Autorotate
23rd May 2004, 00:13
This is a copy of a story that we ran and I thought I would ask for feedback from members.
Two serious accidents with several important aspects in common. Was engine failure the culprit, or was the accident pilot induced?
The Ontario PD had just released its MD500E from the department’s maintenance facility. The helicopter departed Ontario Airport on a post-maintenance flight with two crewmembers on board.
The flight on that day in late June 2002, quickly went wrong. The engine failed during climbout and crashed on Mission Blvd just south of the Airport. The pilot, seated in the left front seat, sustained critical injuries. The mechanic, seated next to the pilot, sustained serious injuries.
Less than four months later and just ten miles from the site of the Ontario accident, a San Bernardino Sheriff’s MD 600 crashed shortly after takeoff. The flight departed for an afternoon patrol mission from the Sheriff’s facility at Rialto Airport. Minutes later, an engine failure brought the aircraft down in a residential neighborhood south of the airport. As in the Ontario accident, the pilot was the more seriously injured of the two-man crew.
The wreckage of each aircraft also was similar. When I saw photographs of the two accidents aircraft side-by-side, I began to notice a pattern. Both photos indicated that the helicopters impacted the ground extremely hard, with the first point of impact on the pilot’s side. In the case of the MD 600, the right skid apparently did not even touch the ground. The photos also show that the rotors were turning quite slowly at the time of impact. Some blades on each helicopter appeared to have little, if any, damage.
Both helicopters were climbing at the time the engines quit, without any warning whatsoever. Both ships had audible and visual low-rotor-rpm warnings that would sound whenever the rotor rpm dropped by approximately five percent from normal. In the case of the 500E, the low-rotor-rpm horn and light activate at approximately 98%, with normal rpm being at 102% to 104% of N2. For the MD 600, the warning activates at approximately 95%, with normal rpm being at approximately 100%. In both cases, the audible and visual low-rotor-rpm warnings activate well before ‘low green’ rotor rpm is reached.
However, in the case of a sudden and complete engine failure, it’s not unusual that the pilots simply don’t recognize such sounds, given the shock and surprise of the event. The engines went from climb power to zero power in an instant. Both pilots were caught completely by surprise.
With the collective positioned for climb power, the reduction in rotor rpm was severe and almost instantaneous. Both pilots reacted as quickly as they could, but clearly not quickly enough. Each pilot said he bottomed the collective while pushing forward on the cyclic to gain or regain airspeed. With the pilot staying ever mindful of the possibility of total loss of power during climbout, bottoming the collective must become a reflexive response. Pitch down and cyclic forward; this is a very common, almost instinctive reaction to such an emergency in a helicopter.
Pushing the cyclic forward can be a deadly mistake at a time like this. In this case only down collective is appropriate. Pushing the cyclic forward doesn’t prevent entry into autorotation, but it does not help it, either. Prompt down collective will cause entry into autorotation regardless of cyclic input, but the resulting minimum rotor speed will be lower with forward cyclic. A positive, smooth aft movement of the cyclic is what may be required, provided that forward airspeed is significant.
Aft cyclic will definitely minimize loss of rotor rpm. But what constitutes ‘significant forward airspeed’? Those who have done a lot of height-velocity testing like Don Armstrong believe that something in the neighborhood of 45+ KIAS is a reasonable threshold. Below about 35 KIAS it would be of little help.
I don’t believe it’s accurate to say that there is a threshold below which aft cyclic may not be beneficial. As I teach at Western Helicopters, any forward airspeed is justification for starting the cyclic back immediately. This is especially true in the takeoff profile. My experience at Western Helicopters indicates that moving the cyclic back never causes a problem, even if it is not significantly effective. So do it, and do it immediately!
The rotor system is not in the autorotation mode of flight unless, and until,the airflow is passing upward through the rotor blades. During powered, forward flight, air is being pulled down through the rotor from top to bottom. Just pushing the collective down does not reverse the airflow, at least not in the critical entry into autorotation. Pushing the collective down does, of course, reduce the drag of the rotor system and thereby reduces the rate of rotor rpm decay. Aerodynamicist Ray Prouty points out that the rotor disk actually tips forward as the blades move to minimum pitch, making it even more imperative to move the cyclic aft very quickly.
