Air conditioning, humidity control, autopilots and auto-brakes. Airborne internet, coffee makers, drink chillers and butt warmers – they make our airplane feel just like home. These niceties permit our attention to be focused on the safe, efficient and enjoyable operation of the vehicle. But, they also make it easy to overlook the significance of operating a machine at Mach numbers in the flight levels – and how harsh the environment is a scant window-thickness away.
In order to keep our brain in a confident, comfortable and conscious state, aircraft environmental systems are made reliable and redundant. The same is true of systems that alert us to, or prevent us from exceeding, velocities that could result in structural or control issues. Concurrent with our goal of keeping the shiny side up and landing on a paved surface as often as possible, we must ensure that supersonic shock waves and our cabin environment are where they belong. This includes control of Mach, cabin pressure, temperature, smoke and fumes and the decibel level of your favorite music.
I used to drive from Phoenix to the North rim of the Grand Canyon to go hunting. The final leg before ascending the Kaibab plateau is a section of desert with an environment similar to that of Death Valley – it’s dangerously toasty; sometimes in the 120-125 Fahrenheit range. When exiting my air-conditioned, Def Leppard-saturated truck to get fuel, the contrast in temperature 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 minus 20 Fahrenheit – another shock when exiting the vehicle. Through the window at 37,000, these contrasts serve to remind me how harsh the environment is just an inch away. Outside your jet, the air is -50C and blowing at 500 mph. If you lose pressurization or heating, that environment is the one in which you must survive. It will be a life-or-death situation and it will be shocking.
A loss of pressurization can occur slowly or quickly. A slow leak could take minutes, or hours, to become apparent, potentially shrouding the threat. Once at a pressure altitude of 25,000 feet, TUC (Time of Useful Consciousness) is 3-5 minutes. At 35,000, it’s ½ to 2 minutes, at 40,000, it’s 15-20 seconds and above 50,000, about 9-12 seconds. A rapid or explosive decompression, usually associated with structural failure of the pressure vessel, requires immediate action. The onset of hypoxia in an explosive scenario can happen in seconds, due to the drop in pressure of the oxygen already in your blood and the lack of oxygen under pressure to replace it. During a rapid or explosive decompression, the times above are approximately half of those listed at each altitude. In a six to ten-seat jet, it’s quite intense and in a fighter-sized jet it can be downright violent. At the altitudes we fly, once the cabin pressure reaches ambient, we may have two minutes, or as little as 10 seconds, to take action.
The altitude chamber is touted as the ultimate source of high altitude training. In an article published in 1992, however, citing Air Force, Navy and NASA studies, the drawbacks of the chamber, especially for those over 40 and more than 14 pounds overweight, were discussed. Those drawbacks include: expanding gas syndrome (extreme bloating), hyperventilation, cerebral hypoxia (hypoxia of the brain), decompression sickness (the bends) and pneumothorax (over-pressurization of the lungs, causing lung failure).
These dangers are encountered in the pursuit of high-altitude training objectives: correct use of the oxygen mask, pressure breathing techniques, communicating while wearing a mask, Valsalva techniques, recognizing personal symptoms of hypoxia and experiencing rapid decompression. The training objectives are valuable but the risks are not insignificant. An alternative to the chamber is PROTE: Portable-Reduced-Oxygen-Training-Enclosure. As the name implies, the system reduces the amount of oxygen within the enclosure without decreasing the pressure as in a barometric chamber. This allows the learning benefits of a reduced oxygen environment without the dangers of large pressure changes.
The negative effects of the altitude chamber are, however, an unambiguous reminder that even if you recognize a pressurization problem or your symptoms of hypoxia, there are medical conditions that can be an immediate, permanent, and occasionally fatal result of a decompression – whether it’s a gradual or rapid onset and whether it occurs in the airplane or in an altitude chamber. The most common after-effect is DCS (decompression sickness) or the bends – a condition in which gasses in solution come out of solution due to a pressure drop (like opening a bottle of champagne). This release from solution results in circulation loss in the joints, lungs, spinal cord, cerebrum, and cerebellum, accompanied by significant, often-debilitating pain, making it difficult or impossible to fly an airplane.
Cancer, genetic defects and fetal damage are possible conditions resulting from radiation. We are irradiated every day on the surface of the planet from terrestrial radiation, cosmic radiation and galactic radiation. At altitude, the less-dense atmosphere provides much less protection from ionizing radiation. This radiation produces electrically charged atoms known as ions. An ion can react with body tissue and cause the above biological effects – and some of the effects are cumulative. A person on the surface is exposed to only 0.05% as much ionizing radiation as received by a person at 39,000 feet. A 13-hour New York to Tokyo flight generates 64.4 uSv. This means it would take 78 flights to get to the yearly-recommended maximum of 5 mSv, about 6.5 one-way trips, or just over three round-trips per month. Studies estimate that the average person’s risk of dying from all forms of cancer is about 220 for every 1,000 or a 22% chance; The American Cancer Society puts it at 22.83%. After 20 years of high altitude flying, the risk increases to 225 for every 1,000 or 22.5% – 23.13% if you factor in the Cancer Society correction. The bottom line is this: if you fly a lot (78 flights of at least three or four hours each), at pretty high altitudes (above 40,000) and at higher (more northerly) latitudes, you are at “very slightly” higher exposure amounts than recommended. And you increase your risk by only about 0.30% (three-tenths of one percent) over those on the ground.
Warp Speed, Scotty
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 1.2, 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. Just as we know this to be untrue, that you don’t perceive the speed, those that have been supersonic or hypersonic will tell you there is little perceptual differences at those velocities either. 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 over-speed warning system to warn of Mach Crit, as well as an automatic system (if the autopilot is engaged) to prevent Mach tuck.
Coffin corner is the altitude, at a constant gross weight and G-force loading (turns or turbulence will increase G), at which the stall speed is equal to the critical Mach number. At this altitude, it’s difficult to maintain stable flight because any reduction in speed will cause the airplane to stall. And because the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation because of shock waves, any increase in speed will cause the airplane to lose lift, or to pitch abruptly nose-down. The “corner” refers to the triangular shape at the top right of a flight envelope chart where the stall speed and critical Mach number lines come together – going either faster or slower results in a stall.
I encourage you to continue this review by studying pressurization malfunctions, in-flight fires, emergency descents, and the effects of mountain wave, clear air turbulence and the Jet Stream. It can be challenging to fly in thin air where the daytime sky darkens and the Mach demon lives; a place not forgiving of rookie mistakes. You’re a well-trained jet pilot; keep your head in the game and don’t let the butt warmer cause you to think like a rookie.