Friday, May 4, 2007

After having a short discussion with Mr Lee about aircraft pressurisation while in flight, I think it would be nice to illustrate the point further. Each plane flies at an average of 33 thousand feet, that's about 10km upwards, sometimes more. But the air pressure up there isn't high enough to ensure we get enough oxygen, so the cabin is pressurised to 10 thousand feet, and that is known as the safe altitude. Should anything go wrong with the pressurisation, the pilots descend to that level at 6000+ feet per minute, while oxygen masks supply everyone with 15 mins of emergency 100% pure oxygen, more than enough for the plane to get down. This difference in pressure inside and outside the plane places some stress on the aircraft fuselage. So below is a video, on how much damage a tiny fracture can cause. This is a true incident, and the video is a segment of Air Crash Investigation by National Geographic. Those who have a fear of flying, or think they may get nightmares, or develop a fear of flying after watching are strongly adviced NOT to watch the video. I will not be responsible for counselling you, so watch at your own discretion.

Aloha Flight 243, Boeing 737-100 or -200



Note that near the ending, after the pilots have landed, the way the co-pilot activates the reverse thrust (a whoosh you hear after a plane touches down, formed by directing engine thrust forward to help slow the plane down) is not accurately depicted. You see the co-pilot using the reverser handles to pull the throttles back to idle and the reverser engages as these levers come up. This is not true for an airliner. Firstly, the reverser handles are locked unless the throttles are on idle. Secondly, thrust is reduced to idle latest when the main gear touches down, and reverse thrust is not activated until the nose gear touches down. So what you should have seen is the throttles already at idle, and the co-pilot pulling the reverser handles upwards to engage reverse thrust.

Next is another video from the same series, this one is something most people don't know about. An airplane has a built in automated survival system, which will keep the airplane in the air, should an upset occur. Find out what it is at the end of this next video. What the pilots have to do can be quite surprising. Also, the first part of this video is quite scary for those who do not like disaster documentaries, so once again, watch only if you dare to.

Aeroflot Flight 593 Airbus A310



This is a long flight, so there is a relief crew, thus you see that there is another captain onboard. Anyway, according to current rules, no one is allowed into the cockpit unless they are part of the flight crew, cabin crew serving the flight deck, or riding in the jump seat/instructor seat. The doors are locked and a safety code is need to open it. This is an accident known as CFIT or Controlled Flight Into Terrain, since the plane is perfectly functional, and the crew, having made several severe mistakes, flew the plane into the mountains. The autopilot shutting down is due to the high stress and unusual flight profile, which is out of the envelope of the controls of the AP. Anyway, the angle of tilt or bank you see on the video is made larger. To experience a G force of twice your weight, the aircraft only needs to be 60 degrees away from the horizontal, which is already excessive. You can try proving that using vectors. At the 90 degrees shown, the G forces increases so sharply and the rate of lift loss is so great, the nose will almost certainly plunge downwards immediately.

Anyway, looking all these, you may start to worry about the safety of flying, especially those who have to fly home every year. Actually, you are more likely to get into problems on your way to the airport (ie on the road) than on the plane itself. If you seriously have doubts you want to clarify, you can look for me, and I will try to explain it to you as best as I can.

Yong Lin
Class Aviation Freak

-posted by 07S6K at 10:01 PM