All helicopter pilots should know that every helicopter rotor system has an rpm below which recovery to normal rpm cannot be attained without the help of the engine. This critical rpm is not marked on the rotor tach but is generally estimated to be around 80% rpm.
The FAA does not require that this catastrophic threshold be defined, or tested to confirm it. Aerodynamic analysis can predict it for a given set of assumptions—weight, altitude, temperature, airspeed, airfoil design—but it must clearly be below the minimum rotor speed limit (redline). Type certification testing involves in-flight demonstration of safe recovery from five percent below the minimum redline rpm, usually done at speeds in the vicinity of Vy. The 80% figure is a reasonable assumption of that catastrophic threshold rpm.
It is my opinion that the rotor systems of both of these helicopters entered this critical area from which normal (in the green range) rotor rpm could not be regained, regardless of altitude, attitude, airspeed or flight maneuvers. Before either pilot reacted, the rotor rpm was below the normal operating range. But this alone isn’At catastrophic — momentary dips below minimum redline rpm can be expected with sudden complete engine failures. But the assumption in certification is that the collective will be lowered promptly to the downstop, and that is borne out during height-velocity testing, where all heights at and above the knee involve an arbitrary delay of at least one second before lowering the collective.
Under similar circumstances, I believe very few helicopter pilots, regardless of experience or flight hours, could have reacted quickly enough to avoid having the rotor rpm drop below the green range. The combination of climb power and total surprise is potentially deadly. This is precisely the challenge to flight instructors—reducing the number of pilots who tend to ignore the risk of engine failure—by instilling good, reflexive reactions to sudden failures.
In the MD 500 series helicopters, sudden engine failure at slow airspeed and high power calls for the immediate application of aft cyclic. Sure, if you have hands on the cyclic and collective at the time the engine quits, then moving collective down to the stop should coincide with the aft cyclic movement. But to delay bringing the cyclic back until the collective is down is wrong.
I base this opinion on my observations of pilots who operate their helicopters with no hands on the collective. This is especially true for pilots flying single-pilot from the left seat, such as in the MD 500 and MD 600. In these machines, pilots routinely remove their left hand from the collective, holding it in position with their leg and/or applying friction to it, while they fly the cyclic with their left hand and use their right hand for something else, such as adjusting the radios.
Although this is certainly not an ideal situation, it’s what occurs in the real world. One hand is always on the cyclic. By all means, lower the collective as soon as you can, but meanwhile, get the cyclic moving aft to preserve as much of the rotor rpm as possible.
At Western, we teach that any autorotation entry that involves pushing the cyclic forward is an automatic failure. One of our teaching techniques involves asking a student to touch the fire extinguisher on the left-front doorpost of the 500D to see if it is vibrating too much, and then we chop the throttle. We demand that the cyclic be started back instantly, while the student gets a hand back on the collective and starts it down. To see how critical this really is, repeat this process in a helicopter with a reciprocating engine, when the rotor rpm falls much more quickly than in something powered by a free-turbine engine.
The height-velocity curve is a constant reminder of the need for airspeed, but at the time the engine quits, the airspeed indicator is the least important instrument in the cockpit. The rotor tach is the only instrument of any importance at a time like this. We must remember that we are flying a rotary-wing aircraft, and all of our wings need to have airspeed above stall in order for them to function. Pitot-tube airspeed is of no consequence whatsoever, unless and until the rotor is back in the green.
It would be wonderful if, at the onset of an engine or driveline failure, all instruments on the panel, except the rotor-tach would go blank and stay that way until the rotor-tach is back in the green. Once the rotor rpm is back in the green, the pilot can attempt to regain airspeed if needed, and if time and altitude allow.
Pushing the cyclic forward at a time like this may well increase the airspeed, but at the expense of rotor rpm. With the rotor rpm in the “never-never” range, from which recovery cannot be made, the helicopter then becomes a falling object. My theory is that, in the early stages immediately after the engine fails, the retreating blades stall quickly, if the collective is not immediately lowered.
