Prop overspeed. How serious?
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Prop overspeed. How serious?
Hi all. I was reading the SK197 TSB report. I found it interesting that they only treated the problem half way leading to secondary problems.
I fly the SF34 and our procedures are a little different from the Q400, but I tried to translate their situation into something that could happen in the little Saab. I've been trying to figure out the mechanics behind a prop overspeed.
I don't have the AOM/AFM available to me so I'm unable to read up on the systems, but I've concluded that either the HMU or the PCU needs to be in fault for a Np overspeed to occur. Am I correct? With a HMU fault that drives the Ng to a higher than normal RPM, the prop would (if not correctde by the PCU) go into an overspeed situation. I know our overspeed protection system works by flaming out the engine temporarily until the Np has been restored below the limit value. If the PCU would be in fault, it can only overspeed by setting a finer pitch than commanded and in this case I cannot see any immediate dangers as the torque and overall loads on the system ought to decrease (like using a high gear to use the engine as a break when driving).
Am I completely lost? I'd appreciate it if anyone with experience on the Saab could help me with explaining what happens during a Np overspeed and what the possible causes could be!
Best regards/ LnS
I fly the SF34 and our procedures are a little different from the Q400, but I tried to translate their situation into something that could happen in the little Saab. I've been trying to figure out the mechanics behind a prop overspeed.
I don't have the AOM/AFM available to me so I'm unable to read up on the systems, but I've concluded that either the HMU or the PCU needs to be in fault for a Np overspeed to occur. Am I correct? With a HMU fault that drives the Ng to a higher than normal RPM, the prop would (if not correctde by the PCU) go into an overspeed situation. I know our overspeed protection system works by flaming out the engine temporarily until the Np has been restored below the limit value. If the PCU would be in fault, it can only overspeed by setting a finer pitch than commanded and in this case I cannot see any immediate dangers as the torque and overall loads on the system ought to decrease (like using a high gear to use the engine as a break when driving).
Am I completely lost? I'd appreciate it if anyone with experience on the Saab could help me with explaining what happens during a Np overspeed and what the possible causes could be!
Best regards/ LnS
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My experience is with a complex turboprop system was on the Emb 120, with the Hamilton Standard 14RF-9 turned by a PW118. I beleve the Ham was an option on the SF34. Early overspeeds with this prop system were attributed to a failure of the pitch change mechanism - the nut that actuated the transfer tube would wear in the thread area, until control was lost of pitch. At that point no amount of governors would alleviate the condition, as they all merely modulated oil pressure on the very mechanism that had failed. In flight, the fuel-topping governor was also irrelavant, as the centrfigual twisting moment was quite sufficient to drive the prop towards fine pitch. One fatal E120 accident was blamed on this failure mode. The metallurgy in the mechanism has been changed, and the problem appears contained at this point.
When looking at the PCU schematic, it also becomes clear that a failure in an oil line in many conceivable locations will result in a prompt overspeed. The troublesome thing about an overspeed is that once it happens, the failure mode is usually of the sort that it can't be fixed with pilot input, be it a feathering solenoid, an electric feather pump, or what have you.
As far as the question "how serious?" When practicing in the sim I've never survived and uncontrollabe overspeed. When the propellor goes to flat pitch at speed the blade angle of attack is negative to the relative wind - it's comparable to selecting reverse. Major drag is created, compounded by massively disrupted airflow over the a large portion of the wing, and horizontal stab. The aircraft is nearly impossible to control in pitch and yaw.
A King Air driver demonstrated the problems nicely when he selected ground fine in flight to expedite a descent. All was well until he advanced the power levers, one engine came out well before the other, control was lost, and the aircraft was lost.
When looking at the PCU schematic, it also becomes clear that a failure in an oil line in many conceivable locations will result in a prompt overspeed. The troublesome thing about an overspeed is that once it happens, the failure mode is usually of the sort that it can't be fixed with pilot input, be it a feathering solenoid, an electric feather pump, or what have you.
As far as the question "how serious?" When practicing in the sim I've never survived and uncontrollabe overspeed. When the propellor goes to flat pitch at speed the blade angle of attack is negative to the relative wind - it's comparable to selecting reverse. Major drag is created, compounded by massively disrupted airflow over the a large portion of the wing, and horizontal stab. The aircraft is nearly impossible to control in pitch and yaw.
