Sikorsky S-76: Ask Nick Lappos
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Nick,
Please may I emphasise that the good advice given in the main thrust of your last post applies ONLY to helicopters with the main rotors turning in the "American" direction i.e. from right to left as the pilot looks out the front.
It is vital for pilots to remember that for aircraft with the "French" main rotor direction i.e. left to right from the pilot's perspective, the OPPOSITE is true with regard to the "lucky" side. "Lucky left / Rotten right only applies to "American" rotor direction of rotation.
I speak as someone who has had spells alternately flying Sikorsky then Aerospatiale / Eurocopter designs for some years (almost every other job!).
For a while I was heavily involved in the RAF Puma Sim project and flew it from the front as well as instructing from the rear of the box, so I have seen how confused some pilots can get during tail rotor malfunctions. I have also since carried out the FSI S-76 sim initial and recurrent courses and similarly the US Army UH-60 Blackhawk sim courses.
From my own experiences, and I've said this before on another recent thread on this forum about tail rotor malfunctions, but I do think it is worth repeating, - the "lucky" side appropriate to any helicopter is the RETREATING BLADE side. All the pilot then has to remember is which side is the retreating blade side - and hopefully he should remember that from the rotor start!
The "lucky" side is the safe side to keep the nose cocked off towards during flight towards a suitable landing area and on the approach. Also it helps to keep any crosswind coming from that same retreating blade side during final approach with a tail rotor control malfunction, if there is a choice of landing direction. Increasing power by raising the collective then brings the nose around to point straight ahead for a landing, by increasing the main rotor torque reaction. Once the nose comes around towards the straight ahead position you have found the power/airspeed combination at which the aircraft should be best to land. If this lies within the normal landing parameters for the aircraft, then the pilot will have an excellent chance of landing without breaking anything.
Two more points: I strongly advocate any pilot suffering a tail rotor malfunction to carry out a "dummy approach" at height if circumstances allow and personally I am not a great fan of messing around with engine controls to control yaw, as this can complicate the issue (I've seen quite a few pilots get it horribly wrong in the sim)although I would agree that it may be the good thing to try if you are on the ground and about to yaw off the edge of the landing area.
Cheers, keep up the good work.
Edited for typos; it's well past my bedtime.
ShyT
[ 02 August 2001: Message edited by: ShyTorque ]
Please may I emphasise that the good advice given in the main thrust of your last post applies ONLY to helicopters with the main rotors turning in the "American" direction i.e. from right to left as the pilot looks out the front.
It is vital for pilots to remember that for aircraft with the "French" main rotor direction i.e. left to right from the pilot's perspective, the OPPOSITE is true with regard to the "lucky" side. "Lucky left / Rotten right only applies to "American" rotor direction of rotation.
I speak as someone who has had spells alternately flying Sikorsky then Aerospatiale / Eurocopter designs for some years (almost every other job!).
For a while I was heavily involved in the RAF Puma Sim project and flew it from the front as well as instructing from the rear of the box, so I have seen how confused some pilots can get during tail rotor malfunctions. I have also since carried out the FSI S-76 sim initial and recurrent courses and similarly the US Army UH-60 Blackhawk sim courses.
From my own experiences, and I've said this before on another recent thread on this forum about tail rotor malfunctions, but I do think it is worth repeating, - the "lucky" side appropriate to any helicopter is the RETREATING BLADE side. All the pilot then has to remember is which side is the retreating blade side - and hopefully he should remember that from the rotor start!
The "lucky" side is the safe side to keep the nose cocked off towards during flight towards a suitable landing area and on the approach. Also it helps to keep any crosswind coming from that same retreating blade side during final approach with a tail rotor control malfunction, if there is a choice of landing direction. Increasing power by raising the collective then brings the nose around to point straight ahead for a landing, by increasing the main rotor torque reaction. Once the nose comes around towards the straight ahead position you have found the power/airspeed combination at which the aircraft should be best to land. If this lies within the normal landing parameters for the aircraft, then the pilot will have an excellent chance of landing without breaking anything.
Two more points: I strongly advocate any pilot suffering a tail rotor malfunction to carry out a "dummy approach" at height if circumstances allow and personally I am not a great fan of messing around with engine controls to control yaw, as this can complicate the issue (I've seen quite a few pilots get it horribly wrong in the sim)although I would agree that it may be the good thing to try if you are on the ground and about to yaw off the edge of the landing area.
Cheers, keep up the good work.
Edited for typos; it's well past my bedtime.
ShyT
[ 02 August 2001: Message edited by: ShyTorque ]
The RAAF had a pretty good way of dealing with stuck pedals that was developed for the Iroquois and adapted for the Squirrel when we got them. The Squirrels had been modded with a twist-grip throttle (not all that satisfactory in some ways) so the technique was easily transferable.
Your point about the possibility of stuffing things up when engine controls are manipulated is a good one, Shy Torque, but with the luxury of practice most people found they could cope with pedals stuck in a wide range of positions.
