Sikorsky S-92: From Design to Operations
The word around about the engine out was that a fuel valve leaked, and in a suction system, air flowed in and cause the engine to quit. Had it been a conventional fuel system with boost pumps, fuel would have been leaking into the cabin, with the possibility fo fire. Basically, the system worked as it should
I would like to hear from you about the fuel system, or if you can point to an on-line document, because the only two turbine engine helos I've flown have boost pumps and I it's pretty strange to me your comments about a new helo like the 92 without boost pumps.
It's a possible fuel leak in the fuel valve the only reason?
Don't you have altitude limitations to fly without boost pumps (like bell-206,bell-412)?
Thanks.
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Aser,
Your question is a good one. The need for boost pumps is a sign of an old technology engine pump, and a less safe design.
Sikorsky hasn't built a helicopter with a pressurized fuel system since the last 61 went down the line.
It will take a generation of people who have not been trained on 40 year old designs before the concept sinks in, however! I would guess you flew a jet ranger and a Huey, both of which have flight manual limitations against flight above about 5,000 feet if the fuel boost pump is not working.
The S-92, like the S-76 and the Hawk family has a "suction fuel" system. Basically, the engine fuel pump is strong enough to pull the fuel up at any altitude, and with any approved fuel. This means that the normal mode for flight leaves all fuel lines with negative pressure, and virtually no ability to leak or spray fuel if they fail, or are punctured, or if a crash separates the lines.
I can't tell you how bad it is to discover a leaking fuel line when your eyes sting from the fuel fumes, and you see fuel mist in the cabin. The aircraft is sudenly in grave danger of an inflight fire, and all you can do is fly to a safe landing, and not flip any switches in the process (to avoid a spark!) Most older fuel systems have from 25 to 40 psi in the fuel lines which run from the fuel tanks to the engine, usually wrapping around the cabin. That is about the same pressure as a household water system, so you can envision the leak from a pin-hole or crack.
The suction fuel needs a special engine pump, a design that Sikorsky championed about 30 years ago. The pump keeps itself wet enough, even when pumping foamy fuel (we call it the vapor to liquid ratio) so that there is no altitude restriction al all. The other, older designs could swap over, but the tests are expensive, and the fuel system might have to be redesigned. Falls under the question I posted earlier, "When is it time to just start a new design?"
Under FAA/JAA scrutiny, the S-92 was taken to its max altitude with fuel that had been heated up to about 140 degrees F (talk about a test pilot earning his pay!) and maneuvered, started and shown to be entirely accepting of the no-boost pump design.
Like that photo of the fuzzy dice, boost pumps are way past their prime! The probability of a fuel fire, in flight or post-crash, is very significantly lower when the fuel cannot spray out.
Your question is a good one. The need for boost pumps is a sign of an old technology engine pump, and a less safe design.
Sikorsky hasn't built a helicopter with a pressurized fuel system since the last 61 went down the line.
It will take a generation of people who have not been trained on 40 year old designs before the concept sinks in, however! I would guess you flew a jet ranger and a Huey, both of which have flight manual limitations against flight above about 5,000 feet if the fuel boost pump is not working.
The S-92, like the S-76 and the Hawk family has a "suction fuel" system. Basically, the engine fuel pump is strong enough to pull the fuel up at any altitude, and with any approved fuel. This means that the normal mode for flight leaves all fuel lines with negative pressure, and virtually no ability to leak or spray fuel if they fail, or are punctured, or if a crash separates the lines.
I can't tell you how bad it is to discover a leaking fuel line when your eyes sting from the fuel fumes, and you see fuel mist in the cabin. The aircraft is sudenly in grave danger of an inflight fire, and all you can do is fly to a safe landing, and not flip any switches in the process (to avoid a spark!) Most older fuel systems have from 25 to 40 psi in the fuel lines which run from the fuel tanks to the engine, usually wrapping around the cabin. That is about the same pressure as a household water system, so you can envision the leak from a pin-hole or crack.