A King Air driver demonstrated the problems nicely when he selected ground fine in flight to expedite a descent. All was well until he advanced the power levers, one engine came out well before the other, control was lost, and the aircraft was lost.
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Thank you very much Cardinal. You've shed some light on a part that seems to have passed unnoticed through my TR course. Considering the fact that pilot input, once the overspeed situation has become uncontrollable, is useless, does this warrant shutting donw the engine as soon as it's detected? Having a prop that isn't featharable and an engine that isn't running isn't the best of prospects. If even the slightest bit of power can be produced, would it not be better to keep it running than to possibly end up with a deap prop still in fine pitch. Although, since the feather position on the CL has to be passed to select fuel off , such a situation I guess would not come as a surprise.
I'll need to revise on the PCU and look at the mechanics behind it for the Saab, but thanks again for your reply!
/LnS
I'll need to revise on the PCU and look at the mechanics behind it for the Saab, but thanks again for your reply!
/LnS
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The SAAB, if equipped with the original Dowty prop, has counterweights to assist in driving to high (coarse) pitch. Thus oil pressure loss does not result in overspeed.
Here my memory is less clear, but I believe this is how the prop is taken into "autocoarsening" mode (supposedly lower drag than fully feathered) in which it windmills slowly. It seemed to me the Dowty prop was thus less likely to encounter a full fine failure than the HSD prop. (25 years ago I had all the answers on these props!
)
Here my memory is less clear, but I believe this is how the prop is taken into "autocoarsening" mode (supposedly lower drag than fully feathered) in which it windmills slowly. It seemed to me the Dowty prop was thus less likely to encounter a full fine failure than the HSD prop. (25 years ago I had all the answers on these props!
)
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On the Ham Standard it doesn't matter much whether the engine is left running or not. In the overspeed case, the prop pitch is out of control, determined only by airspeed - windmilling. Bring the power back, but engine torque only plays a bit part in the equation. A running engine, however, should supply oil pressure, which will be necessary if you have any chance of getting the prop back to feather. Also consider if you have a auxillary prop oil reservoir that is filled by only be an engine driven pump.....without an operating engine you'll need to motor the thing electrically to fill that tank to have oil to play with to advance your feathering agenda. Again, this is very specific to the Ham Standard on the PW118, I'm not sure how the CT7 gets oil to the prop.
Our overspeed checklist called for bringing the power back, getting the airspeed down to as little as 125kts, trying manual feather, electric feather, and then trying to dry motor the engine for the reasons discussed above.
Our overspeed checklist called for bringing the power back, getting the airspeed down to as little as 125kts, trying manual feather, electric feather, and then trying to dry motor the engine for the reasons discussed above.
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I've looked through our abnormal checklist for this and it (contrary to what is siad in the Q400 QRH) that the powerlever should not be moved until made certain that the prop is controllable. We are to try and carefully modulate the conditionlevers towards min to see if the Np is controllable and if so, we'll operate it normally with an increased monitoring on tha engine.
I guess that moving the powerlever could alter the pitch on the prop towards a more fine setting which would worsen the problem (constant speed prop: power reduced, prop goes to fine to correct Np). If it's a oneway controllability issue, this would indeed cause the problem to worsen.
What is intersting in this I conclude is how the prop is brought to feather. The autocoarsen is not feather, but a coarsened pitch. This position will be the least drag position but not fully feathered as I have understood things. It'd be interesting to know if the autocoarsen uses the same mechanical components as in normal prop governing (which I presume it does) or not. And if this ability is lost, will it be possible to feather the prop anyhow?
I realise these are all questions I should be able to read up on myself, but as I said I don't have access to the manuals at present.
/LnS
I guess that moving the powerlever could alter the pitch on the prop towards a more fine setting which would worsen the problem (constant speed prop: power reduced, prop goes to fine to correct Np). If it's a oneway controllability issue, this would indeed cause the problem to worsen.
What is intersting in this I conclude is how the prop is brought to feather. The autocoarsen is not feather, but a coarsened pitch. This position will be the least drag position but not fully feathered as I have understood things. It'd be interesting to know if the autocoarsen uses the same mechanical components as in normal prop governing (which I presume it does) or not. And if this ability is lost, will it be possible to feather the prop anyhow?
I realise these are all questions I should be able to read up on myself, but as I said I don't have access to the manuals at present.