The basic concept was a 60 kt skid ball analysis for a heads-up, followed by a long, low finals decelerating in 10 kt increments until the aircraft nose was pointing a certain limiting number of degrees off the direction of travel.
Then, depending on which way the yaw was, you would (for an American helicopter):
a) For too much left pedal, keep going to a rotating hover, then reduce Nr in small increments which would initially speed up the rotation, but then slow it down as the tail rotor slowed until you could land, or
b) For too much right pedal, slow down until the nose got to be 20 degrees (I think it was) off the direction of travel, then maintain that speed until you were over your big flat grassy landing area, then quickly chop the throttle and run on to an autorotative landing.
With a bit of forethought, you could also steer a bit with judicious (!) amounts of throttle as you ran along, although this was a bit of a mind-blower for some.
Widens your options a bit, I guess, although to coordinate 2 pilots to do it to cope with lever-type throttles would be pretty tricky, I'd imagine.
Your point about the possibility of stuffing things up when engine controls are manipulated is a good one, Shy Torque, but with the luxury of practice most people found they could cope with pedals stuck in a wide range of positions.
The basic concept was a 60 kt skid ball analysis for a heads-up, followed by a long, low finals decelerating in 10 kt increments until the aircraft nose was pointing a certain limiting number of degrees off the direction of travel.
Then, depending on which way the yaw was, you would (for an American helicopter):
a) For too much left pedal, keep going to a rotating hover, then reduce Nr in small increments which would initially speed up the rotation, but then slow it down as the tail rotor slowed until you could land, or
b) For too much right pedal, slow down until the nose got to be 20 degrees (I think it was) off the direction of travel, then maintain that speed until you were over your big flat grassy landing area, then quickly chop the throttle and run on to an autorotative landing.
With a bit of forethought, you could also steer a bit with judicious (!) amounts of throttle as you ran along, although this was a bit of a mind-blower for some.
Widens your options a bit, I guess, although to coordinate 2 pilots to do it to cope with lever-type throttles would be pretty tricky, I'd imagine.
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Arm,
You are right about the difficulty of co-ordinating two pilots during the mainipulation of engine controls; we also used to operate the Puma single pilot which meant the pilot didn't have enough hands, with the ECLs being in the roof panel!
On many larger aircraft in the civilian role it just isn't practical to go messing about with ECLs for practice. On many it isn't allowed, even with an instructor on board, so it can only be practiced in a simulator which means it doesn't get done on a regular enough basis for crews to be really good at it. Some will no doubt argue this one, but I maintain that if it happens for real then there will be some element of the experimental for most line crews.
Personally I would prefer an area of hard surface because if a run-on landing is required and the aircraft does touch down with some yaw it is is a little more forgiving i.e. there is less chance of a wheel or skid digging in and the aircraft rolling over. Of course in UK our grassy areas are usually very soft and muddy which makes our problem worse!
It is a really good idea to keep on discussing tail rotor problems. In the past they were seldom covered in sufficient detail (well done, Sikorsky and FSI for having more recently put some detailed thought into it and come up with useful advice). Those who have never seen one for real or in a simulator (like myself a few years back) need to have the basic principles in the back of their mind. Also, on a selfish basis I like to keep my own dull brain refreshed because I see it as a very cheap life insurance policy.
Two engines is great but there is still only one tail rotor drive shaft and control system!
ShyT
You are right about the difficulty of co-ordinating two pilots during the mainipulation of engine controls; we also used to operate the Puma single pilot which meant the pilot didn't have enough hands, with the ECLs being in the roof panel!
On many larger aircraft in the civilian role it just isn't practical to go messing about with ECLs for practice. On many it isn't allowed, even with an instructor on board, so it can only be practiced in a simulator which means it doesn't get done on a regular enough basis for crews to be really good at it. Some will no doubt argue this one, but I maintain that if it happens for real then there will be some element of the experimental for most line crews.
Personally I would prefer an area of hard surface because if a run-on landing is required and the aircraft does touch down with some yaw it is is a little more forgiving i.e. there is less chance of a wheel or skid digging in and the aircraft rolling over. Of course in UK our grassy areas are usually very soft and muddy which makes our problem worse!
It is a really good idea to keep on discussing tail rotor problems. In the past they were seldom covered in sufficient detail (well done, Sikorsky and FSI for having more recently put some detailed thought into it and come up with useful advice). Those who have never seen one for real or in a simulator (like myself a few years back) need to have the basic principles in the back of their mind. Also, on a selfish basis I like to keep my own dull brain refreshed because I see it as a very cheap life insurance policy.
Two engines is great but there is still only one tail rotor drive shaft and control system!
ShyT
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The S-76 servos are not in the "correct" place for simple unmixed controls to be used. We have a mixer on the top deck to take roll control (for example) and split it out to the two servos that must tilt the swashplate properly to affect a pure roll.
The servos would have been in the perfect place, but we decided to put in 17 degrees of delta 3 in the rotorhead after we had designed the main gearbox casting. To redesign the casting would have delayed the program (the boxes were already being poured) so we just fixed the angle with the mixer on the upper deck.