Retreating blade stall may cause the aircraft to begin a roll to the left. This would account for the fact that the accident aircraft ended up on their left sides. The blades on the advancing side are still producing a certain amount of lift, although they too will quickly stall if the rpm continues to slow.
When the rpm falls below that catastrophic threshold and the entire rotor stalls, regardless of airspeed, the sink rate increases and response to cyclic input is lost. The pilot loses all control of the direction of the flight. With the entire rotor stalled, response to cyclic inputs is virtually non-existent. At this point the pilot is just along for the ride. Directional control is lost, and the helicopter begins rolling to the left. This explains the hard landing on the left side of each aircraft. By extension, we should expect a rightward-rolling tendency on clockwise-rotating rotor systems, such as the Eurocopter AStar.
The exact altitude of the accident helicopters at the time of engine failure is not definitively known, but estimates place them between 250 feet and 500 feet AGL. It is my belief that if the helicopters had been significantly higher above the ground when the engines quit, the crews probably would not have survived. As they descended, both helicopters were picking up speed as their rotors slowed down, and additional altitude would have made the crash forces worse than they were.
Part II on Next Post.
Part II.
The exact airspeed at the time of each engine failure is also unclear. But the height-velocity curve is not a factor in either of these accidents. The main factor is the initial cyclic input being moved forward rather than aft.
No doubt that the NTSB reports will cite engine failure as the cause of these two accidents. Clearly, it initiated an unfortunate chain of events, but the real culprit was rotor rpm. The rpm dropped to the point where recovery was impossible. At that point, the crews were simply along for the ride. Directional control was lost, and because of the very low rotor rpm, there was virtually no kinetic energy remaining in the rotor system. The final pitch pull was of little, if any, value in reducing the crash forces.
In addition, retreating blade stall caused the helicopters to roll to the left before ground contact, greatly decreasing the crashworthiness of the airframe and the effectiveness of the pilot and passenger restraint systems.
The message is this: If the engine fails in forward flight with pitch applied, start the cyclic back immediately. Get the collective down as quickly as possible, but this alone will not stop the decay of the rotor rpm, especially if the pilot is pushing the cyclic forward for any reason.
--------------------------------
Thanks for any comments and opinions.
Ned
Two serious accidents with several important aspects in common. Was engine failure the culprit, or was the accident pilot induced?
The Ontario PD had just released its MD500E from the department’s maintenance facility. The helicopter departed Ontario Airport on a post-maintenance flight with two crewmembers on board.
The flight on that day in late June 2002, quickly went wrong. The engine failed during climbout and crashed on Mission Blvd just south of the Airport. The pilot, seated in the left front seat, sustained critical injuries. The mechanic, seated next to the pilot, sustained serious injuries.
Less than four months later and just ten miles from the site of the Ontario accident, a San Bernardino Sheriff’s MD 600 crashed shortly after takeoff. The flight departed for an afternoon patrol mission from the Sheriff’s facility at Rialto Airport. Minutes later, an engine failure brought the aircraft down in a residential neighborhood south of the airport. As in the Ontario accident, the pilot was the more seriously injured of the two-man crew.
The wreckage of each aircraft also was similar. When I saw photographs of the two accidents aircraft side-by-side, I began to notice a pattern. Both photos indicated that the helicopters impacted the ground extremely hard, with the first point of impact on the pilot’s side. In the case of the MD 600, the right skid apparently did not even touch the ground. The photos also show that the rotors were turning quite slowly at the time of impact. Some blades on each helicopter appeared to have little, if any, damage.
Both helicopters were climbing at the time the engines quit, without any warning whatsoever. Both ships had audible and visual low-rotor-rpm warnings that would sound whenever the rotor rpm dropped by approximately five percent from normal. In the case of the 500E, the low-rotor-rpm horn and light activate at approximately 98%, with normal rpm being at 102% to 104% of N2. For the MD 600, the warning activates at approximately 95%, with normal rpm being at approximately 100%. In both cases, the audible and visual low-rotor-rpm warnings activate well before ‘low green’ rotor rpm is reached.
However, in the case of a sudden and complete engine failure, it’s not unusual that the pilots simply don’t recognize such sounds, given the shock and surprise of the event. The engines went from climb power to zero power in an instant. Both pilots were caught completely by surprise.