The suction fuel needs a special engine pump, a design that Sikorsky championed about 30 years ago. The pump keeps itself wet enough, even when pumping foamy fuel (we call it the vapor to liquid ratio) so that there is no altitude restriction al all. The other, older designs could swap over, but the tests are expensive, and the fuel system might have to be redesigned. Falls under the question I posted earlier, "When is it time to just start a new design?"
Under FAA/JAA scrutiny, the S-92 was taken to its max altitude with fuel that had been heated up to about 140 degrees F (talk about a test pilot earning his pay!) and maneuvered, started and shown to be entirely accepting of the no-boost pump design.
Like that photo of the fuzzy dice, boost pumps are way past their prime! The probability of a fuel fire, in flight or post-crash, is very significantly lower when the fuel cannot spray out.
Just awoken from hibernation in the cellar (been in a box of soil...). Should I butt in and encounter the wrath of Nick once more??? Yea, why not!
The 92 design philosophy and Nick's comments are based on the fact that any leak in the fuel lines up to the engine will cause fuel to be pumped out, possibly into the cabin. This is clearly a bad thing and especially an issue in a crash, so addressing this issue by removing the booster pumps seems a good idea.
But like so many good ideas, there is a down side. The problem is that air has a very much lower viscosity than fuel. So a tiny leak, which would cause a few annoying drips of fuel in a booster pumped system, in a non - booster pumped system can allow enough air to be sucked in to flame the engine out.
I can imagine that it would be harder to check for leaks post-maintenance - in a booster pumped system you just switch on the pumps and sniff around, but in a suction system, you don't really know if air is being sucked in. You can do a ground run, but at low power the suction is less so less air getting in. But once you want takeoff power, will the increased suction pressure cause a problem?
Of course there are ways around this such as pressure checking the installation in situ, but maybe the maintenance industry hasn't caught up with the new concepts yet?
So we have a "good idea" that has a downside that caused a failure fairly early in the life of the 92. In terms of MTBF it doesn't look too good (statistics can be cruel!). I grant you that an engine failure should be less serious than large fuel leak into the cabin, but I would rather have neither.
Maybe the answer is low pressure booster pumps that give only a psi or two at the engine - enough to just maintain positive pressure in the fuel lines?
The 225 also has an engine driven pump that is capable of sucking up fuel without booster pumps with no altitude restriction (big improvement over the L2 which was very dependant on its booster pumps). But interestingly the 225 still has booster pumps working at around 0.7 bar (10 psi to you Nick) - though that is probably still too high to be unconcerned about leaks into the cabin
Regarding the comments about vibration, I think we all now realise that the 92 is rough as old boots at speed, and operators are having to reduce cruise speed to contain its damaging effects. This is a pity as in many ways its a good aircraft - but you only have to glance at the rotor head to realise that its based on old technology. I think Sky will have to bite the bullet and fit a new head and 5 blades in the not too distant future if they want to keep the 92's credibility.
Of course these issues don't seem to be causing sales problems, but I think that's because those that buy them are not those that fly them. However the reputation is starting to become tarnished....
Not being able to miss a dig, I should of course point out that the 225 is very smooth at normal cruise speed (155 kts or so) with or without the active anti-vibration. That system is only needed when flying at low power around 90-120 kts when the 225 otherwise gets a bit rumbly. But why fly at 120 when its smooth as the baby's whatsits at 155?
By the way I fly 92's you will be jealous when I tell you that the 225's autopilot is as far ahead of the L2's as the L2's is ahead of the L's
HC
The 92 design philosophy and Nick's comments are based on the fact that any leak in the fuel lines up to the engine will cause fuel to be pumped out, possibly into the cabin. This is clearly a bad thing and especially an issue in a crash, so addressing this issue by removing the booster pumps seems a good idea.
But like so many good ideas, there is a down side. The problem is that air has a very much lower viscosity than fuel. So a tiny leak, which would cause a few annoying drips of fuel in a booster pumped system, in a non - booster pumped system can allow enough air to be sucked in to flame the engine out.
I can imagine that it would be harder to check for leaks post-maintenance - in a booster pumped system you just switch on the pumps and sniff around, but in a suction system, you don't really know if air is being sucked in. You can do a ground run, but at low power the suction is less so less air getting in. But once you want takeoff power, will the increased suction pressure cause a problem?