It is interesting to hear all the reasons given here, from all the sources. This explains why Lu is so often misdirected. For many esoteric pieces of aerodynamic and programatic trivia, the answers are not found by calling someone at the "factory" or elsewhere!
The servos would have been in the perfect place, but we decided to put in 17 degrees of delta 3 in the rotorhead after we had designed the main gearbox casting. To redesign the casting would have delayed the program (the boxes were already being poured) so we just fixed the angle with the mixer on the upper deck.
It is interesting to hear all the reasons given here, from all the sources. This explains why Lu is so often misdirected. For many esoteric pieces of aerodynamic and programatic trivia, the answers are not found by calling someone at the "factory" or elsewhere!
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I also remember the article about the american 76 driver who after having landed safely had difficulty steering through biased braking on the pedals.
He complained that the FM directions were irrelevant to the actual event. I think (and I would've) he closed the throttles to bring the aircraft to a halt.
He complained that the FM directions were irrelevant to the actual event. I think (and I would've) he closed the throttles to bring the aircraft to a halt.
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Zimmerframe and Collective Bias,
I recall one yaw control problem like this in the long ago past on S-76's (maybe 1980), but it was an oddball, not a typical stuck control/cable/pedal type situation. The tail servo wore a notch in itself after a long time, and the servo just sort of locked itself in the cruise tail pitch position. The pilot (Gulf of Mexico) and I spoke the afternoon of the occurrance. He did a great job of flying home and made a very high speed running landing (about 80 knots as I recall. It might have been possible for him to slow down to the other side of the power required curve and get to 40 knots or so if he had done a near autorotation, but never argue with success! When he bottomed collective on landing, the turn was a mighty one, and he had to chop throttles and stand on one brake to stop the machine. We modified the servo to prevent any recurrence.
Some observations on all this:
1) He really never figured out that it was a yaw problem, because with the yaw/collective mixing as he pressed hard on the pedals, he stopped the collective from going full down. He wasn't sure, but thought he had a stuck collective!
2) When he made his approach, he did what good pilots always do, he sized up what he could do, and made it all come together near the ground. He touched down with no yaw, but at high speed.
3) When he and I spoke, he was not fully convinced that it was a tail servo problem, but they checked the servo and confirmed the guess.
4) The next morning, he told me that his leg really hurt from all the pushing he was doing all the way home, done inadvertantly and instinctively, based on thousands of hours of automatic pedal motions!
I know of no other stuck control problems on the 76 family. Does this one sound like the one you were referring to?
A comment on simulator training - most training sims are not to be believed if you leave the normal flight envelope. Several sims I have flown are quite benign in emergencies that might very well be eye watering. They are excellent procedural guides, but not tools to polish technique or study the effects of large, improbable failures.
I recall one yaw control problem like this in the long ago past on S-76's (maybe 1980), but it was an oddball, not a typical stuck control/cable/pedal type situation. The tail servo wore a notch in itself after a long time, and the servo just sort of locked itself in the cruise tail pitch position. The pilot (Gulf of Mexico) and I spoke the afternoon of the occurrance. He did a great job of flying home and made a very high speed running landing (about 80 knots as I recall. It might have been possible for him to slow down to the other side of the power required curve and get to 40 knots or so if he had done a near autorotation, but never argue with success! When he bottomed collective on landing, the turn was a mighty one, and he had to chop throttles and stand on one brake to stop the machine. We modified the servo to prevent any recurrence.
Some observations on all this:
1) He really never figured out that it was a yaw problem, because with the yaw/collective mixing as he pressed hard on the pedals, he stopped the collective from going full down. He wasn't sure, but thought he had a stuck collective!
2) When he made his approach, he did what good pilots always do, he sized up what he could do, and made it all come together near the ground. He touched down with no yaw, but at high speed.
3) When he and I spoke, he was not fully convinced that it was a tail servo problem, but they checked the servo and confirmed the guess.
4) The next morning, he told me that his leg really hurt from all the pushing he was doing all the way home, done inadvertantly and instinctively, based on thousands of hours of automatic pedal motions!
I know of no other stuck control problems on the 76 family. Does this one sound like the one you were referring to?
A comment on simulator training - most training sims are not to be believed if you leave the normal flight envelope. Several sims I have flown are quite benign in emergencies that might very well be eye watering. They are excellent procedural guides, but not tools to polish technique or study the effects of large, improbable failures.
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Great replies!
Before we digress into an unrelated topic I should mention now that I left out the troubleshooting portion because I was trying to get a feel for what other operators do in their procedures.
I think I should add a little background. WRT the mixing unit in the 76 as it pertains to Collective to Yaw coupling, we all know that in a dead foot or jammed pedal situation, if you were to apply full Right pedal input, the collective would rise, conversly if you were to apply full left pedal, the collective would drop, (maybe).
This known reaction can be applied in the troubleshooting phase as pertains to determining whether you have a jam or a control cable failure. Remember, there are no 76's left without a self-centering device!