With the collective positioned for climb power, the reduction in rotor rpm was severe and almost instantaneous. Both pilots reacted as quickly as they could, but clearly not quickly enough. Each pilot said he bottomed the collective while pushing forward on the cyclic to gain or regain airspeed. With the pilot staying ever mindful of the possibility of total loss of power during climbout, bottoming the collective must become a reflexive response. Pitch down and cyclic forward; this is a very common, almost instinctive reaction to such an emergency in a helicopter.
Pushing the cyclic forward can be a deadly mistake at a time like this. In this case only down collective is appropriate. Pushing the cyclic forward doesn’t prevent entry into autorotation, but it does not help it, either. Prompt down collective will cause entry into autorotation regardless of cyclic input, but the resulting minimum rotor speed will be lower with forward cyclic. A positive, smooth aft movement of the cyclic is what may be required, provided that forward airspeed is significant.
Aft cyclic will definitely minimize loss of rotor rpm. But what constitutes ‘significant forward airspeed’? Those who have done a lot of height-velocity testing like Don Armstrong believe that something in the neighborhood of 45+ KIAS is a reasonable threshold. Below about 35 KIAS it would be of little help.
I don’t believe it’s accurate to say that there is a threshold below which aft cyclic may not be beneficial. As I teach at Western Helicopters, any forward airspeed is justification for starting the cyclic back immediately. This is especially true in the takeoff profile. My experience at Western Helicopters indicates that moving the cyclic back never causes a problem, even if it is not significantly effective. So do it, and do it immediately!
The rotor system is not in the autorotation mode of flight unless, and until,the airflow is passing upward through the rotor blades. During powered, forward flight, air is being pulled down through the rotor from top to bottom. Just pushing the collective down does not reverse the airflow, at least not in the critical entry into autorotation. Pushing the collective down does, of course, reduce the drag of the rotor system and thereby reduces the rate of rotor rpm decay. Aerodynamicist Ray Prouty points out that the rotor disk actually tips forward as the blades move to minimum pitch, making it even more imperative to move the cyclic aft very quickly.
All helicopter pilots should know that every helicopter rotor system has an rpm below which recovery to normal rpm cannot be attained without the help of the engine. This critical rpm is not marked on the rotor tach but is generally estimated to be around 80% rpm.
The FAA does not require that this catastrophic threshold be defined, or tested to confirm it. Aerodynamic analysis can predict it for a given set of assumptions—weight, altitude, temperature, airspeed, airfoil design—but it must clearly be below the minimum rotor speed limit (redline). Type certification testing involves in-flight demonstration of safe recovery from five percent below the minimum redline rpm, usually done at speeds in the vicinity of Vy. The 80% figure is a reasonable assumption of that catastrophic threshold rpm.
It is my opinion that the rotor systems of both of these helicopters entered this critical area from which normal (in the green range) rotor rpm could not be regained, regardless of altitude, attitude, airspeed or flight maneuvers. Before either pilot reacted, the rotor rpm was below the normal operating range. But this alone isn’At catastrophic — momentary dips below minimum redline rpm can be expected with sudden complete engine failures. But the assumption in certification is that the collective will be lowered promptly to the downstop, and that is borne out during height-velocity testing, where all heights at and above the knee involve an arbitrary delay of at least one second before lowering the collective.
Under similar circumstances, I believe very few helicopter pilots, regardless of experience or flight hours, could have reacted quickly enough to avoid having the rotor rpm drop below the green range. The combination of climb power and total surprise is potentially deadly. This is precisely the challenge to flight instructors—reducing the number of pilots who tend to ignore the risk of engine failure—by instilling good, reflexive reactions to sudden failures.
In the MD 500 series helicopters, sudden engine failure at slow airspeed and high power calls for the immediate application of aft cyclic. Sure, if you have hands on the cyclic and collective at the time the engine quits, then moving collective down to the stop should coincide with the aft cyclic movement. But to delay bringing the cyclic back until the collective is down is wrong.