Of course there are ways around this such as pressure checking the installation in situ, but maybe the maintenance industry hasn't caught up with the new concepts yet?
So we have a "good idea" that has a downside that caused a failure fairly early in the life of the 92. In terms of MTBF it doesn't look too good (statistics can be cruel!). I grant you that an engine failure should be less serious than large fuel leak into the cabin, but I would rather have neither.
Maybe the answer is low pressure booster pumps that give only a psi or two at the engine - enough to just maintain positive pressure in the fuel lines?
The 225 also has an engine driven pump that is capable of sucking up fuel without booster pumps with no altitude restriction (big improvement over the L2 which was very dependant on its booster pumps). But interestingly the 225 still has booster pumps working at around 0.7 bar (10 psi to you Nick) - though that is probably still too high to be unconcerned about leaks into the cabin
Regarding the comments about vibration, I think we all now realise that the 92 is rough as old boots at speed, and operators are having to reduce cruise speed to contain its damaging effects. This is a pity as in many ways its a good aircraft - but you only have to glance at the rotor head to realise that its based on old technology. I think Sky will have to bite the bullet and fit a new head and 5 blades in the not too distant future if they want to keep the 92's credibility.
Of course these issues don't seem to be causing sales problems, but I think that's because those that buy them are not those that fly them. However the reputation is starting to become tarnished....
Not being able to miss a dig, I should of course point out that the 225 is very smooth at normal cruise speed (155 kts or so) with or without the active anti-vibration. That system is only needed when flying at low power around 90-120 kts when the 225 otherwise gets a bit rumbly. But why fly at 120 when its smooth as the baby's whatsits at 155?
By the way I fly 92's you will be jealous when I tell you that the 225's autopilot is as far ahead of the L2's as the L2's is ahead of the L's
HC
Nick, Thanks your response, is educational as I was expecting.
I'm flying Bell-412EP.
Certainly the altitude restriction it isn't a big issue, because the possibility of a double boost pump faiure I would like to think that is remote.
But the leaks in a post-crash...
So you don't need the boosts pumps in the 92 even for startup , from a possibly empty line from de cells to the engine pump?
HeliComparator
Don't you would prefer the engine out than the fuel leak?
Having seen the good outcome in the 92 and my little experience with the 412EP(regarding OEI) I would choose the "no-pump" system.
But... what do I know?
Best regards.
Aser
I'm flying Bell-412EP.
Certainly the altitude restriction it isn't a big issue, because the possibility of a double boost pump faiure I would like to think that is remote.
But the leaks in a post-crash...
So you don't need the boosts pumps in the 92 even for startup , from a possibly empty line from de cells to the engine pump?
HeliComparator
The 92 design philosophy and Nick's comments are based on the fact that any leak in the fuel lines up to the engine will cause fuel to be pumped out, possibly into the cabin. This is clearly a bad thing and especially an issue in a crash, so addressing this issue by removing the booster pumps seems a good idea.
there is a down side. The problem is that air has a very much lower viscosity than fuel. So a tiny leak, which would cause a few annoying drips of fuel in a booster pumped system, in a non - booster pumped system can allow enough air to be sucked in to flame the engine out.
Having seen the good outcome in the 92 and my little experience with the 412EP(regarding OEI) I would choose the "no-pump" system.
But... what do I know?
Best regards.
Aser
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Aser,
Yes a fuel leak somewhere in the fuselage (look at the routing of the fuel lines to picture how many corners and wire bundles it can wash over as it goes) is a much bigger problem, by far, than an engine flameout in a Cat A twin.
The concept of suction fuel does not automatically mean that any fuel line leak would mean a flameout. The suction level is very low (about 3 psi at the lowest place in the fuel system, about 1 psi at the top) so that a small leak spot would introduce some air, but typically not enough to cause an engine shutdown. S-76 drivers know that the typical sign of a fuel system leak is a blinking fuel pressure light, showing that bubbles are being eaten by the engine. These bubbles are the air that went in, instead of feul pissing out.