Now wrt the comment about manipulating the throttles, first I want to reiterate that we tried every combination of jam/cable/mixing unit failure and the results were the same, without exception.
In a well coordinated crew, the most important factor is to pre-brief the actions to be taken in handling an emergency. I wasn't suggesting that the PNF simply retard the throttles, but that the PF call for "reduce throttles gently Now" followed by "STOP" as the situation demands.
The 76 Crew that went off the runway actually helped in the post accident analysis that developed this procedure.
Keep the posts coming, it's always great to read your ideas and thoughts.
Cheers
OffshoreIgor
Before we digress into an unrelated topic I should mention now that I left out the troubleshooting portion because I was trying to get a feel for what other operators do in their procedures.
I think I should add a little background. WRT the mixing unit in the 76 as it pertains to Collective to Yaw coupling, we all know that in a dead foot or jammed pedal situation, if you were to apply full Right pedal input, the collective would rise, conversly if you were to apply full left pedal, the collective would drop, (maybe).
This known reaction can be applied in the troubleshooting phase as pertains to determining whether you have a jam or a control cable failure. Remember, there are no 76's left without a self-centering device!
Now wrt the comment about manipulating the throttles, first I want to reiterate that we tried every combination of jam/cable/mixing unit failure and the results were the same, without exception.
In a well coordinated crew, the most important factor is to pre-brief the actions to be taken in handling an emergency. I wasn't suggesting that the PNF simply retard the throttles, but that the PF call for "reduce throttles gently Now" followed by "STOP" as the situation demands.
The 76 Crew that went off the runway actually helped in the post accident analysis that developed this procedure.
Keep the posts coming, it's always great to read your ideas and thoughts.
Cheers
OffshoreIgor
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Good thread !
Three comments;
1. Depending on landing site, area, gross weight, DA etc I might consider leaving one engine at idle to lock torque compensating out. (right stuck,US type)
2. I had the good fortune to train with an ex vietnam, ex air america pilot (the real company)in the early 80s on stuck pedals. We did hours of them to paved runway and sod. While holding the collective (twist throttle) if you point your index finger down and keep it there it gives you an easy visual reference for which way the nose will turn as you monkey with the throttle.(US type)
3. Landing downwind is not good. At all.
Fly safe
Three comments;
1. Depending on landing site, area, gross weight, DA etc I might consider leaving one engine at idle to lock torque compensating out. (right stuck,US type)
2. I had the good fortune to train with an ex vietnam, ex air america pilot (the real company)in the early 80s on stuck pedals. We did hours of them to paved runway and sod. While holding the collective (twist throttle) if you point your index finger down and keep it there it gives you an easy visual reference for which way the nose will turn as you monkey with the throttle.(US type)
3. Landing downwind is not good. At all.
Fly safe
Nick Lappos, I seem to recall that Scotia had an incident in the recent past regarding a tail rotor control failure in a 76C. Can't remember the exact details, I will have to look them up, but the aircraft landed safely at Lossiemouth without incident.
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Zimmer,
What Nick refers to is known as "off model" responses. This is what happens if the simulator programme does not include sufficient (or incorrect) data to accurately represent what happens for real and it is a well-known issue.
A simulator is, after all, a big boys sophisticated computer game built for a particular purpose. The sim "aircraft" does not actually have the properties of a real aircraft unless those properties are accurately included in the programming. I can assure you that the simulator company does not go out in a real aircraft and fail the tail rotor in order to gain data (obvious really, innit).
Therefore, what is programmed in is a representation of what is assumed to happen. Yaw rates and other effects following on from those rates may have to be mathematically derived and are a "best guess".
A few years ago, I worked alongside a simulator software guru who worked on site at a simulator we were using, as a major customer. He had written the programme into the simulator software. We had some particular problems because it was a new setup and we were using the sim for critical failures that no one had tried much before. The simulator response was definitely different to what the real aircraft did. On a number of occasions I explained the differences and stood next to him as he plugged in his keyboard, typed in raw machine code to change the programme and he had modified the response of the aircraft just like that.
Just bear in mind that a simulator is great for procedures and working out techniques but it may not always be completely representative of the actual aircraft in its response. For example, the helicopter simulator I instructed on would fly a full fixed-wing aerobatics sequence, including inverted flight and slow and four-point rolls! I used to demo it for fun and a few pilots came out both shaken and stirred. I always warned everyone that under NO circumstances should the manoeuvres be tried in the real aircraft because they probably wouldn't have got through the first manoeuvre in one piece.
However, simulators are still the best medium we have for training for unusual emergencies but these possible limitations of the system must always be borne in mind.
A "flying by numbers" approach to any particular scenario in a particular simulator may or may not be 100% valid. I have seen that using two different manufacturer's simulators of the same aircraft type may give slightly different ideas of how best to cope with any one scenario in detail!
ShyT
What Nick refers to is known as "off model" responses. This is what happens if the simulator programme does not include sufficient (or incorrect) data to accurately represent what happens for real and it is a well-known issue.
