Crash landings/ditchings
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Hey chornedsnorkack:
Aeronautical engineers go out of their way to design aircraft so that the rudder does not get into “disturbed” airflow very often (hopefully never) – same thing with the elevator. It would be rare to experience situations where a pitch attitude, even up to the stalling AoA, would completely blank out the rudder or elevator. That is why you see some airplanes with very tall vertical stabilizer/rudders and some with very long (wide) horizontal stabilizers/elevators. (The DC-9 was specifically built with a wider horizontal tail/elevator because of the bad experiences the BAC1-11 had with blanking out the elevator and rudder when in a deep stall. The flight crew had no control. So, the –9 was built with wider pitch control surfaces and a hydraulically powered elevator when the cockpit controls were pushed forward to their full extent.)
The reason I say that the pilot has little if any directional control when an airplane has the MLG on the ground and the nose still quite high is, first, as you recognize, there is no nose wheel steering with the nose in the air. Second, there is very little directional control from the rudder because, in addition to at least some of the airflow over the rudder being “disturbed,” the major factor is that the airplane is slowing down rather rapidly, and the resulting rudder authority is getting less and less even more rapidly. I know you recognize that the slower you fly, the greater the cockpit controller position must be (deflecting the flight control surface to a greater degree) to get the desired reaction from the airplane.
Use of the wheel brakes when attempting to brake the airplane aerodynamically is generally not considered. First because the application of the wheel brake will cause a downward pitching (rotational) moment for which the “up” elevator will be less likely to be able to control (even if you are expecting it), and, steering the airplane with differential braking, even with the nose wheel on the ground, is a skill that does not come without some considerable practice.
The flaps, if they are working properly are bussed together, so that they extend and retract equally on both wings. Disconnecting them is not possible. Even if it were, the movement of the flaps, at least on a large airplane, is relatively slow, and it would likely take even more practice to learn how select what flap to move where to accomplish what you wanted – and by then, you would need something else anyway. It would also require at least as much, probably more, practice than steering with the brakes while the nose in the air. Overall, not an option. BUT – I really like your question. It shows you’re thinking about the control available to the pilot and what control is likely to have a response that you’re looking for. That is the mark of a good aviator. The best time to do this thinking and arguing and discussing is on the ground, well prior to being in the air and having to make a split-second decision. Keep it up!
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AirRabbit
Hey Rhodie:
I want to sincerely thank you for your comment. You’ve “made my day!” I very much appreciate the fact that you say you’ve learned from my ramblings – and it assures me that all the research, writing, and editing has had some positive influence – and it makes all of that very much worthwhile. Thank you.
By the way, you say you fly “little” aircraft, with and without engines. From that I assume you’re one of those glider types. My hat is off to you. I’m not sure I’d have the “intestinal fortitude” to fly down final approach, knowing that a missed approach is not an option. I’ll just bet with your familiarity with gliders, you could teach all of us something about aerodynamics as well. Keep safe, my friend!
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AirRabbit
Aeronautical engineers go out of their way to design aircraft so that the rudder does not get into “disturbed” airflow very often (hopefully never) – same thing with the elevator. It would be rare to experience situations where a pitch attitude, even up to the stalling AoA, would completely blank out the rudder or elevator. That is why you see some airplanes with very tall vertical stabilizer/rudders and some with very long (wide) horizontal stabilizers/elevators. (The DC-9 was specifically built with a wider horizontal tail/elevator because of the bad experiences the BAC1-11 had with blanking out the elevator and rudder when in a deep stall. The flight crew had no control. So, the –9 was built with wider pitch control surfaces and a hydraulically powered elevator when the cockpit controls were pushed forward to their full extent.)
