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dangerously toasty, sometimes in the 120 to 130 Fahrenheit range. When exiting my air-conditioned, Def Leppard- saturated truck to get fuel, the contrast was shocking. On another trip to retrieve new motors for the Duke, I journeyed through the Idaho and Montana winter where sections of the route were negative 20 Fahrenheit – another shock when exiting the vehicle.
Through the window at 41,000, the contrasts described above serve to remind us how harsh the environment is just an inch away. Outside of your jet, the air is -50 Celsius, the partial pressure of oxygen will not sustain human life, and it’s blowing at 500 mph. If you lose pressurization or heating, that environment will be the one in which you must survive. It will be a life-or-death situation, and it will be shocking – especially with no coffee or butt warmer.
You’ve never been lost until you’ve been lost at Mach 3.
– SR-71 pilot Paul Crickmore
Mach (from Austrian physicist Ernst Mach) is a measure of speed relative to the speed of sound. Subsonic is a Mach below .75. Transonic is from .75 to 1.20, supersonic is 1.2 to 5.0 and hypersonic is above 5.0. Low altitude pilots that do not use Mach as a measure imagine that jet speeds are
something that certainly must push you into your seat and cause visual distortions. Just as we know this to be untrue, that you don’t perceive the speed, those who have been supersonic or hypersonic will also tell you there are little perceptual differences at those velocities. So, what’s all the hoopla about going really fast? Physics, my dear Watson. There are dangers where the Mach demon lives.
Mach tuck, Mach buzz (aileron buzz) or flutter, Mach Crit (critical), boundary layer separation and coffin corner are among the high-speed demons. Mach Crit is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound but does not exceed it. Mach tuck is the result of the CG shifting aft due to transonic flight which results in a nose down moment. As the Mach number increases further, the resultant nose down attitude causes Mach tuck to increase. Excursions past Mmo may also cause flow separation of boundary layer air over control surfaces. This can create an effect known as aileron buzz and may result in loss of control effectiveness. Your jet likely has an overspeed warning system to warn of Mach Crit as well as an automatic sys- tem (if the autopilot is engaged) to prevent Mach tuck.
What Am I Forgetting?
I’m sure you have heard the adage that certain air- craft sounds can only be heard at night, and some only when you are alone. The intensity of apprehension is
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24 • TWIN & TURBINE / October 2022