What is the glide ratio of an aircraft?

To fully understand how an aircraft flies, I'd like to talk about another fundamental concept: glide ratio!

What is glide ratio for an aircraft?

Many participants in Fear of Flying courses think that in the event of a total engine failure, the plane would fall like a stone! Let's put an end to this legend: an aeroplane glides! (and rather well, as we shall see). We use the term 'glide ratio' for all flying 'objects': paragliders, hang-gliders, helicopters, gliders and planes, of course. The glide ratio of an aircraft is its ability to glide with all engines switched off. Let's take an example: Let's imagine that we turn off the engines of a plane at an altitude of 1000 metres (1 km), and that we observe the distance covered by the plane before it lands. If our plane has glided for 15 km, we would conclude that it has a glide ratio of 15. Glide ratio is therefore a ratio between height and glide distance.

Definition of glide ratio

Glide ratio is the distance an aircraft will travel in glide relative to its altitude, with engines off, from a theoretical speed of 0 km/hour. Note: this never happens, as aircraft are never at 0 km/hour in the sky! In reality, they would travel much further than the theoretical distance calculated by the glide ratio.

What is the purpose of an aircraft's glide ratio?

In the early days of aviation, aircraft glide ratios were around 8-10. Today, commercial aircraft easily reach glide ratios of between 20 and 25. As commercial aircraft fly between 10 and 13 km high, such a glide ratio means that in the event of a total engine failure, these aircraft could fly more than 300 km without any problems! So we're a long way from falling like a stone! For your information, this hypothesis is highly unlikely. Total engine failure is so rare that you have more chance of winning the lottery than of encountering such an incident in your lifetime! To illustrate this glide ratio phenomenon, 2 other examples come to mind: An Airbus A330 on a flight from Portugal to Canada on 24 August 2001 ran out of fuel over the Atlantic Ocean with 306 people on board! The crew managed to land the aircraft on a runway in the Azores with both engines shut down, after gliding for around 20 minutes. An impressive achievement! The second is better known, because it was widely publicised in the media. You may remember this airbus A320 belonging to US Airways, which collided with wild geese in its engines after taking off from La Guardia airport in New York. The collision took place 850 metres above the ground and the plane was able to glide for 3 minutes, enough to ditch into the Hudson River, causing....0 death! Modern aircraft are definitely excellent gliders!