The difference between a 1-3 psi suction, and a 10 to 40 psi fuel spray into the fuselage is the point I am making, and it is an important point, important enough that US Mil spec no longer allows pressurized fuel systems.
The benifits are only available if you have no fuel pressure. A design with boost pumps on is not a suction design, and is inherently more fire prone.
Yes a fuel leak somewhere in the fuselage (look at the routing of the fuel lines to picture how many corners and wire bundles it can wash over as it goes) is a much bigger problem, by far, than an engine flameout in a Cat A twin.
The concept of suction fuel does not automatically mean that any fuel line leak would mean a flameout. The suction level is very low (about 3 psi at the lowest place in the fuel system, about 1 psi at the top) so that a small leak spot would introduce some air, but typically not enough to cause an engine shutdown. S-76 drivers know that the typical sign of a fuel system leak is a blinking fuel pressure light, showing that bubbles are being eaten by the engine. These bubbles are the air that went in, instead of feul pissing out.
The difference between a 1-3 psi suction, and a 10 to 40 psi fuel spray into the fuselage is the point I am making, and it is an important point, important enough that US Mil spec no longer allows pressurized fuel systems.
The benifits are only available if you have no fuel pressure. A design with boost pumps on is not a suction design, and is inherently more fire prone.
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........and in the blue corner
HeliComparator,
Where have you been
Keep it up guys (seriously) it is quite good to get both sides, very interesting.
Do have a question though.
Nick, the 92 was designed and built with the anti vib system installed and approved. Now they apparently are being left on the workshop tables and disgarded.
So why installed /needed in the first place?
How can it fly legally without it?
And what ill effects will the fuselage suffer after a few thousand hours?
BTW, how is the new job?
cdn
Where have you been
Keep it up guys (seriously) it is quite good to get both sides, very interesting.
Do have a question though.
Nick, the 92 was designed and built with the anti vib system installed and approved. Now they apparently are being left on the workshop tables and disgarded.
So why installed /needed in the first place?
How can it fly legally without it?
And what ill effects will the fuselage suffer after a few thousand hours?
BTW, how is the new job?
cdn
Aser
If you mean would I prefer an engine failure to a catastrophic fuel leak / fire in the cabin, then yes of course.
But like I said, I would prefer neither.
It seems fairly unlikely to suddenly get a catastrophic fuel leak / fire in the cabin when the aircraft was OK on despatch. Unless of course you are a military aircraft being shot at (which is what's behind not-biting-Nicks comments about US Military specs I suspect)
The only time you are likely to get a catastrophic fuel leak / fire in the cabin is when you crash - which could be due to that engine failure.
Yes I know its got cat A performance and all that good stuff but you only get cat A performance when you operate it to cat A. Someone once said that the difference between a professional pilot and a private one is that the latter flies outside the "avoid curve" whereas the professional one flies in it. There is a lot of truth in that and of course when you are inside the curve, chances are you have a high power setting - that's just when the suction pressure is at its maximum and that air leak could get you.
It all boils down to probabilty of occurence I suppose. If I have the choice between an engine failure on takeoff every couple of thousand hours or a catastrophic fuel leak / fire once every 50 lifetimes I might go for the latter.
And of course what Nick has failed to mention is that the rogue anti-vibration generator that fell off (along with a big chunk of fuselage skin) ended up perilously close to the sponson fuel tank, so suction system or not its still possible to end up with fuel in the cabin (in this case because the fuel tanks are higher than the cabin floor)
Its good to be back...
HC
ps cdn - I think you'll find that the anti-vibe generators are still installed on the 92's - the vibe level without them is unacceptable. But perhaps they are welding some steel plates around them to take the stress.....(only joking!)
Don't you would prefer the engine out than the fuel leak?
But like I said, I would prefer neither.
It seems fairly unlikely to suddenly get a catastrophic fuel leak / fire in the cabin when the aircraft was OK on despatch. Unless of course you are a military aircraft being shot at (which is what's behind not-biting-Nicks comments about US Military specs I suspect)
The only time you are likely to get a catastrophic fuel leak / fire in the cabin is when you crash - which could be due to that engine failure.
