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Runaway Propellers  
User currently offlineAR385 From Mexico, joined Nov 2003, 6593 posts, RR: 35
Posted (8 years 5 months 2 weeks 2 days 23 hours ago) and read 10171 times:
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I am reading a book about Boeing's stratocruiser and apparently many accidents were caused by a runaway propeller. Can anyone here explain to me what is a runaway propeller and why can it bring down a plane?

17 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (8 years 5 months 2 weeks 2 days 21 hours ago) and read 10157 times:

A propeller which fails in such a way that it remains in flat pitch (high RPM setting). It was a problem with B17s until they added a hydraulic reservoir for feathering propellers after engine failures IIRC.

The propeller will windmill away at a high RPM, but without a running oilpump to keep the bearings lubricated. Eventually, the bearings will overheat and seize. It was not uncommon for prop shafts to break when this happened, sending the propeller careening off and often into the fuselage if it was #2/#3.

A failed CSP mechanism can send a propeller into overspeed very quickly. The worst case scenario is if the propeller starts shedding parts, such as blades, before it is back under control. You want to have an overspeed governor or similar backup mechanism to catch these failure modes and bring the propeller RPM back under control.

Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 2, posted (8 years 5 months 2 weeks 2 days 21 hours ago) and read 10152 times:

I recall that one of the main issues with the Stratocruiser was that the engines were so powerfull that any runaway/seizure would risk real structural damage to the engine mount & wing from massive torques generated. The one that crashed in the Brazillian jungle I believe lost a wing after a prop failure.

I always picture prop pitch / engine RPM as a bit like gears in a car. Fine pitch is like low gear (1st gear in a manual shift car). You get high engine revs for low car speed, but there's plenty of 'pulling power' for rapid acceleration. Also, when you take your foot off the gas, the retardation effect is relatively strong compared to higher gears. So for a plane, you get good acceleration and speed control in fine pitch, but like in a car, as you get faster the engine over speeds and a gear change up / prop pitch change coarser is required.

Obviously modern CSP units aim to keep RPM to a set value so the analogy is only good if you had a car setup with an infinitely variable gearbox that keeps the engine RPM fixed.

Anyway - to picture a runaway prop that has failed to fine pitch, imagine cruising along at 60mph in a manual shift car in top gear and then crunching into 1st gear. The engine screams and there's massive retardation. Obviously in a multi engined plane the failed engine is probably out on a wing somewhere and this large drag will cause very real control problems - much worse than a simple 'engine out & feather scenario'

Anyway - excuse the waffle......



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineVc10 From United Kingdom, joined Nov 2001, 1412 posts, RR: 16
Reply 3, posted (8 years 5 months 2 weeks 2 days 20 hours ago) and read 10139 times:

The way I understand it is that a variable pitch propellor will always want {due to centrifugal force} to go to fine pitch setting, and should the control mechanism fail then it will go to this fine pitch setting where slip stream will now drive it and the engine to sometimes very high RPM, and the now huge centrifugal force is greater than the counter "feathering force", and so you will most probably be unable to feather that engine.

The obvious thing to do is to throttle the engine but if the overspeed is to high then this will have very little affect as the the prop/engine is now being driven rather than driving. What I have always been told is that you need to take action quickly

Throttle/feather--and if this does not achieve the result, then rapidly reduce
both indicated airspeed and altitude to try and reduce the power of the slipstream which is what is driving the prop to this overspeed condition.

It is not only hydraulic controlled props that can suffer from this as on an electric controlled prop if the brake fails then the prop will go to fine pitch very quickly.

Whilst not really as a result of prop overspeed you might find the following site interesting

http://www.conniesurvivors.com/1-ph-tff_bangkok_fire.htm

littlevc10


User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 4, posted (8 years 5 months 2 weeks 2 days 20 hours ago) and read 10133 times:

Quoting Vc10 (Reply 3):
The way I understand it is that a variable pitch propellor will always want {due to centrifugal force} to go to fine pitch setting, and should the control mechanism fail then it will go to this fine pitch setting where slip stream will now drive it and the engine to sometimes very high RPM

A pilot who flies both single and mutli GA aircraft told me that on a single the CSP unit is designed to fail to fine pitch - to facilitate easier landing, but on a multi (except perhaps Cessna 337s and the like) the CSP is designed to fail to the feather (i.e. all the way through fully coarse to the feather position) obviously to minimise asymmetric drag tendancies after a failure.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineVC10 From United Kingdom, joined Nov 2001, 1412 posts, RR: 16
Reply 5, posted (8 years 5 months 2 weeks 2 days 19 hours ago) and read 10120 times:

DH106,
I would not argue with that at all ,but in the case of an overspeed we are normally talking about a failure of the system, which will allow the prop to run out of control to the fine pitch setting.