A simulator is, after all, a big boys sophisticated computer game built for a particular purpose. The sim "aircraft" does not actually have the properties of a real aircraft unless those properties are accurately included in the programming. I can assure you that the simulator company does not go out in a real aircraft and fail the tail rotor in order to gain data (obvious really, innit).
Therefore, what is programmed in is a representation of what is assumed to happen. Yaw rates and other effects following on from those rates may have to be mathematically derived and are a "best guess".
A few years ago, I worked alongside a simulator software guru who worked on site at a simulator we were using, as a major customer. He had written the programme into the simulator software. We had some particular problems because it was a new setup and we were using the sim for critical failures that no one had tried much before. The simulator response was definitely different to what the real aircraft did. On a number of occasions I explained the differences and stood next to him as he plugged in his keyboard, typed in raw machine code to change the programme and he had modified the response of the aircraft just like that.
Just bear in mind that a simulator is great for procedures and working out techniques but it may not always be completely representative of the actual aircraft in its response. For example, the helicopter simulator I instructed on would fly a full fixed-wing aerobatics sequence, including inverted flight and slow and four-point rolls! I used to demo it for fun and a few pilots came out both shaken and stirred. I always warned everyone that under NO circumstances should the manoeuvres be tried in the real aircraft because they probably wouldn't have got through the first manoeuvre in one piece.
However, simulators are still the best medium we have for training for unusual emergencies but these possible limitations of the system must always be borne in mind.
A "flying by numbers" approach to any particular scenario in a particular simulator may or may not be 100% valid. I have seen that using two different manufacturer's simulators of the same aircraft type may give slightly different ideas of how best to cope with any one scenario in detail!
ShyT
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Pitchlink says:
Nick Lappos, I seem to recall that Scotia had an incident in the recent past regarding a tail rotor control failure in a 76C. Can't remember the exact details, I will have to look them up, but the aircraft landed safely at Lossiemouth without incident.
Nick sez:
I am not sure, but I think that was a pedal trim actuator/damper failure, which gave problems to the crew (did it tend to run away in one direction?) The crew handled it well, and all was fine, I believe. It did not require the techniques we are discussing here, I think, but I will defer to someone who is more familiar with the event.
Regarding Zimmerframe's point about simulators, I fully agree with you in their value. My only point is that sims are often only half right, and not particularly representative when improbable failures are being practiced. I do agree that as long as the training is reasonable preparation, they are quite valuable anyway.
Regarding the necessity for two crewmen in a given aircraft, the need to manipulate throttles in a very remotely probable event does not justify the second person, in my opinion. If this were true, we would be spending thousands of dollars a year "just in case" when that cash might be well spent preventing more common occurrences. If one looks at helicopter accident statistics, you can see what drives our game, and it is an eye opener. The mishaps are almost always operational in nature (about 2/3 to 3/4). By operational, I mean what are sometimes called "pilot error" (a term I use with great care) accidents. Without extending the thread way off base, I feel that much of what happens to cause accidents is related to the whole operational scenario, from training and equippage, to weather, procedures, displays and the like, usually (and perhaps improperly) all lumped into "pilot error".
I think we all need to work on fixing what actually happens to cause mishaps, and not sweat to much what might happen in the "extremely remote" probabilities, such as tail rotor failures.
I was the program manager on the joint Honeywell/Sikorsky/FAA program which certified the EGPWS system that is now available on the S-76. This might help reduce CFIT accidents to a memory, and this would cut about 30% of all helo accidents.
Here is a web site that has a great PDF file report on helo accidents offshore, based on 1400 aircraft, 1.4 million hours and 2 years. I believe the data is quite representative of helo operations anywhere, even military:
http://www.ogp.org.uk/pubs/300.pdf
Great stuff, and fodder for a bunch of comments from PPRUNERs.
Nick Lappos, I seem to recall that Scotia had an incident in the recent past regarding a tail rotor control failure in a 76C. Can't remember the exact details, I will have to look them up, but the aircraft landed safely at Lossiemouth without incident.
Nick sez:
I am not sure, but I think that was a pedal trim actuator/damper failure, which gave problems to the crew (did it tend to run away in one direction?) The crew handled it well, and all was fine, I believe. It did not require the techniques we are discussing here, I think, but I will defer to someone who is more familiar with the event.
Regarding Zimmerframe's point about simulators, I fully agree with you in their value. My only point is that sims are often only half right, and not particularly representative when improbable failures are being practiced. I do agree that as long as the training is reasonable preparation, they are quite valuable anyway.
Regarding the necessity for two crewmen in a given aircraft, the need to manipulate throttles in a very remotely probable event does not justify the second person, in my opinion. If this were true, we would be spending thousands of dollars a year "just in case" when that cash might be well spent preventing more common occurrences. If one looks at helicopter accident statistics, you can see what drives our game, and it is an eye opener. The mishaps are almost always operational in nature (about 2/3 to 3/4). By operational, I mean what are sometimes called "pilot error" (a term I use with great care) accidents. Without extending the thread way off base, I feel that much of what happens to cause accidents is related to the whole operational scenario, from training and equippage, to weather, procedures, displays and the like, usually (and perhaps improperly) all lumped into "pilot error".
