What's up With 41,000 Feet?


Insurance companies and safety experts-self appointed and otherwise-are constantly worrying that owner-flown jets...particularly the coming crop of new light jets, can be flown at 41,000 feet by a single pilot who may not be very experienced. What's up with that? What's so special about 41,000 feet? The answer is that FL 410 is the traditional extreme upper edge of the operating envelope for civilian jets, either airline or business jet. The very first jets were certified to 41,000 feet even though they couldn't get there unless they were virtually out of fuel or carried little payload. While the 707s, DC-8s, Hawker 125s, JetStars and other first and second generation jets ripped along in the high 20s and low 30s, FL 410 loomed over them as a possibility that apparently required on-the-edge airplane performance, and way above average pilot skill to enjoy.

The fact that there were some high altitude upsets leading to crashes in the early airline jets amplified the impression that the high altitudes were no place for amateurs. And those upsets occurred at altitudes far below 41,000 feet, when pilots who had thousands of hours experience in the large piston airplanes lost control of their highly swept wing jets.

Until the coming of reduced vertical separation minimums (RVSM) a couple of years ago, the usable flight levels in U.S. domestic airspace stepped up by twos above 29,000 feet. You couldn't fly at FL 300, the next step was FL 310. All the high flight level numbers were odd, but FL 310 and 350 and 390 went west, with 290, 330 and so on going east. The reason for the 2,000-foot separation is that the FAA determined, correctly, that altimeters and pitot-static systems on the early jets were not accurate enough in the very thin air of the high flight levels to safely separate cruising airplanes by only 1,000 feet.

Now, RVSM rules require digital electronic air data computers that are smart enough to correct errors in the pitot-static system, so altitude measurement at high altitudes is very precise and must be demonstrated to be accurate in each individual airplane by a test flight with an independent height measuring unit onboard. In addition, an altitude keeping system-read autopilot-must be used above FL 290 to keep the airplane on altitude with only a few feet of tolerance.

The fact that you had to climb the stairs two at a time to get to 41,000 feet, I think, added to its almost mythical qualities and its implied challenge. A jet could be doing just fine at 37,000, or even 39,000, but just not have enough power, or enough wing performance, to make that big and final step.

The first two civilian airplanes that I can think of that routinely climbed to FL 410 were the Gulfstream II and the original Learjet 23. The G-II was the biggest business jet and the Learjet was the smallest. But both had gobs of power for their weight. The Lear rocketed up to the magic altitude using the highest power-to-weight ratio that I can think of in any business jet, while the stately Gulfstream used the same combination of characteristics that still prevail in that family to get on top of other airplanes-an enormous wing and the most powerful engines available.

The Gulfstream had a smooth introduction into service and has posted an excellent safety record from the beginning. The early Learjets didn't fare as well. The costly Gulfstream-the most expensive business jet-was, logically enough, flown by the most experienced and well-trained crews. The Lear was the entry-level jet and, rightly or wrongly, was assumed to be flown by less experienced and careful pilots. There were even a handful of owner-pilots at the controls of Lears, something unheard of in a large cabin jet such as the Gulfstream. The fact that both airplanes could and did climb to 41,000 feet, and one-with the most professional crews-had no difficulty, and the other with the presumed less expert pilots had some problems, confirmed the mythical status of flying at FL 410. You had better be the best to go up there.

The challenge of high altitude flight is real, but the solutions come from expanded understanding of aerodynamics and thus a proper design, not from superhuman pilot skills. Because air density decreases exponentially with altitude, the seemingly small climb from the mid-30s to FL 410 causes a dramatic change in the amount of air the wing and engines have to work with. With power output dropping dramatically, the wing needs to operate at an ever increasing angle of attack to hold the airplane up. At some weight there won't be enough power available to accelerate the airplane, the AOA will continue to increase just to hold altitude and the airplane will be hanging there, unable to speed up and a stall is inevitable.

The other critical issue at FL 410 is the effect of Mach. Even though the airplane is moving through the air at well under the speed of sound, air accelerating to pass over the wing and tail can easily come close to, or exceed, Mach 1. When that happens a transonic shock wave develops and airflow over the wing can go crazy, splitting off in all directions. In the worst case at high altitude an increase in speed can cause Mach buffet, possibly a nose-down tuck and a loss of control. In the same conditions slowing down can cause low speed buffet and a stall. This is the so-called coffin corner, where a jet that's too heavy for its available wing lift and engine power is stuck with no way out. Many of the early jets could reach the coffin corner if their crews tried to climb too high at too high a weight. The existence of a coffin corner just enhanced the industry perception that extreme altitudes were available only to the most expert pilots.

But all of this is old news. It really only matters to older design jets. High altitude flight is still demanding, but the mystery is gone. The airplane performance margins for newer designs at 41,000 feet are wide, and any reasonably competent pilot can stay within them.

Learjet resolved its high altitude issues many years ago with better pilot training and modifications to the wing. Gulfstream continued to make ever bigger and better wings optimized for high altitude, and the G400 and G500 series airplanes blast right through FL 410 on their way to 45,000 feet, or FL 510, in the case of the G-500 series. But I think the airplane that really ended the superstition about FL 410 and above is the straight-wing Citation.

When the Citation 560 Ultra came along with its enhanced straight wing and more powerful engines, it could climb directly to FL 410 at max takeoff weight. Even higher at lighter weights. When it got to high altitude the margin between high and low speed buffets was usually 40, 50 or more knots. I remember marveling at how this otherwise modest airplane climbed, and when level you could make steep turns of 45 degrees bank at FL 410 without any rumble or complaint from the wing. High altitude flight had been a fingertip kind of flying operation in the past, but the Ultra didn't care. It was happy there.

Since then many jets have made FL 410 and above their routine cruise altitudes. Learjet was the first to certify to 51,000 feet with its Longhorn model 28, but it could only get there with just enough fuel left for descent. Now, the Gulfstream 550 can be at FL 510 with several hours of fuel in the tanks, and the Citation X can also cruise at that level. Even the light jets such as the Citation CJ2 and CJ3 blow through FL 410 on their way to a routine cruise at FL 450.

So, insurance companies and safety mavens, get over it. The bogyman at FL 410 has been slain by advanced aerodynamics, not superhuman pilot skills. Any pilot who can get through the type rating requirements to fly one of the newly designed jets will be just fine at the high altitudes. And the record shows that. There has not been a high altitude loss of control in a business jet in many, many years.

If the safety experts must worry about high altitude safety, they have the wrong airplanes in their sights. It won't be the newly designed light jets, but some older airplanes that don't have the margins when flying near their certified ceilings. The light jets and their pilots will be just fine up high. If you must worry, stay focused on safety in the terminal area where nearly all jet accidents happen. Enhanced design can make an airplane easy to fly up high, but there is no aerodynamic solution for a confused pilot who flies into the ground.