I base this opinion on my observations of pilots who operate their helicopters with no hands on the collective. This is especially true for pilots flying single-pilot from the left seat, such as in the MD 500 and MD 600. In these machines, pilots routinely remove their left hand from the collective, holding it in position with their leg and/or applying friction to it, while they fly the cyclic with their left hand and use their right hand for something else, such as adjusting the radios.
Although this is certainly not an ideal situation, it’s what occurs in the real world. One hand is always on the cyclic. By all means, lower the collective as soon as you can, but meanwhile, get the cyclic moving aft to preserve as much of the rotor rpm as possible.
At Western, we teach that any autorotation entry that involves pushing the cyclic forward is an automatic failure. One of our teaching techniques involves asking a student to touch the fire extinguisher on the left-front doorpost of the 500D to see if it is vibrating too much, and then we chop the throttle. We demand that the cyclic be started back instantly, while the student gets a hand back on the collective and starts it down. To see how critical this really is, repeat this process in a helicopter with a reciprocating engine, when the rotor rpm falls much more quickly than in something powered by a free-turbine engine.
The height-velocity curve is a constant reminder of the need for airspeed, but at the time the engine quits, the airspeed indicator is the least important instrument in the cockpit. The rotor tach is the only instrument of any importance at a time like this. We must remember that we are flying a rotary-wing aircraft, and all of our wings need to have airspeed above stall in order for them to function. Pitot-tube airspeed is of no consequence whatsoever, unless and until the rotor is back in the green.
It would be wonderful if, at the onset of an engine or driveline failure, all instruments on the panel, except the rotor-tach would go blank and stay that way until the rotor-tach is back in the green. Once the rotor rpm is back in the green, the pilot can attempt to regain airspeed if needed, and if time and altitude allow.
Pushing the cyclic forward at a time like this may well increase the airspeed, but at the expense of rotor rpm. With the rotor rpm in the “never-never” range, from which recovery cannot be made, the helicopter then becomes a falling object. My theory is that, in the early stages immediately after the engine fails, the retreating blades stall quickly, if the collective is not immediately lowered.
Retreating blade stall may cause the aircraft to begin a roll to the left. This would account for the fact that the accident aircraft ended up on their left sides. The blades on the advancing side are still producing a certain amount of lift, although they too will quickly stall if the rpm continues to slow.
When the rpm falls below that catastrophic threshold and the entire rotor stalls, regardless of airspeed, the sink rate increases and response to cyclic input is lost. The pilot loses all control of the direction of the flight. With the entire rotor stalled, response to cyclic inputs is virtually non-existent. At this point the pilot is just along for the ride. Directional control is lost, and the helicopter begins rolling to the left. This explains the hard landing on the left side of each aircraft. By extension, we should expect a rightward-rolling tendency on clockwise-rotating rotor systems, such as the Eurocopter AStar.
The exact altitude of the accident helicopters at the time of engine failure is not definitively known, but estimates place them between 250 feet and 500 feet AGL. It is my belief that if the helicopters had been significantly higher above the ground when the engines quit, the crews probably would not have survived. As they descended, both helicopters were picking up speed as their rotors slowed down, and additional altitude would have made the crash forces worse than they were.
Part II on Next Post.
Part II.
The exact airspeed at the time of each engine failure is also unclear. But the height-velocity curve is not a factor in either of these accidents. The main factor is the initial cyclic input being moved forward rather than aft.
No doubt that the NTSB reports will cite engine failure as the cause of these two accidents. Clearly, it initiated an unfortunate chain of events, but the real culprit was rotor rpm. The rpm dropped to the point where recovery was impossible. At that point, the crews were simply along for the ride. Directional control was lost, and because of the very low rotor rpm, there was virtually no kinetic energy remaining in the rotor system. The final pitch pull was of little, if any, value in reducing the crash forces.
In addition, retreating blade stall caused the helicopters to roll to the left before ground contact, greatly decreasing the crashworthiness of the airframe and the effectiveness of the pilot and passenger restraint systems.
The message is this: If the engine fails in forward flight with pitch applied, start the cyclic back immediately. Get the collective down as quickly as possible, but this alone will not stop the decay of the rotor rpm, especially if the pilot is pushing the cyclic forward for any reason.
--------------------------------
Thanks for any comments and opinions.
Ned