/LnS
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Originally Posted by Cardinal
A King Air driver demonstrated the problems nicely when he selected ground fine in flight to expedite a descent. All was well until he advanced the power levers, one engine came out well before the other, control was lost, and the aircraft was lost.
Do you have a reference on that? I've heard the same, but no details. I'm kind of surprised, because the King Air series is very similar to the BE-99, and the -27/28 and -36 on the BE-99 use a beta valve system that makes it virtually impossible to achieve Beta while in flight- while the governor is modulating pitch, the beta valve cannot redirect oil pressure to change the FTG setting. In fact, the system is so reliable, they stopped putting in low-pitch stops sometime before the -27/28 series (Sorry, I'm not too well versed on the systems of the -99 and -99A or any of the King Airs, where they differ)
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Unfortunately I don't have a reference, the story comes only from oral tradition. I just performed a cursory search of the NTSB database and was unable to come up with anything similar.
As for low pitch stops, the PT6A-65/67 were equipped with an electronic low pitch stop that modulated between flight idle at 13 degrees-ish and ground idle at 5 degrees. Selection of the beta range in-flight was very possible. (Several accidents in the landing phase and my own "experience" point to this). As I believe you suggest, the cases I've seen of this have all been in an underspeed condition, i.e., off the governor. I can't say for certain what would happen at higher speeds.
As for low pitch stops, the PT6A-65/67 were equipped with an electronic low pitch stop that modulated between flight idle at 13 degrees-ish and ground idle at 5 degrees. Selection of the beta range in-flight was very possible. (Several accidents in the landing phase and my own "experience" point to this). As I believe you suggest, the cases I've seen of this have all been in an underspeed condition, i.e., off the governor. I can't say for certain what would happen at higher speeds.
Cardinal:
As you might have read, this also happened on the Martin B-26 back in WW2 ("One a day in Tampa Bay"). Read about what IPs did to control the plane when a prop oversped right after takeoff. The short wings (+loading) required a very high Vmc.
Do SF-340 props have NTS protection? Can overspeed cause the shaft to decouple at the gearbox, as with the Allison engines?
From what I've read, at first the Curtis electric props were difficult to maintain, and the Instructor Pilots were also very new. This problem might have happened on the C-46 Commando, flying through the worst weather in the world, over the highest mountains (the "Hump")
.
As you might have read, this also happened on the Martin B-26 back in WW2 ("One a day in Tampa Bay"). Read about what IPs did to control the plane when a prop oversped right after takeoff. The short wings (+loading) required a very high Vmc.
Do SF-340 props have NTS protection? Can overspeed cause the shaft to decouple at the gearbox, as with the Allison engines?
From what I've read, at first the Curtis electric props were difficult to maintain, and the Instructor Pilots were also very new. This problem might have happened on the C-46 Commando, flying through the worst weather in the world, over the highest mountains (the "Hump")
.
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From what I've read, at first the Curtis electric props were difficult to maintain.....
Hollow steel blades were severely prone to cracking, and if one let go, you could say goodbye to the whole engine in a heartbeat...as in, torn right off the wing.
Malfunctioning large propellors can be very serious business, and need to be treated with utmost respect....least you end up quite dead.
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TangoBar/cardinal...
Don't know if this helps ( or hinders)... I do remember reading an accident report ... might be as far back as early 90sor late 80's... believe in US mag 'Flying ' of maybe a twotter that came to grief due to the capts habit of selecting ground fine on a regular basis as a way of leaving descent very late...got along with it for quite some time apparantly before getting bitten.
Don't know if this helps ( or hinders)... I do remember reading an accident report ... might be as far back as early 90sor late 80's... believe in US mag 'Flying ' of maybe a twotter that came to grief due to the capts habit of selecting ground fine on a regular basis as a way of leaving descent very late...got along with it for quite some time apparantly before getting bitten.
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SF34 overspeed and NTS protection
The Saab 340 has as last prtection an overspeed govenor wich is not a part of the PCU. For NTS you have the autocarsen system wich has an auto relight feature sensed by P3 pressure drop.
I never heard of an decoupled shaft at the Gearbox of an CT7 engine.
Hope this will help a little
I never heard of an decoupled shaft at the Gearbox of an CT7 engine.
Hope this will help a little
A question regarding Cardinal's post about the KingAir why would someone choose a grnd setting in the air? do you have an ntsb report reference or at least the year? how was the blame assigned?
