From: Ray ®	09/11/2001 17:20:07
Subject: re: AVIATION FAQ	post id: 490482
Why do I see jet contrails in some places and not others? [The contrails are generally formed at high altitudes, not near major airports where aircraft are usually lower.] From: Steve (Avatar) Contrails are normally off the wing tips. Well, some are, and these are relatively short lived. The long lasting contrails are produced by two mechanisms (1) burning jet fuel produces water, lots of it. This water vapour is pumped out behind the aircraft and condenses or sublimates to form a condensation trail (2) Air in the upper troposphere can be supersaturated due to the lack of condensation or freezing nuclei. Shove a big jet through it belching out lots of particles and you get instant cloud. This is the type of contrail that will gradually spread out. Why do I get sore eyes on long flights? [High altitude air has very low humidity.] From: Ray ® The gases in the cabin of commercial aircraft are just compressed from the outside air. The main difference with the outside air at 35 000 ft compared to sea level is the relative humidity is much lower at higher altitudes (ie the air is much drier). This is why people on long flights get sore eyes, throats etc. Considering the thousands of bird/aircraft collisions which occur each year, and the potential for one of them to be catastrophic, and with the millions of dollars spent in various ways to avoid them, why is it not feasible to install some sort of dome shaped, wire screen in front of the engines ? The best reason I could find for not doing this is because of the restriction in air flow the screens would cause at the jet intake. A mesh screen decreases the effective intake area in two ways: the screen material itself blocks air from entering, and the extra drag and turbulance caused by the airflow around the screen acts to decrease the flow of air through the screen. To compensate for this, the intakes of a jet engine would have to be twice the size they are now -- which would increase the cost and weight of the engine, neither of which is acceptable. Airplanes and their components are designed to survive a strike by a three pound bird, and a plane has to be able to land after losing an engine to be used in commercial service. The probability is very small that a serious strike would completely disable the airplane, and the focus is on avoiding any sort of strike if possible. That's why the efforts are made around airports to reduce the number of large birds. Kevin Reed, Engineer http://www.madsci.org/posts/archives/sep2000/969809917.Eg.r.html