Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Miles Jordan on 26/08/2009 11:30:04
-
Miles Jordan asked the Naked Scientists:
What would it feel like if you're on an airliner that suddenly loses power in all engines? Would the plane start to glide gradually downward or would it suddenly plummet?
What do you think?
-
I assume you are referring to a total loss of power while mid flight.
The plane would not plummet to the ground, it would continue to glide, but would suffer greatly reduced speed, which would result in a rapid decent, until the air speed was no longer sufficent to sustain flight. The chances of all power loss are remote and planes can fly even if an engin cuts out.
Watch this http://www.youtube.com/watch?v=62CWpNkPzcw (http://www.youtube.com/watch?v=62CWpNkPzcw)
-
I assume you are referring to a total loss of power while mid flight.
The plane would not plummet to the ground, it would continue to glide, but would suffer greatly reduced speed, which would result in a rapid decent, until the air speed was no longer sufficent to sustain flight. The chances of all power loss are remote and planes can fly even if an engin cuts out.
Watch this http://www.youtube.com/watch?v=62CWpNkPzcw (http://www.youtube.com/watch?v=62CWpNkPzcw)
Not really true, Don. The plane would just glide. The pilot would steepen the angle of descent to keep the plane from stalling but it is nothing like a plummet! If he was near a runway he could safely land it. The space shuttle has a much steeper descent than a typical airliner (when near stall conditions) but that is the only way it can land - it glides in with no engines.
-
The only snag is that you don't get a second go at landing right!
-
There are planes that have plummeted.. is that because of some other gross inconsistency? Pilot pushes nose down or something... Surely there's a point of no return from a plane just being a large glider to it falling nose first to the earth.
-
The shuttle travels very much faster than a commercial airliner http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html (http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html).
There are planes that have plummeted.. ...... Surely there's a point of no return from a plane just being a large glider to it falling nose first to the earth.
I would agree with that.
-
There are planes that have plummeted.. is that because of some other gross inconsistency? Pilot pushes nose down or something... Surely there's a point of no return from a plane just being a large glider to it falling nose first to the earth.
An aircraft can 'plummet' for a couple of reasons. The most common example of 'plummeting' is when an aircraft flies into vertical windshear and this is where it flies into a volume of air that is rapidly moving downwards, carrying the aircraft down with it. Less commonly, an aircraft can 'plummet' if it flies into horizontal windshear, where the air is moving in the same direction as the aircraft. In this case, the airspeed of the aircraft can drop to the point where it is insufficient to produce lift.
Funnily enough, if the nose of an aircraft is pointing to Earth, you're soon going to have enough airspeed to regain lift and pull out of the dive, structural limitations permitting. It's actually when the nose isn't pointing towards the Earth and you're falling that's the problem.
As we're talking about airliners here - they were mentioned in the original question - what would happen if the engines stopped is that the aircraft would start to slow down due to drag and friction. As the aircraft slows down, the wings will produce less lift, so if the pitch of the aircraft is unchanged the aircraft will start to sink. However, airliners need a lot of power to keep them in the air, so they lose speed and lift rapidly, and at a certain point the aircraft will be going too slow, even though it's descending, for the wings to produce enough lift to keep the aircraft flying and it will 'stall'. At this point, the aircraft would then 'plummet' or simply fall out of the sky.
If the engines failed, the pilot could try to compensate for the loss of lift due to the reduction of airspeed by increasing the pitch i.e. by raising the nose and increasing the angle of airflow over the wings, which increases the amount of lift they produce, but the downside to this is that this also increases the drag, so the aircraft slows down even more rapidly and stalls even sooner.
What the pilot has to do then, is achieve a rate of descent, at the optimum pitch, such that (s)he gains/maintains sufficient airspeed over drag and friction to preserve lift.
-
The shuttle travels very much faster than a commercial airliner http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html (http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html).
There are planes that have plummeted.. ...... Surely there's a point of no return from a plane just being a large glider to it falling nose first to the earth.
I would agree with that.