The reason I say that the pilot has little if any directional control when an airplane has the MLG on the ground and the nose still quite high is, first, as you recognize, there is no nose wheel steering with the nose in the air. Second, there is very little directional control from the rudder because, in addition to at least some of the airflow over the rudder being “disturbed,” the major factor is that the airplane is slowing down rather rapidly, and the resulting rudder authority is getting less and less even more rapidly. I know you recognize that the slower you fly, the greater the cockpit controller position must be (deflecting the flight control surface to a greater degree) to get the desired reaction from the airplane.
Use of the wheel brakes when attempting to brake the airplane aerodynamically is generally not considered. First because the application of the wheel brake will cause a downward pitching (rotational) moment for which the “up” elevator will be less likely to be able to control (even if you are expecting it), and, steering the airplane with differential braking, even with the nose wheel on the ground, is a skill that does not come without some considerable practice.
The flaps, if they are working properly are bussed together, so that they extend and retract equally on both wings. Disconnecting them is not possible. Even if it were, the movement of the flaps, at least on a large airplane, is relatively slow, and it would likely take even more practice to learn how select what flap to move where to accomplish what you wanted – and by then, you would need something else anyway. It would also require at least as much, probably more, practice than steering with the brakes while the nose in the air. Overall, not an option. BUT – I really like your question. It shows you’re thinking about the control available to the pilot and what control is likely to have a response that you’re looking for. That is the mark of a good aviator. The best time to do this thinking and arguing and discussing is on the ground, well prior to being in the air and having to make a split-second decision. Keep it up!
_______
AirRabbit
Hey Rhodie:
I want to sincerely thank you for your comment. You’ve “made my day!” I very much appreciate the fact that you say you’ve learned from my ramblings – and it assures me that all the research, writing, and editing has had some positive influence – and it makes all of that very much worthwhile. Thank you.
By the way, you say you fly “little” aircraft, with and without engines. From that I assume you’re one of those glider types. My hat is off to you. I’m not sure I’d have the “intestinal fortitude” to fly down final approach, knowing that a missed approach is not an option. I’ll just bet with your familiarity with gliders, you could teach all of us something about aerodynamics as well. Keep safe, my friend!
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AirRabbit
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Running out of airspeed
Second, there is very little directional control from the rudder because, in addition to at least some of the airflow over the rudder being “disturbed,” the major factor is that the airplane is slowing down rather rapidly, and the resulting rudder authority is getting less and less even more rapidly. I know you recognize that the slower you fly, the greater the cockpit controller position must be (deflecting the flight control surface to a greater degree) to get the desired reaction from the airplane.
So, you say that by the time a plane runs out of the lift to avoid touchdown, and falling to 3-point attitude, the control forces for yaw are unacceptably small - it is the controllability in yaw that runs out first, before lift and pitch control?
Does a plane need adequate airspeed and RAT pressure for braking? For steering with nose wheel? Or does a plane roll completely free and uncontrolled before full stop?
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Hey chornedsnorkack:
If the RAT (Ram Air Turbine) was deployed, yes, that would be less effective as well. But, understand that a RAT is an emergency device – deployed when either the electric or hydraulic system(s) is/are malfunctioning or unavailable – and it is not generally used unless there is such an emergency; and not all aircraft have such a mechanism. Because it is a “Ram Air” device, any time there is less air to “ram” into it, it’s not going to function as well; and you can diminish the air flowing directly into the intake either by slowing down or by changing the angle the air has to follow to get into the intake.
First, let me be clear – I don’t advocate “running out of lift while avoiding the touchdown.” Doing that, in my not-so-humble opinion, is abdicating the duty and responsibility of the pilot. The pilot lands the airplane; not the other way around. I believe that it borders on irresponsibility to attempt to slow the airplane by keeping it off the ground – when most of the time this is strictly for the “imperceptible” touchdown and the applause of the passengers. Horse-hockey! Unless you know exactly what you are doing AND you have a whole bunch of runway ahead of you that you are not ever going to use, the goal should always be to put the airplane on the ground and get it stopped the best and safest way possible. Can you do it the other way? Sure, but please note, I’m being very very careful about acknowledging what has to be in place before I agree that this may be done!
