TransAsia ATR crash in Taiwan.
Discussion
Chuck328 said:
Four engines probably not representative but, on the C130 if you lost say an outboard at take off, what was the roll rate like? I say that because I'll stick my neck out here and say it was nothing to do with VMCA.
My point being that if it was a Vmca1 problem then there is a high likelihood of mis-handling.For the non-flyers, and those who can not seem to understand the difference between an abreviation/initialisation and an acronym
(and to expand upon Testaburger's earlier comments which threw me at first, hence my reply, because he talked about VMC [which to me immediately translates as Visual Meteorological Conditions] and not Vmca (Velocity Minimum Control Airborne). On a multi-engined a/c, loss of an engine will create an assymtric thrust and this will apply a yawing moment to the a/c. As the a/c yaws the outside wing is moving faster through the air than the inside wing. It will therefore produce more lift and the a/c will roll into the yaw ie towards the dead engine. As the a/c rolls, the result of the change in the lift vector (which is effectively perpendicular to the wing) means that the a/c will start to descend; eventually the nose will drop and a spiral dive will ensue.
Controlling this yaw is a function of the fin/rudder and depends upon the rudder authority and moment arm. Additionally banking away from the dead engine will help to stop the yaw (because, just like roll is a secondary effect of yaw, so yaw is a secondary effect of roll) - however, go too far at too low a speed and you have effectively got crossed (pro-spin) controls. However this control is aerodynamic - the fin/rudder produce what is effectively 'sideways' lift. Since lift is proportional to the square of the speed, the slower you go the less control you will have.
Thus there will be a minimum speed at which control can be maintained, go any slower and the a/c will contiue to yaw and roll towards the dead engine. This speed is termed Vmca - Velocity Minimum Control Airborne. This is defined as the minimum speed at which a sufficient margin of control remains for the average pilot to keep the a/c straight while applying full rudder and up to 5 degrees of bank towards the live engine(s). Vmca1 is one engine inoperative, Vmca2 is 2 engines inoperative. The point being that full rudder must be applied and it must be applied before the speed falls to Vmca - otherwise you are outside the parameters.
To say that the assymetric effect of a prop a/c will be worse than on a jet a/c is not necessarily true - it will depend on the amount of thrust being produced on the live side, the amount of drag caused by the windmilling/seized/feathered engie on the dead side and the relative moment arms; there are/were jets that were far more lethal than any prop in the assymetric regime, Canberra for eg.
However, there are exacerbating factors in a prop a/c. These are the propwash effect, torque reaction, P-factor and gyroscopic precession. Essentially the propwash rotates around the a/c and impacts upon the fin/rudder applying a yawing moment. Torque reaction is the tendency to roll in the opposite direction to the way the prop rotates. P-factor is the result of the down going prop blade having a greater angle of attack to the relative airflow than the upgoing blade, P-factor comes into play when the prop is not meeting the airflow head on, eg during pitching manoeuvres. Precession is the result of the gyroscopic effect of the prop - if you pitch the a/c nose up it will try to turn with no input from the pilot - it is often confused with P-factor. Propwash effect, torque reaction and P-factor are additive, precession is subtractive.
The upshot of this is that, if all the engines rotate in the same direction then one of the engines will be the most critical in terms of the yaw/roll effect if that engine is lost.
For eg on Albert (C130) the props rotate in a clockwise direction when viewed from the rear. Thus the propwash effect makes Albert want to turn left and the torque reaction makes the a/c want to roll left. Hence the No1 engine (Port Outer) is the most critical in terms of engine failure because its loss will set up a left yawing moment and, being an outboard, it has the greatest moment arm.
Never having flown an ATR I don't know which engine is the more critical.
Another critical factor is whether the prop is feathered. If not and it's stationary then it will pruduce unwanted drag; if it's windmilling it will also produce drag (negative torque) which will exacerbate the yaw. AFAIK the ATR has autofeather, C130 had a negative torque sensing system that would coarsen the prop automatically (and ultimately the prop would decouple from the engine if the negative torque became too great).
All this is moot however during normal operation and correct handling. For aircraft operating to 'Scheduled Performance Class A', then the rotate speed off the runway, ie the speed at which the pilot pulls the stick back, is greater than Vmca1 by a factor of 1.05. Furthermore, V2 (which is the minimum climb speed for the best climb gradient, one engine inoperative) is greater than Vmca1 by a factor of 1.1. Thus, should you lose a donk above stop/go decision speed (V1) the a/c should be able to get airborne and climb away safely.
