Can a plane land safely if it loses power during approach?

Question

British Airways Flight 38 experienced a loss of power during approach to Heathrow and ended up crashing 270m short of the runway. The aircraft was on glideslope when it lost power. The name "glideslope" suggests it should be able to glide to a safe landing. So why didn't it?

Suppose a commercial aircraft (e.g. 7X7, A3X0) lost all engines (say, due to birdstrike) shortly before landing. How likely would a safe landing on the planned runway be?

It would make sense for the approach angle to be such that a safe landing is very likely. So if not, why not?

EDIT: To clarify (due to @rbp's comment): I want to know how far is the typical approach angle from being able to glide in? And why is the approach angle not chosen so that gliding is possible?

Answer 1

A standard “glide-slope” is 3–3.3°. 3° is 5.2% or 1:19, 3.3° is 5.8% or ~1:17.

A modern airliner will have lift to drag ratio between 17:1 and 20:1 in clean configuration and at best glide speed, which is usually somewhere in range 220–250 knots indicated, possibly even a bit more depending on weight. So it would be just barely able to follow the glide-slope if it was configured for best glide.

But on final approach it will at least need gear down and that increases the drag considerably. It will also already be slower than the best glide speed and have some flaps deployed and retracting them and accelerating to the best glide speed will take some altitude. Therefore if all engines quit on final, the aircraft is definitely landing short of the intended point.

For small aircraft I've read advice that if the runway is long enough, you should normally plan to use the middle part of the runway exactly so that you have a margin when engine quits on short final, accompanied with account of incident where it proved useful right there and another in the next chapter. However airliners need more runway, so they can't afford leaving much reserve on the near end, plus they usually follow the ILS or at least PAPI and those lead to fixed spot 1000 ft past threshold.

Answer 2

On "glideslope" doesn't mean the aircraft can glide to a safe landing.

The glideslope is a 3-degree approach path, which is considered the "normal" landing path.

Gliders, which don't have engines, and many warbirds are flown such that they can glide to the runway from any point on the approach.

Answer 3

This varies for every plane and in every situation. Generally speaking every plane has a "Best Glide" speed. This is the speed through the air at which you will gain the largest ground cover for a given vertical distance traveled. Most planes have a published glide chart like this one for a PA-28. The published glide ratio is usually achieved at the best glide speed or or specified rates may be achived at a specific air speed.

(source)

You can also see that this chart is specifically for 0 flaps, gear up, and 0 wind. All of those factors will decrease the distance you can travel while gliding as they increase drag. The very need of flaps is to increase the approach angle without increasing the speed.

The glide slope is generally around 3 Degrees but more precisely it is at an angle which guarantees clearance from all obstructions that may be in the approach path so it may actually vary.

If you are on slope and you plane has a glide ratio that allows for an at or better angle you may be able to make the runway. However the term does not imply that it allows you to "glide" to a landing should the plane lose power.

On top of all that you will generally be landing into the wind, with that in mind if you have a strong head wind down the runway and you lose power you may may fall short due to an altered ground speed.

There is a nice debate on it here but smaller planes like a Diamond DA-20 has a glide ratio of 11:1 and a Cessna 172 has a glide ratio of 9:1 if memory serves. Various places around the internet seem to say bigger jets have in the 17:1 - 20:1 ratio range so they may even glide better than smaller planes. One difference with bigger jets is that they have larger speed ranges and if best glide speed is at 300KT the plane may be well below that on short final and by the time it pitches down to gain the speed it will be on the ground. Compare this to a Piper Warrior that has a 73KT best glide speed and a 70KT approach speed (sometimes I even come in at 75KT on a long runway in a warrior). It can be far easier to come to best glide in a smaller plane than a big jet. Keep in mind that on final approach you will have flaps down, gear down, and possibly slats down if your plane has them. That will greatly change what this chart would look like. I These charts are mainly published so you know how far you can go if you lose an engine in flight.

Answer 4

Many aircraft are able to do an unpowered glide at the 3$^\circ$ angle that the typical (ILS) glideslope is. However that requires the aircraft to be in clean configuration (no flaps, no gear) and at a speed well above landing speed.

When the aircraft is configured for landing (flaps + gear down) it requires a fair amount of power to maintain on the glideslope.

It would make sense for the approach angle to be such that a safe landing is very likely. So if not, why not?

If the glideslope was steep enough to support an unpowered approach, then there would no margin to slow down the aircraft if it were approaching fast (one can't apply less power). It becomes difficult to stabilize the aircraft before landing. Also turbine aircraft need to be above idle power during the approach to ensure a fast response in case of a missed approach.

In case of a steep approach at near idle power, additional drag would be required to slow down. Most aircraft can only provide this by extending the spoilers. Unfortunately this also reduces the lift produced by the wing, something which is usually avoided on approach. A few transport aircraft such as the BAe 146/ RJ70 / RJ85 / RJ100 and Fokker F70/F100 are equipped with air brakes. They aid in slowing down the aircraft on steep approaches without compromising the lift. Steep approach modifications to the Airbus A318 includes changes to the spoiler logic such that they will extend only to 30% to prevent loss of lift. Propeller aircraft usually have less difficulty with steep approaches since the propeller can be used as an effective air brake. For these reasons the diversity of aircraft at London City airport (glideslope 5.5$^\circ$) is limited to a few models.

Currently there is an experiment running on Heathrow airport that features a increased glideslope (3.2$^\circ$ instead of 3$^\circ$) to reduce noise. Due to the steeper approach the aircraft require less power to maintain speed which relieves people living nearby of some noise.

This small increase in glideslope angle does not require special training for the flight crew or changes to aircraft but it may have a negative effect on the airport's capacity. Since the steeper slope reduces the deceleration capability of aircraft it might lead to an increase in go-arounds (e.g. because the aircraft's speed isn't stabilized at 1000ft, or the separation between successive aircraft is compromised).

Answer 5

The approach path of an airplane making powered vs unpowered (gliding) approach would be very different - normal powered approach is around 3 degrees steep, unpowered approach will be much steeper. While a small piston engine plane can perform power-off approach and still be agile enough to power up on short final for a go-around, a big commercial jet can't do it. Furthermore, the chance of a complete engine failure in a multiengine airplane is fairly small. So it is much safer for multiengine aircraft to execute fairly shallow powered approaches (3 deg).

Answer 6

I think a main difference between piston engines and jet engines is the time it takes for the jet engine to spool up. Where a piston engine has near instantaneous power; a jet engine will take several seconds to produce full power.

It is this reason why a jet pilot prefers to have power in the mid range during an approach somthat if they have to perform a go-around the time it takes to spool up and produce full power is reduced. If the angle was such that the engines were at glide power during the approach a go-around would be a much more dangerous.

If the glideslope were increased so its engine can be at idle during the dedcent, any failure that would cause the jet to land with less than full drag will inherently cause the jet to increase speed during the descent. The pilots would be forced to use a shallower descent angle to keep the speed under control. If the approach wasn't designed for a shallower approach it could cause the jet to get dangerously to terrain.