I meanthere can be fatal consequence for not following prescribed limitations. but was Beechcraft's prop system also to blame in any way? just curious it sounds like a good example of why the POH/AFM is followed despite not foreseeing in your mind's eyes any untoward consequences.
I meanthere can be fatal consequence for not following prescribed limitations. but was Beechcraft's prop system also to blame in any way? just curious it sounds like a good example of why the POH/AFM is followed despite not foreseeing in your mind's eyes any untoward consequences.
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Again, no, we don't have the reference. As to why? Well, if you really wan't to get down in a hurry. Keep in mind most turboprop pilots develop a deep instinctive connection between "slow" and "beta/reverse." Every landing, every turn, every stop while taxiing includes the use of the prop. After a few thousand hours it get's pretty hardwired. Once when floating slightly too far down a wet runway I found myself edging into ground fine without conscious thought. Everything worked out, but my body did one thing while my brain knew perfectly well it was a bad idea. Beta at altitude is probably a pre-meditated affair, but you know what happens if we get away with it once...
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Had a PT6a engine blow apart due to: CSU failed, the overspeed occured so quick that the overspeed governor and then the FTG also failed, all within a split second. Apparently the system couldn't handle the force. Engine destroyed and cokpit filled with smoke thru the bleed system. No control problems.
As for pulling throttles out of the flight range during flight, a Casa 212 turboprop was on final to runway 21R at DTW in 1987.
The Captain was apparently high and or fast, and despite the danger he pulled the throttles just behind the flight idle stops, hoping to slow the plane down. The plane quickly yawed/rolled left and it crashed into the end of one concourse (old F concourse?). He and some of the passengers died.
The Captain was apparently high and or fast, and despite the danger he pulled the throttles just behind the flight idle stops, hoping to slow the plane down. The plane quickly yawed/rolled left and it crashed into the end of one concourse (old F concourse?). He and some of the passengers died.
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Reference to the CASA incidents here..
http://www.ntsb.gov/recs/letters/1990/A90_181_182.pdf
and the Fokker 50 at Luxembourg incident..
http://aviation-safety.net/database/...?id=20021106-0
http://www.iasa.com.au/folders/Safet...embourg-0.html
Seems to be some question over the reliability of inflight beta prevention on several types?
http://www.ntsb.gov/recs/letters/1990/A90_181_182.pdf
and the Fokker 50 at Luxembourg incident..
http://aviation-safety.net/database/...?id=20021106-0
http://www.iasa.com.au/folders/Safet...embourg-0.html
Seems to be some question over the reliability of inflight beta prevention on several types?
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You should probably take a look at this NTSB report:
http://www.ntsb.gov/ntsb/brief.asp?e...06X00741&key=1
A SF34 Captain moved the power levers below flight idle and the result was overspeed / overtorque ... the rest is history
It's very simple to develop the NP overspeed in the saab 340, there is a bottoming governor on the CT7-9B (saab 340b, don't know the CT7 design, i i only fly the B) - when engaged the power lever change the blade angle. That alone can result in an overtorque and in some condition in overspeed. Hope i managed to give you the information needed.
Kids, don't try this at home
http://www.ntsb.gov/ntsb/brief.asp?e...06X00741&key=1
A SF34 Captain moved the power levers below flight idle and the result was overspeed / overtorque ... the rest is history
It's very simple to develop the NP overspeed in the saab 340, there is a bottoming governor on the CT7-9B (saab 340b, don't know the CT7 design, i i only fly the B) - when engaged the power lever change the blade angle. That alone can result in an overtorque and in some condition in overspeed. Hope i managed to give you the information needed.
Kids, don't try this at home
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S20, you seem to know the ssab quite well. The overspeed protection system, where are the sensors placed? I mean does it sense enginge overspeed or prop overspeed?
/LnS
/LnS
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The CT7 has an overspeed sensor in the power turbine, to protect it against a loss of load (shaft breakage, etc.) However, if the prop were to fail to fine pitch at a high IAS, it could probably drive up to the overspeed trip even with the core engine (gas generator) shut down. In that case the o/s trip is of no help.
However - as I mentioned before - I see that as very unlikely with a counterweight prop such as Dowty. The counterweights will always attempt to drive the blade toward coarse pitch.
However - as I mentioned before - I see that as very unlikely with a counterweight prop such as Dowty. The counterweights will always attempt to drive the blade toward coarse pitch.