Yes I know its got cat A performance and all that good stuff but you only get cat A performance when you operate it to cat A. Someone once said that the difference between a professional pilot and a private one is that the latter flies outside the "avoid curve" whereas the professional one flies in it. There is a lot of truth in that and of course when you are inside the curve, chances are you have a high power setting - that's just when the suction pressure is at its maximum and that air leak could get you.
It all boils down to probabilty of occurence I suppose. If I have the choice between an engine failure on takeoff every couple of thousand hours or a catastrophic fuel leak / fire once every 50 lifetimes I might go for the latter.
And of course what Nick has failed to mention is that the rogue anti-vibration generator that fell off (along with a big chunk of fuselage skin) ended up perilously close to the sponson fuel tank, so suction system or not its still possible to end up with fuel in the cabin (in this case because the fuel tanks are higher than the cabin floor)
Its good to be back...
HC
ps cdn - I think you'll find that the anti-vibe generators are still installed on the 92's - the vibe level without them is unacceptable. But perhaps they are welding some steel plates around them to take the stress.....(only joking!)
Last edited by HeliComparator; 21st Aug 2005 at 19:33.
cdn
OK I have to bow to your insider knowledge but I am still surprised, because I have felt the vibration level on a 92 with the system switched off, and its bad.
HC
OK I have to bow to your insider knowledge but I am still surprised, because I have felt the vibration level on a 92 with the system switched off, and its bad.
HC
It all boils down to probabilty of occurence I suppose. If I have the choice between an engine failure on takeoff every couple of thousand hours or a catastrophic fuel leak / fire once every 50 lifetimes I might go for the latter.
Helicomparator: you are very right about the cat A stuff , but let's assume it's the case.
Anyway because the fuel leak,(boost pump or not) it's so improbable, and the crashs (whatever the cause) happens you agree that a no-boost-pump system it's evolution, no?
Regards.
Aser
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Most cars built in the last 10 years or so have a little device called an inertia switch that trips off the electric fuel injection pump in the unfortunate event of an accident impact. Just thought I'd mention it
Didn't the Wessex have these too?
Didn't the Wessex have these too?
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Here's a little info just to clear a couple of things.
The S-92 has two fuel prime pumps for APU run and engine starting.
ASB 92-18-001 revA allows the removal of one force generator and the continued operation with the remaining two force generators. The force generator mechanical units that have the cracks in the mounting area are located in the right hand cabin wall, about a foot from the floor. It did not drive itself through the roof, some wiring and, eventually, the top of the fuel tank.
The S-92 has two fuel prime pumps for APU run and engine starting.
ASB 92-18-001 revA allows the removal of one force generator and the continued operation with the remaining two force generators. The force generator mechanical units that have the cracks in the mounting area are located in the right hand cabin wall, about a foot from the floor. It did not drive itself through the roof, some wiring and, eventually, the top of the fuel tank.
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Nick said ~ "Forty years from now, people will lament that retirement of the S-92, and a great new Sikorsky team will roll out the S-124. I know I won't be there to see it, dammit!"
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Interesting discussion ref risk of air leak flaming out engine on suction fuel systems versus in-flight fire from fuel leak in pressurised systems. Of course real issue is trying to quantify those risks.
Anyone know any statistics? And if not, there may be enough of an experience pool here to form a view. How many air leak engine flame outs does anyone know of? And then how many pressurised fuel system leaks causing in-flight aircraft fires?
Anyone know any statistics? And if not, there may be enough of an experience pool here to form a view. How many air leak engine flame outs does anyone know of? And then how many pressurised fuel system leaks causing in-flight aircraft fires?
The other danger with pumps is sparks, as Boeing et al know all about ( I recall a BHL Jet Ranger suffered a 'tank explosion' too a few years a go, no injuries).
Of course, this was being built decades before the Puma was even though of, and was still being built all the way through Super Puma production and even into Super Puma Mk II times!
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Did it suck or blow? I think it sucked.