1013 hPa (there is no display of the 0,25 in the cockpit) is used as an ISA reference, if the pressure is high it's more dense, if it's low it's less dense (same with temperature), this changes what's called the pressure altitude, by about 30 feet per hPa, if the pressure is 1000 hPa the pressure altitude is 390 feet, this affects performance. There are a lot of calculations before an aircraft takes off to figure out the runway needed if an engine fails, is there enough to either stop or get airborne if one engine fails. What will the climb rate be and will it clear any obstacles. Every large airplane pilot does this before any takeoff.
I used to live in Abu Dhabi, every airline came thru in the middle of the night when it was coolest. (This is long range aircraft, going to Europe or to the Far East). If they had a mechanical and got delayed, things got frantic once the sun came up because the airplane would be too heavy for the runway available as the temperature climbed from say 30C to 43C, so they had to start taking stuff off, normally cargo, recompute the performance and leave before it got hotter. Most people don't realize most long range heavy jets are operating near the limits of their performance which is always calculated as if one engine quits on the runway at a critical moment when the distance to get airborne is the same as the distance to stop. If 2 engines quit you have a real problem (as in the landing on the hudson) even in a 4 engine aircraft. In a 747 or similar they start dumping fuel the moment one engine quits so the weight is reduced in case a second fails. The fuel comes out a pipe on the rear tip of each wing. Once enroute above 18000 ft in the US and above 4-5000 ft in Europe everyone flies along a line of constant pressure not a constant altitude, although it's displayed that way. This way a plane at 20000 ft will be 1000 ft lower than one at 21000 ft in the same spot. Everyone is flying around with 1013 hPa set in the Kohlsman window. If you fly into a low pressure area you'll actually be lower than your set altitude and vise versa. Mike Payne On 05/09/2011, at 09:35 , Kilopascal wrote: > According to this paragraph from Wikipedia: > > Aircraft operating at higher altitudes (at or above what is called the > transition altitude, which varies by country) set their barometric altimeters > to a standard pressure of 29.92 inHg (1 atm = 29.92 inHg) or 1,013.25 hPa (1 > hPa = 1 mbar) regardless of the actual sea level pressure, with inches of > mercury used in the U.S. and Canada. The resulting altimeter readings are > known as flight levels. > > It appears from the statement highlighted in red that the sea level pressure > is not factored in. > > There is nothing magical about sea level pressure. Any pressure can be > established as a reference. All you need to do is factor in all the > variations from the reference to determine where you are above the ground. > > > [USMA:51071] Re: planes collide > > Martin Vlietstra > Sun, 04 Sep 2011 14:42:48 -0700 > > I believe that the aviation industry uses air pressure to calibrate > altimeters. Changing the standard atmosphere from 101.325 kPa to 100 kPa > will result in an apparent 130 m difference in reference altitude. I don't > think that they are willing to take the risk with this change.