The shuttle also has much smaller wings. All commercial aircraft that I know of will glide. The only reason they will go out of control is because of pilot error or an important bit (like a wing or tailplane) falling off. Some of the most modern military aircraft would have problems because they are not designed to be naturally stable, but if the electronics keeps going they can glide too.
The object is to keep the airspeed comfortably above stalling speed and this is controlled by the steepness of the descent. The pilot can also extend the flaps a bit to give a little more lift at the expense of drag. If there is a runway nearby (or if not the flattest bit of ground possible), the pilot can plot a course to effect a safe landing. It does indeed take judgement because he won't get a second go, but there are electronic aids that can help. It is obviously important not to get too low so erring on the high side is safer - using more flaps can slow you down and beyond a certain point increase the descent rate so it is possible to drop the aircraft more quickly if needed. As the craft approaches the runway the pilot will pull back and hold off the craft until the airspeed falls and it eventually drops on to the runway at close to stall speed. This is not very different from a powered landing but without a second chance.
Recovering from a vertical dive in a commercial airliner is not so easy. The phrase "I wouldn't start from here" comes to mind. It may well be possible (I'm not sure) but I suspect the airframes are not strong enough to take the strain in some cases and it probably needs a lot of initial altitude.
I notice a post from Lee whilst I'm typing this but I'll post anyway.
-
I heard that one of the characteristics of Concorde was that it "Glides like a streamlined brick". So I guess it depends on the aircraft. I gather that landing a helicopter without the engines is quite a common stunt.
-
Many years ago I was flying a small aircraft and when approaching the airport got the runway switched. It was quite windy and so the original runway was such that I would be landing into the wind. This is good because you then have a slower ground speed and as you get near the ground the wind drops a bit; this lowers your airpeed which makes it easy to put the plane down, even if a bit bumpily. But as it was getting dark they switched to the only runway with lights and it only had them in one direction. This meant a downwind landing and, to make matters worse, the runway sloped down towards the far end. I couldn't get the plane onto the ground. I held off as long as I could until the far fence was approaching then decided to open the throttle and go round again. It took three failed attempts but on the last one I decided to push the plane onto the ground. It bounced a bit but stayed down and I managed to brake before the end of the runway.
This is an example of windshear on landing.
-
Gliding like a brick is a common perjorative for high speed aircraft. It really means they have a steep descent slope but they will still glide. I doubt that Concorde is worse than the shuttle.
-
A spot of googling confirms that the space shuttle glides roughly twice as badly as Concorde.
I'd not have liked to be there when they measured the glide angles.
http://en.wikipedia.org/wiki/Gliding_(flight)
http://goliath.ecnext.com/coms2/gi_0199-7092383/Gliding-with-precision-when-your.html
-
... But as it was getting dark they switched to the only runway with lights and it only had them in one direction. This meant a downwind landing and, to make matters worse, the runway sloped down towards the far end.
Yikes! - that is not a good situation to find oneself put in. I expect you would have preferred the original runway, in to the wind, even if it meant a low-light approach and landing. Glad you got down ok.
-
Damn right. I didn't have the confidence to tell them that I would use the original runway anyway. I think this would have been the right decision as it wasn't that dark. But I was a young man and thought the Air Traffic Control knew best and I should do what they say. But then I didn't know how hard (and dangerous) it was until I'd tried it. I was not very experienced.
-
They were just covering their arses. If you'd gone in it wouldn't have been their fault.
-
I find flying an amazing feat. I am not the best passenger, and avoid flying if I can (I have travelled three days in a car to avoid three hours on a plane).
Your story is incredible graham... and I am always amazed at the seeming calmness of pilots in such situations..
maybe I just watch too much 'Air Crash Investigations'
-
I remember the Piper J5 I purchased many years ago; it was a side-by-side two seater with an auxiliary fuel tank behind the two seats. Fuel was gravity feed so if you were on the aux tank and pointed the nose up, the engine quit. This happened to me several times. Small planes are designed so that their trim holds them level at the trimmed airspeed. With the same trim and lower speed the nose drops automatically. So in each case of my engine out experience, the nose dropped, the fuel flowed again and the wind milling engine restarted.
It only happened once on take off. Thankfully the runway was long enough for me to set it back down.