I think we all need to work on fixing what actually happens to cause mishaps, and not sweat to much what might happen in the "extremely remote" probabilities, such as tail rotor failures.
I was the program manager on the joint Honeywell/Sikorsky/FAA program which certified the EGPWS system that is now available on the S-76. This might help reduce CFIT accidents to a memory, and this would cut about 30% of all helo accidents.
Here is a web site that has a great PDF file report on helo accidents offshore, based on 1400 aircraft, 1.4 million hours and 2 years. I believe the data is quite representative of helo operations anywhere, even military:
http://www.ogp.org.uk/pubs/300.pdf
Great stuff, and fodder for a bunch of comments from PPRUNERs.
Great thread.
Nick makes some great points. I fully agree with his comments about simulators. They are fantastic for procedural training but, like all things, rubbish in equals rubbish out. In other words, the representative ability of the sim is dependant upon the quality of programming. My understanding (Nick will be able to help here) is that an actual aircraft is extensively fitted out with accelerometers, control position indicators, and other data sensors and taken through its paces. The data thus collected is used for the sim program. Accordingly, the quality (like everything else) is dependent upon the budget of the data-gathering phase. Unusual flight conditions are programmed by mathematical extension of collected data and supposition. Therefore, you cannot really "prove" specific reactions to emergencies in the simulator, but the simulation will provide preparation. Nick said something in his article on testing the S92 that is applicable: when faced with actual observed data disagreeing with computed prediction of behavior, the observed data always wins. Also, the lack of vestibular stimulation generated in simulators make them ideal for IFR/Night, Airline type flying (gentle attitude changes and turn rates) and less applicable for low flying, turn and burn stuff, such as autos, tail problems etc, where the pilot includes vetibular clues in his/her effort to control the aircraft.
In response to Arm out the window's excellent post, I would like to add that the practice of bleeding off tail rotor RPM for excessive power pedal problems is, as you say, extremely effective for the UH-1H. However, I have found it to be far less effective on types that actually have a tail rotor that works! For example, in the UH 60 you have to bleed main rotor right off before you see a good reaction (to the point that generators may trip off line). In the B212 (and I presume the B205A1 and maybe the B412), the tail rotor is so effective (at low DAs)that with a decent left pedal stuck forward case, you are unlikely to bleed enough RPM off to help your tail problem before you lose so much main rotor RPM that you risk losing control. This is more so in the IFR B212 that has a form of collective to yaw coupling non-existent in the VFR version.
Forgive me being slightly off the thread topic of the S76.
[ 05 August 2001: Message edited by: helmet fire ]
Nick makes some great points. I fully agree with his comments about simulators. They are fantastic for procedural training but, like all things, rubbish in equals rubbish out. In other words, the representative ability of the sim is dependant upon the quality of programming. My understanding (Nick will be able to help here) is that an actual aircraft is extensively fitted out with accelerometers, control position indicators, and other data sensors and taken through its paces. The data thus collected is used for the sim program. Accordingly, the quality (like everything else) is dependent upon the budget of the data-gathering phase. Unusual flight conditions are programmed by mathematical extension of collected data and supposition. Therefore, you cannot really "prove" specific reactions to emergencies in the simulator, but the simulation will provide preparation. Nick said something in his article on testing the S92 that is applicable: when faced with actual observed data disagreeing with computed prediction of behavior, the observed data always wins. Also, the lack of vestibular stimulation generated in simulators make them ideal for IFR/Night, Airline type flying (gentle attitude changes and turn rates) and less applicable for low flying, turn and burn stuff, such as autos, tail problems etc, where the pilot includes vetibular clues in his/her effort to control the aircraft.
In response to Arm out the window's excellent post, I would like to add that the practice of bleeding off tail rotor RPM for excessive power pedal problems is, as you say, extremely effective for the UH-1H. However, I have found it to be far less effective on types that actually have a tail rotor that works! For example, in the UH 60 you have to bleed main rotor right off before you see a good reaction (to the point that generators may trip off line). In the B212 (and I presume the B205A1 and maybe the B412), the tail rotor is so effective (at low DAs)that with a decent left pedal stuck forward case, you are unlikely to bleed enough RPM off to help your tail problem before you lose so much main rotor RPM that you risk losing control. This is more so in the IFR B212 that has a form of collective to yaw coupling non-existent in the VFR version.
Forgive me being slightly off the thread topic of the S76.
[ 05 August 2001: Message edited by: helmet fire ]
Back in the very early 80's when I was with Okanagan, we had a fatal S76 accident off Thailand (IIRC), which led to the development and installation of the mousetrap to centre the t/r following a cable failure. I'm well out of currency on type, is this mod. still around, and what (if any) effect would it have on the circumstances under discussion?
