT have made it over the trees. But I’m glad I didn’t try. If it doesn’t feel right for ANY reason, better to abort. Remember, there are no emergency takeoffs, only emergency landings.
by Rob Spencer
Rob has accumulated over 1000 flight hours since 1984 and holds a commercial pilot license with privileges in airplane single engine land and sea, glider, and an instrument rating. He is a volunteer tow pilot and commercial glider pilot member of Franconia Soaring Association, New Hampshire’s only soaring club. Look for more information at www.franconiasoaring.org