Flying the Plane
Tuesday, 27 October 2009 15:10
Dear Captain Lim,
I’m just wondering, generally what is the gliding speed (with total engine failure) of a commercial airliner eg A340 or B777?
I have heard so much about ELT being picked up by aircraft say flying at 28000 feet. What is the formula for calculating the range of the ELT?
When is the signal heard or signal lost? Roughly, what is the distance apart between signal heard and signal lost? Is this distance fixed?
The best gliding speed of any commercial jet liner is the speed that gives the best “lift/drag” ratio. Yes, a plane can glide at any speed following total engine failure as long as it is not below the stalling speed. But there is only one speed that gives it the maximum gliding range. On the Airbus, it is the “Green Dot” speed indicated on the airspeed indicator.
Following total engine failure, the recommended gliding speed for both the Airbus A340 and Boeing 777 is in the region of around 250 knots. But remember, this is not the speed to fly at initially as the pilot would want to attempt to relight any failed engines in the air. So they prefer to glide at 300 knots, hoping to get the engines started until it becomes hopeless whereby they would then fly at the best gliding speed to reach a good spot for a prepared crash landing.
Please see my previous article about glide landing on a jet here.
The ELTs (Emergency Locator Transmitters) are installed on planes to enable others to detect their location in the event of an emergency crash landing in jungles or water.
How far then, can a plane be located if it crashed into the jungles and the ELT is activated? Well, you can use this formula for calculating the theoretical VHF reception distance in nautical miles: 1.23 multiplied by the square root of the altitude (feet). If the plane were at 28000 feet, that would be around 205 miles.
In practice, the actual practical range is generally around 10% or less than this, depending on transmitter power, the effect of terrain and how good the receiver is. When submerged the ELT’s range is quite drastically affected, since the water absorbs radio waves.
However, I believe the ELTs are equipped with an acoustic "pinger" that runs for 30 days that can be received by acoustic sensors for a long distance in water, since water transmits sound well.
The duration of the ELT is around 48 hours and should be heard if a rescue plane is within the range. Obviously, the power of the transmission weakens as the battery runs low and the distance of the reception will deteriorate as well.
P.S. My apologies to many readers - I have not been answering any questions lately as I was very, very busy.