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John Eacott:The "bear trap "or tail rotor centreing spring, sets a negative 2 degree setting for the tail-rotor blades in the event of a double cable failure, allowing balanced flight at 135 and 40 knots . So one could do a run on landing at 40 with no or few problems.
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Concur Nick and Helmets comments on the fidelity of simulators V aircraft and the some times very poor first simulations of some types. In my limited experience caused by commercial pressures of one manufacturer not wanting to give accurate flight data to another. I know of test pilots going straight from the aircraft to the sim to 'fine tune' the sim responses.
BUT having used them for years and instructed in them for too many they do have a very valid use particularly with regards to the big mind bu$$£*!^$ faults such as Tail rotor snags. If you don't try it in the sim and experience 'best guess' symptoms your first try will be when it happens for real
Helmet.. Having spoken to pilots, read reports on tail rotor failures and experienced loss of t/r effectiveness in a Gazelle (Fenestron stall......Oh hush my mouth!!) the rates of yaw experienced (150deg/sec+) would not be duplicated by any serviceable sim. More to the point would not be properly processed by the human vestibular apparatus except as a drastic disorientation where the visual apparatus would be required to try to make sense of the world.
And another comment.... To my understanding the drooping of rotor speed is going to have very little effect on tail rotor thrust if stuck BUT (Again) reducing main rotor speed will reduce total rotor thrust! to maintain the flight path the collective will need to be raised to maintain lift. If collective is increased rotor drag increases and requires more torque to maintain the same flight path thus 'using up' some of the excessive tail rotor thrust.
Right that's it for the mo. Time to give some poor stude a beasting in the Sim!!
(Most of my victims have actually appreciated a couple of hours playing with different Tail Rotor faults.....or so they said at the time!!)
BUT having used them for years and instructed in them for too many they do have a very valid use particularly with regards to the big mind bu$$£*!^$ faults such as Tail rotor snags. If you don't try it in the sim and experience 'best guess' symptoms your first try will be when it happens for real
Helmet.. Having spoken to pilots, read reports on tail rotor failures and experienced loss of t/r effectiveness in a Gazelle (Fenestron stall......Oh hush my mouth!!) the rates of yaw experienced (150deg/sec+) would not be duplicated by any serviceable sim. More to the point would not be properly processed by the human vestibular apparatus except as a drastic disorientation where the visual apparatus would be required to try to make sense of the world.
And another comment.... To my understanding the drooping of rotor speed is going to have very little effect on tail rotor thrust if stuck BUT (Again) reducing main rotor speed will reduce total rotor thrust! to maintain the flight path the collective will need to be raised to maintain lift. If collective is increased rotor drag increases and requires more torque to maintain the same flight path thus 'using up' some of the excessive tail rotor thrust.
Right that's it for the mo. Time to give some poor stude a beasting in the Sim!!
(Most of my victims have actually appreciated a couple of hours playing with different Tail Rotor faults.....or so they said at the time!!)
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Main rotor rpm nudges can be effective at the tail end of a procedure, if the aircraft is almost lines up and just a tweek is needed to get the machine aligned with its direction of travel (a very good idea, skids or wheels, since a roll over can be noisy, expensive and painful).
(All directions are consistent with American and British rotor direction, front blades going to the left)
The rpm is a double whammy, increasing the rpm will act like a boost of left pedal and decreasing the rpm will act like right pedal.
Here is why. If the main rotor thrust is held constant as rpm is changed (for example, collective is lowered slightly with up rpm), then a raise in rpm will lower the main rotor torque exactly as the percent rpm increase. This is because the power needed by the rotor is almost constant, and the power is the torque times the rpm. More rpm means that less torque is needed for constant power. An rpm increase makes the current tail rotor pitch setting (which is stuck) produce somewhat more anti-torque, which produces a left nose movement. Also, the rpm increase will further increase the tail rotor thrust for even more left pedal equivilent.
If the main rotor rpm is reduced, the main torque will increase, requiring more anti-torque, and absorbing some of the anti-torque the fixed-pitch tail rotor is putting out. This will cause a right nose swing. As above, the rpm change changes the tail thrust, reducing it in this case and making the right swing even more pronounced.
The tail rotor thrust changes by the square of the rpm change, and the main torque changes linearly.
(All directions are consistent with American and British rotor direction, front blades going to the left)
The rpm is a double whammy, increasing the rpm will act like a boost of left pedal and decreasing the rpm will act like right pedal.
Here is why. If the main rotor thrust is held constant as rpm is changed (for example, collective is lowered slightly with up rpm), then a raise in rpm will lower the main rotor torque exactly as the percent rpm increase. This is because the power needed by the rotor is almost constant, and the power is the torque times the rpm. More rpm means that less torque is needed for constant power. An rpm increase makes the current tail rotor pitch setting (which is stuck) produce somewhat more anti-torque, which produces a left nose movement. Also, the rpm increase will further increase the tail rotor thrust for even more left pedal equivilent.