On the big multi-engined piston powered airliners like the Connie thay had a system called Auto-Feather which once selected would automatically feather a prop should the engine power fall by more than certain % of its selected power setting. In fact if this system was not working and you had to rely on the crew to feather, there was a reduction required in the aircraft's RTOW.

This might be what the Cessna has, as it takes quite an effort to drive a prop into the feather position when it is rotating, however I have noticed that on some of the smaller Turbo prop aircraft, their blades go to the feather position when they shut them down on the ground,so there is a good chance you are correct
However these are a bit modern for a old git like me

littlevc10  old 


User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 6, posted (8 years 5 months 2 weeks 2 days 18 hours ago) and read 10103 times:

Quoting VC10 (Reply 5):
DH106,
I would not argue with that at all ,but in the case of an overspeed we are normally talking about a failure of the system, which will allow the prop to run out of control to the fine pitch setting.

Agreed, but most if not all of the oil-based systems (as opposed to electric) are sprung to one extreme and use ported oil pressure to drive them the other way. So they're pretty much 'fail safe' - a leak results in the prop being driven to the 'safe by design' pitch stop.

I guess the electrical type may be prone to fail-to-fine under some electrical failure circumstances.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 7, posted (8 years 5 months 2 weeks 2 days 15 hours ago) and read 10047 times:

Quoting Vc10 (Reply 3):
The way I understand it is that a variable pitch propellor will always want {due to centrifugal force} to go to fine pitch setting,

Yes, if aerodynamic moments or other features do not drive them to another setting. Smart propeller designers of double-operating propellers, where oil pressure drive the prop towards fine as well as coarse pithc, include some counterweights which will drive the propellers to a suitable setting by centrifugal force should all other means of propeller control be lost.

Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 8, posted (8 years 5 months 2 weeks 2 days 14 hours ago) and read 10041 times:

Quoting DH106 (Reply 4):
A pilot who flies both single and mutli GA aircraft told me that on a single the CSP unit is designed to fail to fine pitch - to facilitate easier landing, but on a multi (except perhaps Cessna 337s and the like) the CSP is designed to fail to the feather (i.e. all the way through fully coarse to the feather position) obviously to minimise asymmetric drag tendancies after a failure.



That's what the FAA insists the correct answer is...and what us Yanks are taught in our commercial ground schools.

If you examine the design of a constant-speed propeller mechanism, the pitch mechanism is inverted for a single-engined aircraft vs. a multi-engined one (at least in the GA world...). IIRC, the presence of oil pressure in a single works to move the propeller towards a coarse-pitched setting, meaning that the propeller will fail into a fine-pitched (high RPM) setting, which is a fail-safe setting in a single-engined aircraft (the engine can always produce power with the prop control at high RPM). In a twin, the presence of oil pressure works to "unfeather" the propeller blades. So, the failure mode in a twin is that the propeller will revert to a feathered position (drag is the enemy in a light twin with an engine out!).

EDIT: another spelling mistake nixed in the bud...

[Edited 2006-07-07 19:02:53]


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 9, posted (8 years 5 months 2 weeks 2 days 14 hours ago) and read 10029 times:

The exception being multi pistons and turboprops without a free turbine, where a mechanism is in place to keep the props from feathering on shutdown so the starter won't have to spin a feathered prop when starting the engine.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 10, posted (8 years 5 months 2 weeks 2 days 14 hours ago) and read 10013 times:

Quoting FredT (Reply 9):
The exception being multi pistons and turboprops without a free turbine, where a mechanism is in place to keep the props from feathering on shutdown so the starter won't have to spin a feathered prop when starting the engine.