Hence my assertion that, if in the case of this crash it was a Vmca1 problem, there is a high likelihood of mis-handling.
To answer Chuck's Q, loss of the No1 on Albert on a normal take off was no real problem, everything would work as Scheduled Performance Class A said it should. However on a Tactical Take Off where you would be rotating around 25kts below normal rotate speed then the loss of an outboard seriously concentrated the mind. You''d have major, large control inputs - full opposite rudder, full opposite aileron, a good handfull of nose down and you would fully retard the opposite outboard. Even then you might not survive. I crashed the sim on a number of occasions.
Edited by Ginetta G15 Girl on Thursday 5th February 14:37
I had to go have a good think to work out how you got Visual flight conditions from VMC, going through visual flight rules and the like before working it out (having got Testaburgers meaning through context).
M being meteorological for anyone else who was a little confused.
Interesting explanation, thank you.
M being meteorological for anyone else who was a little confused.
Interesting explanation, thank you.
Ginetta G15 Girl said:
Chuck328 said:
Four engines probably not representative but, on the C130 if you lost say an outboard at take off, what was the roll rate like? I say that because I'll stick my neck out here and say it was nothing to do with VMCA.
My point being that if it was a Vmca1 problem then there is a high likelihood of mis-handling.For the non-flyers, and those who can not seem to understand the difference between an abreviation/initialisation and an acronym
(and to expand upon Testaburger's earlier comments which threw me at first, hence my reply, because he talked about VMC [which to me immediately translates as Visual Flight Conditions] and not Vmca (Velocity Minimum Control Airborne). On a multi-engined a/c, loss of an engine will create an assymtric thrust and this will apply a yawing moment to the a/c. As the a/c yaws the outside wing is moving faster through the air than the inside wing. It will therefore produce more lift and the a/c will roll into the yaw ie towards the dead engine. As the a/c rolls, the result of the change in the lift vector (which is effectively perpendicular to the wing) means that the a/c will start to descend; eventually the nose will drop and a spiral dive will ensue.
Controlling this yaw is a function of the fin/rudder and depends upon the rudder authority and moment arm. Additionally banking away from the dead engine will help to stop the yaw (because, just like roll is a secondary effect of yaw, so yaw is a secondary effect of roll) - however, go too far at too low a speed and you have effectively got crossed (pro-spin) controls. However this control is aerodynamic - the fin/rudder produce what is effectively 'sideways' lift. Since lift is proportional to the square of the speed, the slower you go the less control you will have.
Thus there will be a minimum speed at which control can be maintained, go any slower and the a/c will contiue to yaw and roll towards the dead engine. This speed is termed Vmca - Velocity Minimum Control Airborne. This is defined as the minimum speed at which a sufficient margin of control remains for the average pilot to keep the a/c straight while applying full rudder and up to 5 degrees of bank towards the live engine(s). Vmca1 is one engine inoperative, Vmca2 is 2 engines inoperative. The point being that full rudder must be applied and it must be applied before the speed falls to Vmca - otherwise you are outside the parameters.
To say that the assymetric effect of a prop a/c will be worse than on a jet a/c is not necessarily true - it will depend on the amount of thrust being produced on the live side, the amount of drag caused by the windmilling/seized/feathered engie on the dead side and the relative moment arms; there are/were jets that were far more lethal than any prop in the assymetric regime, Canberra for eg.
However, there are exacerbating factors in a prop a/c. These are the propwash effect, torque reaction, P-factor and gyroscopic precession. Essentially the propwash rotates around the a/c and impacts upon the fin/rudder applying a yawing moment. Torque reaction is the tendency to roll in the opposite direction to the way the prop rotates. P-factor is the result of the down going prop blade having a greater angle of attack to the relative airflow than the upgoing blade, P-factor comes into play when the prop is not meeting the airflow head on, eg during pitching manoeuvres. Precession is the result of the gyroscopic effect of the prop - if you pitch the a/c nose up it will try to turn with no input from the pilot - it is often confused with P-factor. Propwash effect, torque reaction and P-factor are additive, precession is subtractive.
The upshot of this is that, if all the engines rotate in the same direction then one of the engines will be the most critical in terms of the yaw/roll effect if that engine is lost.
