ThomasMore
02-08-2008, 09:43 PM
Large transport pilots* plan their takeoffs considering something called a balanced-field length. (*I mean the transports are large, not the pilots...but some of the pilots are pretty large, too.)
During takeoff, the pilots have to assume that an engine will quit during the takeoff roll. After the engine failure, the airplane has to be able to either stop on the remaining runway (accelerate-stop), or be able to continue the takeoff and climb out on the remaining engine(s), again without running out of runway or hitting terrain after takeoff (accelerate-go).
The decision speed is called V1, and it is calculated for every takeoff based on airport altitude, outside air temperature, airplane weight, and airplane performance. V1 is not the takeoff speed, but the stop/continue speed.
Below V1, if the engine quits, the pilots can hit the brakes and stop on remaining runway. Brakes might catch fire or tires blow, but those are acceptable. If the data are correct, the plane should not run off the end.
As the airplane continues to accelerate, the airplane's momentum continues to increase, and the remaining runway rapidly dwindles.
Above V1, the plane now has too much momentum to stop on the remaining runway. But if an engine quits, the remaining good engine(s) will give the airplane enough performance to keep accelerating to takeoff speed on the remaining runway, and to climb clear of all obstacles.
If the runway isn't long enough for the airplane to do that with the altitude/temperature/weight/performance available, the pilots are prohibited from taking off. They would have to find a longer runway, wait for the outside air temperature to cool down, or offload weight until they reach a point where the airplane meets the required engine-failure takeoff performance.
2075
V-speeds displayed on center pedestal
V1 is decision speed
VR is the speed the pilot "rotates,"
or pulls the nose into the air.
V2 is the takeoff safety speed,
after the aircraft has become airborne.
That is why airliners can't always fill every seat, or have to make fuel stops: If the pilots loaded all the passengers and fuel they wanted, they would be too heavy to safely stop or take off with an engine failure.
That is also why transport aircraft usually use only about 2/3 of a runway for takeoff. The rest of the runway is there to allow them to either stop or keep accelerating after an engine fails.
The long preamble above is to put the following video in its proper context. A Russian Ilyushin Il-76 (http://en.wikipedia.org/wiki/Ilyushin_Il-76) transport plane (NATO code name "Candid") is seen taking off from the 8,800 foot long runway at Canberra, Australia. You can hear the Australian controllers' voices, as well as radio communications with the Russian pilots. All four engines appear to be developing full power. Whether the pilots did their homework, or just ignored the results, is something of a mystery.
<object width="425" height="373"><param name="movie" value="http://www.youtube.com/v/aWtdtuspnoM&rel=0&color1=0x2b405b&color2=0x6b8ab6&border=1"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/aWtdtuspnoM&rel=0&color1=0x2b405b&color2=0x6b8ab6&border=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="373"></embed></object>
During takeoff, the pilots have to assume that an engine will quit during the takeoff roll. After the engine failure, the airplane has to be able to either stop on the remaining runway (accelerate-stop), or be able to continue the takeoff and climb out on the remaining engine(s), again without running out of runway or hitting terrain after takeoff (accelerate-go).
The decision speed is called V1, and it is calculated for every takeoff based on airport altitude, outside air temperature, airplane weight, and airplane performance. V1 is not the takeoff speed, but the stop/continue speed.
Below V1, if the engine quits, the pilots can hit the brakes and stop on remaining runway. Brakes might catch fire or tires blow, but those are acceptable. If the data are correct, the plane should not run off the end.
As the airplane continues to accelerate, the airplane's momentum continues to increase, and the remaining runway rapidly dwindles.
Above V1, the plane now has too much momentum to stop on the remaining runway. But if an engine quits, the remaining good engine(s) will give the airplane enough performance to keep accelerating to takeoff speed on the remaining runway, and to climb clear of all obstacles.
If the runway isn't long enough for the airplane to do that with the altitude/temperature/weight/performance available, the pilots are prohibited from taking off. They would have to find a longer runway, wait for the outside air temperature to cool down, or offload weight until they reach a point where the airplane meets the required engine-failure takeoff performance.
2075
V-speeds displayed on center pedestal
V1 is decision speed
VR is the speed the pilot "rotates,"
or pulls the nose into the air.
V2 is the takeoff safety speed,
after the aircraft has become airborne.
That is why airliners can't always fill every seat, or have to make fuel stops: If the pilots loaded all the passengers and fuel they wanted, they would be too heavy to safely stop or take off with an engine failure.
That is also why transport aircraft usually use only about 2/3 of a runway for takeoff. The rest of the runway is there to allow them to either stop or keep accelerating after an engine fails.
The long preamble above is to put the following video in its proper context. A Russian Ilyushin Il-76 (http://en.wikipedia.org/wiki/Ilyushin_Il-76) transport plane (NATO code name "Candid") is seen taking off from the 8,800 foot long runway at Canberra, Australia. You can hear the Australian controllers' voices, as well as radio communications with the Russian pilots. All four engines appear to be developing full power. Whether the pilots did their homework, or just ignored the results, is something of a mystery.
<object width="425" height="373"><param name="movie" value="http://www.youtube.com/v/aWtdtuspnoM&rel=0&color1=0x2b405b&color2=0x6b8ab6&border=1"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/aWtdtuspnoM&rel=0&color1=0x2b405b&color2=0x6b8ab6&border=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="373"></embed></object>