Second, after the airplane touches down, the AoA will certainly have changed – even if the RoD was very small, there was an AoA dependent on the angle of the wing chord and the relative wind. After touchdown the relative wind will necessarily change and that will change the wing’s AoA. Also, friction with the ground (even without wheel brakes or ground spoilers, etc.) will cause the airplane to slow. This also decreases the lift produced by the wings. If the nose is held off for any appreciable amount of time after landing (and aerodynamic braking would be the only reason why this might happen) certainly, the additional drag both form (or parasite) drag and induced drag (although, in ground effect, the induced drag is lessened) will also act to slow the airplane.
Without doing an analysis of the aerodynamic forces (“lift”) produced by the vertical stabilizer / rudder and the horizontal stabilizer / elevator it would be hard to say which loses more first. Suffice it to say that directional control is probably more important than aerodynamic braking – most runways are not extremely wide (except when landing something like a Cessna 152 on a commercial aiirport runway) and controlling the diretion the airplane's heading is a very serious issue. This is particularly true if there is a crosswind. I’m sure you’re familiar with the control inputs necessary to maintain the longitudnal axis of the airplane aligned with the centerline of the runway to land in a crosswind. That requires a given amount of rudder input already – with the airplane developing the amount of aerodynamic force available during the final approach, flare, and touchdown. The required amount of rudder deflection will only have to increase as the aerodynamic effectiveness of the rudder is reduced. You can always put the nose on the ground and use nose wheel steering or differential braking, or a comination of the above for directional control.
As I indicated above, a RAT is an emergency device. Even if all hydraulic fluid were lost and a RAT (if the airplane had one) were deployed to get the gear down and locked, etc., the RAT is going to be less effective as the airspeed is slowed to land and particuarly so after landing. However, almost all larger airplanes have hydraulic accumulators that provide some hydraulic pressure (most provide 3000 psi) to exercise the brakes. Most emergency procedures advise turning off the anti-skid system with a loss of hydraulics. If left on, the anti-skid system could cycle through several times in just seconds and deplete the accumulator pressure, leaving you with no brakes. Some airplanes have a pneumatic (air pressure) system to back-up the hydraulic accumulator back-up system.
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AirRabbit
Yes, as the plane loses airspeed, all aerodynamic forces decrease. The lift available decreases, the control forces for pitch decrease, the control forces for yaw also decrease - and so does the hydraulic pressure from the RAT, right?
you say that by the time a plane runs out of the lift to avoid touchdown, and falling to 3-point attitude, the control forces for yaw are unacceptably small - it is the controllability in yaw that runs out first, before lift and pitch control?
Second, after the airplane touches down, the AoA will certainly have changed – even if the RoD was very small, there was an AoA dependent on the angle of the wing chord and the relative wind. After touchdown the relative wind will necessarily change and that will change the wing’s AoA. Also, friction with the ground (even without wheel brakes or ground spoilers, etc.) will cause the airplane to slow. This also decreases the lift produced by the wings. If the nose is held off for any appreciable amount of time after landing (and aerodynamic braking would be the only reason why this might happen) certainly, the additional drag both form (or parasite) drag and induced drag (although, in ground effect, the induced drag is lessened) will also act to slow the airplane.
Without doing an analysis of the aerodynamic forces (“lift”) produced by the vertical stabilizer / rudder and the horizontal stabilizer / elevator it would be hard to say which loses more first. Suffice it to say that directional control is probably more important than aerodynamic braking – most runways are not extremely wide (except when landing something like a Cessna 152 on a commercial aiirport runway) and controlling the diretion the airplane's heading is a very serious issue. This is particularly true if there is a crosswind. I’m sure you’re familiar with the control inputs necessary to maintain the longitudnal axis of the airplane aligned with the centerline of the runway to land in a crosswind. That requires a given amount of rudder input already – with the airplane developing the amount of aerodynamic force available during the final approach, flare, and touchdown. The required amount of rudder deflection will only have to increase as the aerodynamic effectiveness of the rudder is reduced. You can always put the nose on the ground and use nose wheel steering or differential braking, or a comination of the above for directional control.
