April 1, 2003
WILLIAM K. KERSHNER
The Navy has its Grampaw Pettibone, an ancient aviator who has infinite experience and knowledge, and a sometimes sarcastic tongue when offering advice on flight accidents and problems. We civilians have Grampappy Altovogel (a modified German phrase for old bird), and after nearly 60 years of flying he is willing to let us have the benefit of his aeronautical experience. He stopped by our airport for coffee. (I didn't know people were still flying the Curtis Robin). I took notes and learned a few things that I'm passing on.
Shadows — If the sun is in the proper position (high) and you have to taxi by another high-wing airplane, looking at the shadows of your and the other airplane's wing is a good way to judge the clearance distance. If the shadows won't make contact, neither will your wing tips. (It's even better if you have a wing walker, when in doubt.) If it's a cloudy day, or at night, or the sun is very low, a wing walker is a very good idea.
If you are reasonably close to a scattered or broken cloud layer (but still legal and safe), the approximate wind direction and speed close to your altitude may be seen by watching the movement of the cloud shadows on the ground.
When you are on the downwind leg (or elsewhere in the traffic pattern) and the sun is right, check your airplane's shadow. You could see a shadow (very close to you) of another airplane in your blind spot.
Crosswind — During the runup before takeoff in a fixed-pitch propeller airplane you notice that the engine is rough on either or both magnetos. (The rpm drop is correct on either mag.) Try turning the airplane so that it is not being run up directly crosswind in a strong wind. If turning the airplane makes no difference, better taxi back to the hangar and talk to a mechanic.
Stroboscopic effect — Your buddy (male or female) has the same-model airplane you have, but on those breakfast flights he or she keeps running off and leaving you behind. He swears that he's only using 2,200 rpm. You are using 2,200 rpm but can't keep up. So you buy a new prop (same pitch) and have the engine overhauled, but it's still happening. On the next cross-country you fall in safely behind (your usual position) and look through your prop disk at his prop disk. Change the rpm until the other prop "stops." Check what your tachometer indicates (2,400 rpm!) and you can confront him after landing. A lot depends on tachometer accuracy or pilot-parallax on both airplanes, but this experiment will give you something to start with, anyway.
Brake check — Recently a tricycle-gear airplane ran off the end of the runway at one of the local airports. The pilot indicated that he was high and fast, and as he applied brakes one of the pedals "went to the floor." He had not checked the firmness of the pedals while approaching the airport, didn't know how to brake with one brake, and kept some power on even as he departed the hard-surface runway. If you press on the brake pedals as you fly to the airport and find one or both "go to the floor," you can (1) go to another airport with longer runways and/or better maintenance (be careful taxiing in); (2) make a short-field, full-flaps approach and landing at the original destination; or (3) do (1) and (2) and know how to brake with one brake and taxi slowly after you're on the ground. At least you'll have some warning before touching down.
Another source of fuel — If for some reason the carburetor is not putting fuel into the engine in the usual sense — the front-seat passenger pulled the mixture control out by the roots in a fit of rage (I'm writing this and will use my own reasons for the failure) — for some airplanes the primer with its different route into the cylinders may be used to "stretch" the glide toward better terrain. Set the throttle to about one-half, pull the primer out, let it fill, and give the engine a surge of power as you push it in. Repeat. This results in a stretch in distance, if a somewhat jerky progression. Maintain the pilot's operating handbook-recommended best-glide speed during this procedure. You're not helping the engine, but it might be a lifesaver under the right conditions.
Density altitude — You can calculate the approximate density altitude at which you're operating if you know the following: standard temperature at sea level (59 degrees Fahrenheit or 15 degrees Celsius) and the standard lapse rate (3.5 degrees F and 2 degrees C per thousand feet of altitude).
With that in mind, add 1,000 feet to the pressure altitude for every 15 degrees F or 8 degrees C above standard for that altitude.
Here's a sample problem. An airplane will be taking off at an elevation (and pressure altitude) of 2,000 feet. The temperature is 97 degrees F or 36 degrees C (rounded off). The standard temperature at 2,000 feet elevation is 52 degrees F or 11 degrees C. The actual temperature is 45 degrees F or 25 degrees C above standard. The density altitude is 2,000 plus 3,000 feet, for an answer of 5,000 feet. (Our airport, located at 2,000 feet, has had that density altitude several times in the summer.) Water vapor or high humidity particularly hurts a reciprocating engine as compared to a jet. Remember the four Hs for takeoff: high, hot, and humid hurt. (A high elevation, a high temperature, and humid air hurt aircraft performance.)
Weight and the takeoff — Weight hurts the takeoff more than you might realize. Extra weight increases the takeoff run by the square of the added value. As an example, if you overload an airplane by 20 percent, or a factor of 1.2 times the gross weight, the takeoff run won't be 20 percent longer but will be 44 percent longer or (1.2) 2 = 1.44. This, of course, assumes that all other factors are equal. Also, you should be aware that ... but that's for another time.
William K. Kershner has been flying since 1945, including a stint in the Navy, and instructing since 1949. He is the author of numerous aviation textbooks, including the Student Pilot's Flight Manual and the Instrument Flight Manual. Visit his Web site ( www.kershnerflightmanuals.com).
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