Proficient Pilot

CG at the aft limit

January 1, 2007

Barry Schiff has been teaching flight for more than 40 years.

 A local flight instructor called me on the landline last month. He asked if he could buy me a cup of coffee at the airport cafe. "There's something about stall training that I don't understand. I'd really appreciate your input." "Sure," I said. "No problem. How about tomorrow morning at the Spitfire Grill at nine?" The next morning a young and somewhat anguished instructor looked at me from across the table. "I'm embarrassed to admit this," he began, "but I don't get it. I don't understand how a pilot can inadvertently cause a stall that results in an accident. "The nose is so high during stall entry that I don't see how a pilot can do this without realizing that the aircraft is in jeopardy." He paused. "Yes, I know that you can stall an airplane with the nose near or below the horizon, but this is difficult to demonstrate without trying really hard. I am trying to say that I don't comprehend how even a ham-fisted pilot can stall an airplane without intending to do so." I understood his consternation. I recall having had a similar discussion with one of the Ancients when I was a new instructor (although the aircraft in which I instructed during the 1950s stalled more easily than modern designs). "What type of aircraft do you use for training? I asked. "Cessna 172s," he said. Aha, I thought. That's part of the problem. "My guess is that you teach and practice stalls with only two of you in the airplane and a relatively forward center of gravity. Right?" "Of course." He answered in a manner that said this obviously was the time-honored and recommended way to teach stalls. "Have you ever stalled a Skyhawk when it was fully loaded and with a center of gravity near the aft limit?" I suspected that he had not, and his reply confirmed my suspicion. "You know, airplanes typically behave differently when heavy and with a CG at the aft limit. They are not as docile. "When you have time, let's take up a 172 with a couple of instructors in the back seat and some weight in the baggage compartment. I think you'll discover that the airplane has surprisingly different stall characteristics; stall entry and aircraft behavior will not be quite what you expect. "Stalling speeds will be higher, but you will discover that it is easier to stall the airplane as well. This is because the stick force — the amount of pull on the control wheel — is less than what you are used to. Stick (or wheel) forces decrease noticeably as the CG moves aft. In other words, it takes fewer pounds of pull to create a given G-load as the center of gravity moves aft." (If the CG is far enough aft, you can pull the wings off with your fingertips.) The instructor seemed intrigued by this. I was greeted the next morning by three young men eager to participate in this "experiment." One presented me with a weight-and-balance computation. We would be departing Santa Monica Airport with the airplane loaded to its maximum-allowable gross weight and the center of gravity at the aft limit. I asked the instructor to perform a few maneuvers to give him a chance to get used to the adverse loading conditions. He acknowledged that the airplane was more sluggish and that it took noticeably less back pressure to maintain altitude during a steep turn. I then asked him to perform a climbing, turning departure stall using full power and partial flaps. He commented that the airplane stalled quicker and more abruptly than usual; the nose was not as high as he had been conditioned to expect; stall behavior (the pitching and rolling moments) was not quite what he expected; and less pull was required on the control wheel to induce the stall. He added that it took more forward pressure on the wheel to affect recovery compared to simply releasing back pressure as when recovering from a stall at light weights and a forward CG. We then did other types of stalls of which accelerated stalls provided him some of the most surprising aircraft behavior. At no time did we perform a conventional straight-ahead, power-off stall because this rarely leads to an accidental stall. Afterwards, all three instructors had a better understanding of how unsuspecting pilots can get themselves into a world of hurt as the result of inadvertent stalls. A six- or eight-place airplane, of course, behaves even more differently during a stall at high gross weight and with an aft CG. I suggested that the instructors conduct some flight reviews with a full complement of willing passengers and baggage to educate pilots about what to expect during stalls under adverse loading conditions. The worst-case scenario, I explained, occurs during a departure with a heavy load and an aft CG at high-density altitude. This is when pilots try to coax performance from an airplane that simply might not be available. Departing in mountainous terrain exacerbates the problem because it is more difficult for a pilot to envision the natural horizon. Some pilots are genuinely startled when performing stalls in an adversely loaded airplane. A few have confessed that they were shocked at how unexpectedly their airplanes behaved. One told me that it was like flying a totally different machine. In a way, he was right.