Does a plane need adequate airspeed and RAT pressure for braking? For steering with nose wheel? Or does a plane roll completely free and uncontrolled before full stop?
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AirRabbit
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Crash landing
Since the title of the thread is Crash landings/ditchings, it is an issue.
A lot of controlled crash landings happen because an airplane lacks engine thrust. Because why should an airplane with adequate thrust for level above ground flight perform a crash landing?
And the engine is ultimately the source of much of the power available on the plane - pumping air, generating electricity, pumping hydraulic fluids...
All of which systems are affected once all engine power is lost.
A glider is a plane which has no functional engines - but it has airspeed (kinetic energy) and altitude (potential energy) and is able to use controls to convert between one and the other, while undergoing irreversible loss of the total.
And the RAT-s are devices to convert kinetic (and potential) energy of the planre to other usable forms of power, like hydraulic pressure. So, on a crash-landing plane like a glider, they are very vital.
As you mentioned, there are also hydraulic accumulators, so it is important to use them wisely.
A lot of controlled crash landings happen because an airplane lacks engine thrust. Because why should an airplane with adequate thrust for level above ground flight perform a crash landing?
And the engine is ultimately the source of much of the power available on the plane - pumping air, generating electricity, pumping hydraulic fluids...
All of which systems are affected once all engine power is lost.
A glider is a plane which has no functional engines - but it has airspeed (kinetic energy) and altitude (potential energy) and is able to use controls to convert between one and the other, while undergoing irreversible loss of the total.
And the RAT-s are devices to convert kinetic (and potential) energy of the planre to other usable forms of power, like hydraulic pressure. So, on a crash-landing plane like a glider, they are very vital.
As you mentioned, there are also hydraulic accumulators, so it is important to use them wisely.
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Capt Pit Bull:
Your comment,
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PLEASE SOMEBODY MAKE THE SUFFERING STOP.
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Obviously your flight experience is quite limited.
Your comment,
____________________________________________________
PLEASE SOMEBODY MAKE THE SUFFERING STOP.
____________________________________________________
Obviously your flight experience is quite limited.
Last edited by wsherif1; 8th Oct 2005 at 19:40.
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Hey chornedsnorkack.
Again, not a lot of airplanes have a RAT or similar device. A lot of aircraft have an auxiliary power unit (APU) that can supply at least some electrical, hydraulic, and/or pneumatic system operation. I am not a glider pilot (at least not voluntarily), but from what I know, I don't think gliders have a lot of "systems" aboard - except for communications, perhaps navigation, and oxygen. Additonal "systems" I would think would be extra weight that they probably don't want.
You are correct that when there is an engine problem, at least a majority of the "systems" on board are affected -- sometimes rendered useless -- and sometimes there are other back-up systems.
Again, not being a glider pilot I may be wrong, but it would seem to me that a RAT on a glider would be frivilous at best. Since there is no engine power available, once the RAT were deployed, there would be a significant increase on the drag and the pilot would have to descend at a much higher rate to effectively rotate the RAT to gain whatever it is the RAT would supply. I would think that would severely limit what the pilot could do about staying airborne or getting to a preferred landing site. (Now watch, someone is going to come along and show all of us his/her glider, complete with deployable RAT and a hydraulic system to raise/lower gear and flaps….)
Perhaps one of the most publicized “crash” landings with no engines operating but having full APU operational capability was the Southern Airways accident in New Hope, Georgia. I believe it was in the late 1970’s – probably 1977 or 1978. Both engines on the DC-9 failed due to high quantities of water ingestion (large thunderstorm line) but the APU was operational and supplied full electric and necessary hydraulic system pressure to operate the airplane and communicate with the ground. However, having no engines means coming down – somewhere close – and they did – on a road north of Atlanta.