If the main rotor rpm is reduced, the main torque will increase, requiring more anti-torque, and absorbing some of the anti-torque the fixed-pitch tail rotor is putting out. This will cause a right nose swing. As above, the rpm change changes the tail thrust, reducing it in this case and making the right swing even more pronounced.
The tail rotor thrust changes by the square of the rpm change, and the main torque changes linearly.
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As I understand it increasing rotor speed to increase tail rotor thrust for a low power problem (Low power balances stuck tail) will only have a limited benefit. Too much increase in rotor speed will exponentially increase rotor drag for the lift gained requiring more torque to keep rotor speed....but you are trying to keep torque down so a small + but too much and you'll be into the -'s.
Using a little more power and opposite cyclic to counter the yaw and using a cross wind to counter the resultant drift may get a reasonable run on speed and ROD if not I'd go for an engine off (My auto's are probably survivable)
ps. The Sea King has had several snapped and jammed cables as well as a drive failure or two
Using a little more power and opposite cyclic to counter the yaw and using a cross wind to counter the resultant drift may get a reasonable run on speed and ROD if not I'd go for an engine off (My auto's are probably survivable)
ps. The Sea King has had several snapped and jammed cables as well as a drive failure or two
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I think another way to say what Nick stated is so;
,
Imagine the aircraft at a five foot hover with a stuck left pedal.
While maintaining that height above the ground the mainrotor is developing a value of torque. Call it 30.
If you lower the rotor rpm (say 5 percent) and want to maintain that five feet you must increase the collective.
So you've increased the pitch in the main rotor blades (brought the torque value back to 30)and left the tailrotor blade pitch at it's smaller setting and happily, travelling slower.
The lift equation has as it's largest factor airspeed. In fact it's airspeed squared.
Same output from mainrotor,
Less output from tailrotor.
,
Imagine the aircraft at a five foot hover with a stuck left pedal.
While maintaining that height above the ground the mainrotor is developing a value of torque. Call it 30.
If you lower the rotor rpm (say 5 percent) and want to maintain that five feet you must increase the collective.
So you've increased the pitch in the main rotor blades (brought the torque value back to 30)and left the tailrotor blade pitch at it's smaller setting and happily, travelling slower.
The lift equation has as it's largest factor airspeed. In fact it's airspeed squared.
Same output from mainrotor,
Less output from tailrotor.
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Tgrendl,
I like your way of illustrating the issue, so let me try again:
If while hovering at 100% rpm at torque of 80%, your tail rotor becomes stuck at a fixed pitch setting. If you creep the rpm down to 95%, while holding hover height, the torque will rise to about 84% (1.05 x 80), because the power is a constant, and power is torque times rpm (84 torque times 95 rpm = 80 torque times 100 rpm).
Because the torque is now higher (and the tail rotor has less rpm for the stuck pitch position), the tail rotor thrust is insufficient, so the aircraft will rotate to the right.
In the same circumstance, if the pilot sneaks the rpm up to 105% and holds altitude with reduced collective pitch, the torque will go down to about 75%, and the tail rotor will now be producing excess anti-torque, so the aircraft will rotate to the left.
In other words, the rpm can be used as a yaw control if well handled!
The effects are used in the Huey stuck pedal procedure we have all practiced at one time or another, mostly because the bicycle chain tail rotor control on a Huey was more prone to failure at one time. I do not believe this is true any longer, but I defer to those who have more knowledge.
One comment on all this stuff - we tend to focus on the emergencies that we can practice, so I have seen people do 5 of these stuck pedal procedures in one flight, and 5 or ten engine failures for the remainder. If one studies accident statistics, CFIT and the like are more likely to bite us. Why don't we practice not hitting the ground!
I like your way of illustrating the issue, so let me try again:
If while hovering at 100% rpm at torque of 80%, your tail rotor becomes stuck at a fixed pitch setting. If you creep the rpm down to 95%, while holding hover height, the torque will rise to about 84% (1.05 x 80), because the power is a constant, and power is torque times rpm (84 torque times 95 rpm = 80 torque times 100 rpm).
Because the torque is now higher (and the tail rotor has less rpm for the stuck pitch position), the tail rotor thrust is insufficient, so the aircraft will rotate to the right.
In the same circumstance, if the pilot sneaks the rpm up to 105% and holds altitude with reduced collective pitch, the torque will go down to about 75%, and the tail rotor will now be producing excess anti-torque, so the aircraft will rotate to the left.
In other words, the rpm can be used as a yaw control if well handled!
The effects are used in the Huey stuck pedal procedure we have all practiced at one time or another, mostly because the bicycle chain tail rotor control on a Huey was more prone to failure at one time. I do not believe this is true any longer, but I defer to those who have more knowledge.
One comment on all this stuff - we tend to focus on the emergencies that we can practice, so I have seen people do 5 of these stuck pedal procedures in one flight, and 5 or ten engine failures for the remainder. If one studies accident statistics, CFIT and the like are more likely to bite us. Why don't we practice not hitting the ground!