So close to 51,000 feet
Today we took a Learjet 60 on a test flight after it got a pair of newly rebuilt engines ($2.1 million total). The airplane's owner requested that we try to take it all the way to its maximum certified altitude of 51,000 feet above sea level. We tried, but even with minimal fuel, most of the soft drinks and various publications, tool kit, and other cabin supplies removed, albeit with a maintenance representative on board, we were not quite light enough for the slightly above standard air temperature up there. But we did (the other pilot and I) fly higher than we've ever been before, 49,300 feet (really, "Flight Level 493", minutes after this was taken), just 700 feet short of FIFTY-THOUSAND FEET UP (9.34 statute miles), when a warning light came on indicating a hot air duct overheat, forcing us to pull one engine to idle and a give up trying to slowly inch our way upwards in the extremely rareified air. Note the indicated airspeed displayed vertically on the left, about 177 knots, yet our true airspeed was close to 400. Note the stalling speed at 146 knots and the never-exceed speed of 191 knots. Had we been able to continue climbing the remaining 1700 feet, the minimum and maximum speeds would have grown much closer; in any case, we needed to stay above Mach .70, and that was getting more and more difficult as the engines ran out of thrust in the thinner air and as drag increased with the required increase in angle-of-attack. Had the air temperature been just a couple degrees cooler (LL corner= -56C, a bit warmer than standard for any altitude above 35,000-36,000 feet where the tropopause is), we could have made it. While the airplane is approved for 51,000, it doesn't have the aerodynamics, i.e. wing design, for altitudes much above 43,000, except under the best conditions, that is, an unpractically light fuel load, no payload (passengers) and not a fraction of a degree warmer than standard temperature outside. 51,000 is more of a marketing gimmick. What it really means is that the pressurization (life-support) system can maintain a sufficient cabin altitude at that height. We both wore oxygen masks during this attempt, just in case of a pressurization problem. A failure, albeit extremely unlikely, would be especially deadly without it, and likely fatal regardless (similar to surfacing too quickly after deep scuba diving) unless it were a reletively slow leak allowing us time to descend as fast as possible, which wouldn't be fast enough without supplemental oxygen supplied through a sealed mask.