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AirRabbit
Again, not a lot of airplanes have a RAT or similar device. A lot of aircraft have an auxiliary power unit (APU) that can supply at least some electrical, hydraulic, and/or pneumatic system operation. I am not a glider pilot (at least not voluntarily), but from what I know, I don't think gliders have a lot of "systems" aboard - except for communications, perhaps navigation, and oxygen. Additonal "systems" I would think would be extra weight that they probably don't want.
You are correct that when there is an engine problem, at least a majority of the "systems" on board are affected -- sometimes rendered useless -- and sometimes there are other back-up systems.
Again, not being a glider pilot I may be wrong, but it would seem to me that a RAT on a glider would be frivilous at best. Since there is no engine power available, once the RAT were deployed, there would be a significant increase on the drag and the pilot would have to descend at a much higher rate to effectively rotate the RAT to gain whatever it is the RAT would supply. I would think that would severely limit what the pilot could do about staying airborne or getting to a preferred landing site. (Now watch, someone is going to come along and show all of us his/her glider, complete with deployable RAT and a hydraulic system to raise/lower gear and flaps….)
Perhaps one of the most publicized “crash” landings with no engines operating but having full APU operational capability was the Southern Airways accident in New Hope, Georgia. I believe it was in the late 1970’s – probably 1977 or 1978. Both engines on the DC-9 failed due to high quantities of water ingestion (large thunderstorm line) but the APU was operational and supplied full electric and necessary hydraulic system pressure to operate the airplane and communicate with the ground. However, having no engines means coming down – somewhere close – and they did – on a road north of Atlanta.
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AirRabbit
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ATR
Can anyone explain why so many passengers perished in the ATR ditching near Palermo? 19 out of over 30 aboard?
A crash with multiple fatalities and multiple survivors is probably the best test of safety - multiple survivors mean that it´s not a hopeless case, and multiple fatalities mean that the matter was serious...
A crash with multiple fatalities and multiple survivors is probably the best test of safety - multiple survivors mean that it´s not a hopeless case, and multiple fatalities mean that the matter was serious...
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I'm not familiar with this particular accident, but I think it would largely depend on the cause of death of those who did not make it. Did they survive the crash and drown because they could not get out of a sinking airplane? Did they survive the crash, get out of the airplane, and die because of the injuries sustained in the crash? Did they survive the crash, get out of the airplane and drown? Did they survive the crash, get out of the airplane and die of exposure? etc., etc.
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AirRabbit
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AirRabbit
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I've heard of a few ditchings, some with, some without power, and I think in all of them one wing has touched the waves first and caused a cartwheel affect, breaking up the fuselage.
Does anyone know of a "successful" ditching?
I have heard of two ditchings, both I think in Africa, one I think was a cargo 747, where an approach was made over water, and in both cases the crew mistook lights in the water (ie presumably boats) for the runway and landed, power available, gear down, in the water. In the cargo 747 accident I understand that the crew had time to climb out of the flight deck windows before the aircraft sunk, but although the engines were ripped off on contact with the water, there was no cartwheel effect.
So I am wondering. On a commercial jet, should we really keep the gear up on ditching? I reckon that it would be pretty much impossible to keep the wings exactly level, within say a 2-3 foot boundary, which would correspond with a fairly smooth sea, bearing in mind that the aircraft may well also have a crosswind.
I wonder if the "normal approach and landing" to water would be better, being that the only relatively successful ditchings I've heard of did just that.
If one wingtip hits first then the aircraft will cartwheel. However, if one wheel hits first, the second would probably hit fairly soon after, bringing the aircraft back towards straight, and then the 2 wheels/ gear would act on the aircraft to both slow it down and keep it straight, until the gear broke off. But as the aircraft lowered into the water, the engines would probably hit next, followed by the wings.
Does anyone have any knowledge of any ditchings, with or without gear down, successful or not?
Does anyone know of a "successful" ditching?
