The video of the 747 crashing after takeoff from Bagram Air Base in Afghanistan is hard to watch. The airplane had climbed only a few hundred feet and was flying quite slowly when it rolled a little to the left and then rolled off to the right, obviously out of control, as in the beginning of a spin. The only thing similar I can remember was video of a USAF B-52 starting to spin out of the bottom of a really steep turn years ago. In both cases, the airplane was quite low to begin and hit the ground shortly after control was lost.
The B-52 was to be flown in some sort of demonstration and the pilots were really throwing the big airplane around. In that steep turn, the angle-of-attack was apparently not minded and the airplane did what any airplane, large or small, will do when AOA (or alpha) is allowed to reach the stalling point.
The 747, on the other hand, was on a routine mission to fly military equipment to another air base. Certainly nothing unusual was planned but something unusual certainly happened.
As pilots will do, after watching the video I came up with an idea on what I thought might have happened. If the center of gravity moves aft, stability deteriorates. Far enough aft, there is no pitch stability and it can finally reach an aft point where the airplane will stall regardless of what the pilots do with the controls.
The 747 had a loadmaster as a crewmember and with a lot of eyes on the weight and balance, I doubt seriously if the airplane was loaded with the center of gravity beyond the aft limit. This would mean, in this scenario, that the heavy load of vehicles could have shifted aft. To do so, it would have had to be improperly secured and that has certainly been known to happen.
Apparently this subject had come up because in a statement, National Air Cargo, operator of the 747, said that the airplane was loaded at another base and stopped at Bagram only to refuel. After the original loading, the statement said the cargo was inspected and was found to be properly loaded and secured and had passed all the necessary inspections. The cargo was again inspected prior to departure from Bagram.
The NTSB is investigating and someday we will learn the probable cause. For now, for me, it raises an important question. Most pilots know that overloading an airplane is bad but a lot of pilots do it anyway. They know that some performance and a little structural integrity is sacrificed when an airplane is flown overweight and they are willing to risk that. What I don’t think many pilots fully understand is that a transgression in the balance of an airplane can be lethal, especially if the aft limit of the CG range is exceeded.
This got a lot of attention a number of years ago, in 1977. The Commonwealth of Pennsylvania leased and then crashed one of the first turboprop Piper Cheyennes. Control of the airplane was lost shortly after a departure in instrument meteorological conditions. The investigation revealed that the CG was as much as 3.2 inches aft of the limit. There were eight adults on board which could explain this. Two would have had to be seated aft of the standard club arrangement.
Pitch stability had been a big question in the certification of the Cheyenne. In fact, to meet the regulations, Piper had to include an active stability augmentation system (SAS). Manufacturers had long used downsprings and bobweights in pitch systems to buttress longitudinal stability but the Cheyenne’s SAS was a first in light general aviation airplanes. Some pointed to it as a weakness in the design.
The Cheyenne was basically a piston-powered Navajo converted to a turboprop with a lot more horsepower. Horsepower is destabilizing in pitch, thus the problem.
The Cheyenne SAS kicks in when the airspeed drops below 125 knots. Then it starts applying forward pressure on the control wheel through a spring and at 100 knots it reaches the maximum push. The effect of this is to provide artificial control feel.
The stability requirements call for an airplane to always seek a trim speed and to return to that speed when disturbed. To go progressively slower or faster than the trim speed requires more pull or push.
At some point an aft CG condition can cause an airplane to reach what is called the stick-free neutral point where there is no feel in pitch. You don’t have to exert force to go slower or faster, you just have to move the elevator control. As you would expect, the aft CG limit is set ahead of the stick-free neutral point. It would be possible to control an airplane with the CG aft of the stick-free neutral point but it is difficult.
I have flown both a simulator and a variable-stability Navion operated by Princeton University and looked at both possible and impossible aft loadings and at best it will make you sweat and at worse control will be lost. It is a bad feeling, one you surely would not want to replicate without a way out.
After some original controversy about the SAS, the Cheyenne flew on. The pilots flying the airplanes apparently understood there was no margin in the aft CG limit and most kept it forward of that limit. Many disabled the SAS system because they didn’t like the way it messed with the controls when the airspeed dropped below 125 knots. I don’t think that ever contributed to an accident.
All stayed quiet until December 15, 1983, when The Wall Street Journal published a sensationalist, paper-peddling article that implied that the Cheyenne was unsafe because it lacked longitudinal stability. The report cast doubt on the FAA certification process for all airplanes.
There was so much wrong with the article that we at FLYING worked to set the record straight. Mac McClellan and I traveled to Florida, where the Cheyenne was produced at the time, and flew the FAA-mandated stability tests in each model of the airplane. It was an interesting exercise and I learned a lot about the stability characteristics of the airplane.
The hardest test to pass comes in a climb with the airplane trimmed for the best-rate-of-climb speed, with takeoff flaps, and with full power. Here the airplane must have a stable stick force curve, meaning pull for slower and push for faster, it must seek the trim speed when away from it and the controls are slowly released, and any change is speed must result in a stick force that is clear to the pilot.
At the time we flew, the Cheyenne IA was the basic airplane. It differed from the original, with 500 hp per side as opposed to 620 and with an aft CG limit two inches farther forward. In the full power climb test, this airplane had light stick forces but it had acceptable control feel and was easily controllable.
Next up was the Cheyenne II, which was the current name for the original with 620 hp per side. With the SAS operative it met the requirements though not quite as well as the Cheyenne I. With the SAS inoperative, it was at the stick-free neutral point in that full power climb and was flyable only if you knew what to expect and how to deal with it.
At the time, Piper was also building a Cheyenne IIXL with a longer fuselage than the basic Cheyennes and to keep from using an SAS system the company decided instead to limit the 620 hp engines to 500 hp for climb. Even at that, in the 500 hp per side climb the airplane was close to the stick-free neutral point and the return to a trim speed was not strong. It was the weakest of the Cheyennes that we flew.
Piper was also building the even longer-body Cheyennes III and IV and the WSJ article suggested they also had longitudinal stability problems. Nothing could have been farther from the truth. The original prototype did have problems and Piper addressed them with a new design that included a huge tail. When one of these airplanes is viewed from behind the tail almost looks bigger than the airplane.
What this means to pilots in every day flying is that any airplane will be less stable with the CG aft. A pilot who learns in a Skyhawk might find it a much different airplane after he gets a certificate and starts taking friends and relatives for an airplane ride.
The wider the CG range, the more pronounced this can become. The V-tail Bonanza had a narrow range and had to be loaded with care but the handling qualities didn’t change a lot within the CG range. The P210 that I flew for 28 years had a wide CG range and went from an airplane with strong longitudinal stability to one that was honestly hard to fly accurately when the CG moved aft. It was so bad that I arbitrarily limited loading to two inches ahead of the aft limit, or, 50 inches aft of datum. In flight testing for supplemental type certificate mods to the airplane, the last time I looked nobody had been able to certify to the 52 inches aft that Cessna had certified. I know that one, and possibly two, P210s were lost in testing at that 52 inch aft limit.
There is one other thing to consider on the 747 in Afghanistan. There are bad guys with guns there so most operators climb to gain altitude as quickly as possible. The result is less airspeed margin above a stall than would be found in a normal climb. One report said there were thunderstorms in the area and the possibility that wind shear caused that loss of control has to be considered.