Let It Snow
Fifteen minutes from your destination at 7,500 feet, and 500 feet below the clouds, you notice that snow is beginning to fall, and quite heavily. You’re not instrument rated. You begin an immediate descent at a nominal 500 feet per minute. Although the speed that snowflakes fall varies, it averages out at roughly five mph. Assume that the snow is widespread and falls uniformly at that rate, your destination runway is at sea level, and you decide it’s safe to make a straight-in arrival, to avoid possible instrument conditions. How much time, if any, will you have to land before snow begins falling on the runway and visibility could go below VFR minimums?
- You won’t make it. (And if it’s heavy snow, you might as well be inside the clouds.) The snow will reach the runway five minutes ahead of you.
- You’ll have plenty of time, and you’d have over ten minutes to land, taxi in, and tie down. You could even make a normal pattern entry and relax.
- You’d probably have about two minutes to spare.
- You would make it, but the snowfall would be right above you, all the way down.
Answer: Most snowflakes fall rather slowly, due to their relatively large surface area and low weight. Their average speed of about five miles per hour would about equal a 440 fpm descent. Descending from 7500 feet at 500 fpm takes 15 minutes, which would put you over the numbers just at the conclusion of your descent (if no other traffic intervened). During those 15 minutes, you would gain about a 900 foot lead on the snowfall, which would give you about two minutes to land and clear the runway. The answer is C. That’s cutting it a bit close, but it could work. It would probably be smarter to descend more quickly, land at a closer airport, or both.
Noises Off
You’re descending at a relatively high speed in calm air, and since there was no forecast for nor little chance of turbulence, you let the airspeed climb into the yellow arc, but fail to keep your attention focused on the airspeed indicator. Then you hear this buzzing noise. What might that mean?
- You probably had a bee, wasp, or other insect as a stowaway.
- Your mike is stuck.
- One or more Dzus fasteners have come loose, most likely on the cowling.
- You’d better slow down!
Answer: Although aileron buzz usually exists in transonic airplanes, and is generally not an automatic precursor to flutter, it can precede structural failure in even an ultralight aircraft. True, the ailerons of certificated lightplanes are not supposed to exhibit aileron buzz even at never-exceed speed (red line), but it can–and has–happened. The reason can be an imbalance condition or problem in the control cables, for example. The “buzz” usually refers to a rapid oscillation of a control surface, most often an aileron, which doesn’t usually attain a large amplitude or become dangerous, and is caused in most cases by shock-induced separation of the boundary layer at high transonic speeds. Nonetheless, you have just become a test pilot, and the only correct thing to do is to slow down (choice D). Again, such events have usually occurred with high speed operations and at high altitudes.
Tales From the Crypt
What is the “coffin corner”?
- the dead end of an uphill one-way Idaho back-country airstrip
- a narrow airspeed range, within which one can be on the verge of flying both too fast and too slow, at the same time
- a politically incorrect term for the NTSB reference library
- a region formerly part of Hungary, now Romania, above which many battles were waged between Soviet and eastern European fighters during WW II
It has nothing to do with overestimating the influence of gravity against brevity, sarcastic barbs against morbid preoccupation, or Transylvanian transgressions. It’s about speed. As we know, an airplane’s indicated airspeed decreases as its altitude increases, due to the greatly reduced air density, while its actual or true airspeed actually increases. Because the air is much less dense, the angle of attack has to be increased to maintain the same lift, but because of that and the already high true airspeed, the air must become more greatly accelerated over the wing, attaining speeds near Mach 1, which induces flow separation and buffeting. Eventually, the gap between a “low speed” and a “high speed” or Mach related stall continues to narrow, the higher you go, and finally stall speed and high speed Mach buffet merge in a way, and any sudden roll, increase in angle of attack, load factor, gusts, or turbulence could result in a loss of control. Theoretically, according to Barry Schiff, an airplane like a U-2 in a turn at high altitude could experience a stall buffet on the inside wing, and at the same time, Mach buffet on the outside one. Manifestations often include more typically low speed problems such as Dutch roll (coupled oscillations of roll and yaw), adverse yaw, and stalling. Undoubtedly attracted by both the potential alliteration and the opportunity to wax sepulchral, aeronautical engineers didn’t miss their chance to label this region the “coffin corner”. The answer is B.