I have heard of two ditchings, both I think in Africa, one I think was a cargo 747, where an approach was made over water, and in both cases the crew mistook lights in the water (ie presumably boats) for the runway and landed, power available, gear down, in the water. In the cargo 747 accident I understand that the crew had time to climb out of the flight deck windows before the aircraft sunk, but although the engines were ripped off on contact with the water, there was no cartwheel effect.
So I am wondering. On a commercial jet, should we really keep the gear up on ditching? I reckon that it would be pretty much impossible to keep the wings exactly level, within say a 2-3 foot boundary, which would correspond with a fairly smooth sea, bearing in mind that the aircraft may well also have a crosswind.
I wonder if the "normal approach and landing" to water would be better, being that the only relatively successful ditchings I've heard of did just that.
If one wingtip hits first then the aircraft will cartwheel. However, if one wheel hits first, the second would probably hit fairly soon after, bringing the aircraft back towards straight, and then the 2 wheels/ gear would act on the aircraft to both slow it down and keep it straight, until the gear broke off. But as the aircraft lowered into the water, the engines would probably hit next, followed by the wings.
Does anyone have any knowledge of any ditchings, with or without gear down, successful or not?
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Airbus Girl wrote:
While what you say may sound logical, I can assure that it is not. If a ditching is made with the landing gear extended, once the gear hits the water, the relatively free forward movement of the airplane (including the gear) will be dramatically changed because of the position of the gear relative to the center of mass of the airplane. The rearward force on the gear dragging in the water in combination with the forward momentum of the airplane will cause an instantaneous rotational moment around that gear that will “spear” the nose into the water with all (or almost all) of the forward momentum changed into angular momentum. With this increased downward (angular) momentum established, there is a greater likelyhood of structural failure at the front of the airplane (the cockpit). Aircraft attempting to ditch with the gear extended have been seen to immediately submerge, nose down, tail up, and submerge completely – with only bubbles left on the surface.
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AirRabbit
So I am wondering. On a commercial jet, should we really keep the gear up on ditching? I reckon that it would be pretty much impossible to keep the wings exactly level, within say a 2-3 foot boundary, which would correspond with a fairly smooth sea, bearing in mind that the aircraft may well also have a crosswind.
I wonder if the "normal approach and landing" to water would be better, being that the only relatively successful ditchings I've heard of did just that.
If one wingtip hits first then the aircraft will cartwheel. However, if one wheel hits first, the second would probably hit fairly soon after, bringing the aircraft back towards straight, and then the 2 wheels/ gear would act on the aircraft to both slow it down and keep it straight, until the gear broke off. But as the aircraft lowered into the water, the engines would probably hit next, followed by the wings.
I wonder if the "normal approach and landing" to water would be better, being that the only relatively successful ditchings I've heard of did just that.
If one wingtip hits first then the aircraft will cartwheel. However, if one wheel hits first, the second would probably hit fairly soon after, bringing the aircraft back towards straight, and then the 2 wheels/ gear would act on the aircraft to both slow it down and keep it straight, until the gear broke off. But as the aircraft lowered into the water, the engines would probably hit next, followed by the wings.
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AirRabbit
Last edited by AirRabbit; 21st Oct 2005 at 22:13.
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About 2 years ago, an RAF crew ditched a Nimrod (mil version of the Comet airliner). Landed wheels up flaps up.
Aircraft stayed fundamentally intact, though there was a big fusleage split round about mid-length.
All 7 crew got out with nothing worse than broken bones.
It can be done.
And the pilot got an AFC for it.
Sven
Aircraft stayed fundamentally intact, though there was a big fusleage split round about mid-length.
All 7 crew got out with nothing worse than broken bones.
It can be done.
And the pilot got an AFC for it.
Sven
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Does anyone have any knowledge of any ditchings, with or without gear down, successful or not?
Since the airplane was brand new, they fished it out of the bay, UAL scrubbed it down and gave it a D check, and (the very definition of an excellent landing) JAL used it again.
Report here
