Hello Captain. I am not a pilot, but I find aviation fascinating. I have been reading your weekly column with interest. My question is; how does a pilot know when to start the descent from cruising altitude to land at destination airport ?
Question asked by Carlos.
The most efficient way to descend an airplane is to stay at cruising altitude for as long as possible, then reduce engine thrust to idle and descend all the way down to a runway, without increasing engine thrust. At idle power, jet engines produce little thrust and consume ten times less fuel that at cruise power.
So the airplane basically glides toward the airport. Airliners have a relatively small wing compared to gliders, but they are aerodynamically very clean, so they can fly fast and consequently glide quite well. A typical glide ratio of an airliner is approximately 18:1. That means that for every 18 feet of forward movement the airplane will descent one foot. This ratio translates to 3 nautical miles for every 1,000 feet of altitude loss. If an airliner is cruising at 35,000 feet, to be able to land at an airport at an elevation close to sea level, the pilot must start the descent 105 Nautical Miles, or 190 kilometers from the airport.
A pilot can easily make this mental calculation. There are however several factors that complicate this formula and the pilot has to make appropriate adjustments. Modern airliners have computers that can make this calculation very precisely, based on data entered by the pilot into the computers and on actual wind and environmental conditions. The computers help the pilot to fly the airplane more efficiently, burning less fuel and thus also reducing emissions.
A bit more about Captain Dan:
Captain Dan Baz enrolled in Flight School at the age of 16.
He has completed studies in Aircraft Engineering and Master of Business Administration. He has been at the controls of many different types of aircraft, from single engine Cessnas to large intercontinental jets on global routes.
Over the last four decades he has flown thousands of hours up in the blue sky.
Have a question for the Captain ? Send it to Captain Dan Baz firstname.lastname@example.org Every week a question from the readers will be selected and answer posted in this column.