For eg on Albert (C130) the props rotate in a clockwise direction when viewed from the rear. Thus the propwash effect makes Albert want to turn left and the torque reaction makes the a/c want to roll left. Hence the No1 engine (Port Outer) is the most critical in terms of engine failure because its loss will set up a left yawing moment and, being an outboard, it has the greatest moment arm.
Never having flown an ATR I don't know which engine is the more critical.
Another critical factor is whether the prop is feathered. If not and it's stationary then it will pruduce unwanted drag; if it's windmilling it will also produce drag (negative torque) which will exacerbate the yaw. AFAIK the ATR has autofeather, C130 had a negative torque sensing system that would coarsen the prop automatically (and ultimately the prop would decouple from the engine if the negative torque became too great).
All this is moot however during normal operation and correct handling. For aircraft operating to 'Scheduled Performance Class A', then the rotate speed off the runway, ie the speed at which the pilot pulls the stick back, is greater than Vmca1 by a factor of 1.05. Furthermore, V2 (which is the minimum climb speed for the best climb gradient, one engine inoperative) is greater than Vmca1 by a factor of 1.1. Thus, should you lose a donk above stop/go decision speed (V1) the a/c should be able to get airborne and climb away safely.
Hence my assertion that, if in the case of this crash it was a Vmca1 problem, there is a high likelihood of mis-handling.
To answer Chuck's Q, loss of the No1 on Albert on a normal take off was no real problem, everything would work as Scheduled Performance Class A said it should. However on a Tactical Take Off where you would be rotating around 25kts below normal rotate speed then the loss of an outboard seriously concentrated the mind. You''d have major, large control inputs - full opposite rudder, full opposite aileron, a good handfull of nose down and you would fully retard the opposite outboard. Even then you might not survive. I crashed the sim on a number of occasions.
Edited by Ginetta G15 Girl on Thursday 5th February 14:31
That said, let's not speculate until we have all the facts, eh chaps?
pip-pip and all that.
Legend83 said:
Ginetta G15 Girl said:
Wow, are you an accountant?
That is so boring.
WACC, LIBOR, ASB, XBRL, ZBA, SOX etc etc.That is so boring.
This maybe a really numpty question and I apologise, what's the different between a "feathered" prop and a "windmilling" prop (I can guess what windmilling props do, but not feathered).
Horrendous accident either way, must have been truly terrifying.
Huge well done to the pilot though, for missing so many other objects and at least getting it to the river.
Horrendous accident either way, must have been truly terrifying.
Huge well done to the pilot though, for missing so many other objects and at least getting it to the river.
Feathered is where the blades are angled to not interrupt the air flow (minimise drag).
Wind milling ones are in the same position as they would be in operation so turn in the air and create drag:
See: http://en.wikipedia.org/wiki/Propeller_%28aeronaut...
Wind milling ones are in the same position as they would be in operation so turn in the air and create drag:
See: http://en.wikipedia.org/wiki/Propeller_%28aeronaut...
Edited by KTF on Thursday 5th February 16:01
AshVX220 said:
This maybe a really numpty question and I apologise, what's the different between a "feathered" prop and a "windmilling" prop (I can guess what windmilling props do, but not feathered).
Horrendous accident either way, must have been truly terrifying.
Huge well done to the pilot though, for missing so many other objects and at least getting it to the river.
Feathering the blades means that they can be rotated parallel to the airflow to reduce drag, I think. Horrendous accident either way, must have been truly terrifying.
Huge well done to the pilot though, for missing so many other objects and at least getting it to the river.
Correct.
Windmilling is not good as it creates massive drag and can even result in the prop "running away" - as happened a lot during World War 2 if damage prevented the pilot from being able to feather the prop.
If you get a runaway prop it can either fly off or rip the engine off its bearings.
Neil Armstrong always claimed that the most frightening experience he ever had in his career was when a B-29 he was flying co-pilot had a prop come off and slash through the fuselage, slicing nearly all of the control lines. they just about got it down.
Windmilling is not good as it creates massive drag and can even result in the prop "running away" - as happened a lot during World War 2 if damage prevented the pilot from being able to feather the prop.
If you get a runaway prop it can either fly off or rip the engine off its bearings.
Neil Armstrong always claimed that the most frightening experience he ever had in his career was when a B-29 he was flying co-pilot had a prop come off and slash through the fuselage, slicing nearly all of the control lines. they just about got it down.
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