When I was a re-fueler at LRU, I used to love it when YV would taxi out their Beech 1900 in the morning. They would cycle the props about 3 times...it sounded like they were moving the propellers into beta (I'm not sure if the P&W PT6's on the Beech 1900D had a full beta setting...they might have just been cycling the props into or close to the feather position). It was a very cool sound  cloudnine 



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 11, posted (8 years 5 months 2 weeks 2 days 12 hours ago) and read 9963 times:
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Quoting FredT (Reply 9):
The exception being multi pistons and turboprops without a free turbine, where a mechanism is in place to keep the props from feathering on shutdown so the starter won't have to spin a feathered prop when starting the engine.

...And another exception being the Champion 402 Lancer....which, I believe, is the one and only production multi-engine airplane with fixed-pitch propellers:






2H4





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User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 12, posted (8 years 5 months 2 weeks 2 days 12 hours ago) and read 9958 times:

Quoting 2H4 (Reply 11):
...And another exception being the Champion 402 Lancer....which, I believe, is the one and only production multi-engine airplane with fixed-pitch propellers:

Wow, there was one of these rotting on the ramp at LRU when I was in college/working as a lineboy in the early '90s (well, maybe not rotting. But certainly suffering sun damage to it's fabric-covered parts  Wink ). The story I got from the FBO owner was that these were used as Army multiengine trainers...I'm not sure if that's the case or not. It is definitely an obscure type!

BTW the same FBO owner had a soft spot in his heart for multi-engine converted Navions (another rare type!).



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineJHSfan From Denmark, joined Apr 2004, 469 posts, RR: 2
Reply 13, posted (8 years 5 months 2 weeks 2 days 11 hours ago) and read 9934 times:

A couple of links to show what a damaged C-130 looks like when one of the props had gone through the fuselage:
Mechanical failure!
Lockheed C-130 runaway prop damage

- JHSfan



Look at me, I´m riding high, I´m the airbornmaster of the sky...
User currently offline2H4 From United States of America, joined Oct 2004, 8956 posts, RR: 60
Reply 14, posted (8 years 5 months 2 weeks 2 days 8 hours ago) and read 9897 times:
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DATABASE EDITOR




Quoting KELPkid (Reply 12):
The story I got from the FBO owner was that these were used as Army multiengine trainers

What I want to know is this.....if one were to take his/her multi checkride in a twin with two fixed-pitch props, would his/her rating carry a centerline thrust restriction?

After all...in that airplane, it would be impossible to demonstrate all engine securing procedures to the examiner...




2H4





Intentionally Left Blank
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 15, posted (8 years 5 months 2 weeks 1 day 4 hours ago) and read 9766 times:

Since the original post referred to the Boeing Stratocruiser, and yours truly, quite a long time ago, operated one as a co-pilot, the following is offered....

The Stratocruiser was offered with two types of propellers depending on the original customers specification, namely,
The Hamilton Standard Hydromatic or the Curtis Electric.

The main problem with the Curtis Electric (using a stephead electric motor to control blade pitch) was not the electric blade actuation (which was quite complicated, but reliable) but the hollow blades.
These were prone to blade fracture, which resulted in partial (usual case) blade separation, creating a terrible unbalanced condition, which can (and did) result in the engine being torn from its mounts.

Most original airlines that ordered the Stratocruiser with Curtis Electric props had the Hamilton Standard Hydromatic props fitted later.

The Hydromatic prop, if it loses prop govenor oil pressure, normally (but not always) will drift toward the low pitch (high RPM) blade position.
In order to feather this propellor, an electric motor driven pump was used.
It was called the feather pump, and supplied engine oil, under pressure to the prop, to drive the blades toward the feathered position.
IF the feather pump would not operate, the prop could absolutely NOT be feathered.

Therefore, if normal prop govenor oil pressure was lost, the result could well lead to a runaway condition, and the feather pump was used to feather the propeller.
However, if a runaway prop could not be feathered, either because the feather pump could not supply sufficient oil pressure, the pump failed to operate or the oil supply to the pump (or the propeller) was interrupted, the EMERGENCY procedure was to pull the respective engine fire handle (to, among other things, cut off oil to the engine), in hopes of causing the engine to seize, because....the airplane could not maintain altitude with one prop windmilling (runaway prop) at heavy weights.
Even at lighter weights, the ability to maintain altitude was limited, and in any case, the anticipated range (as in an ocean crossing) in such a condition would be severely compromised.

By the way, for those interested, the Stratocruiser was a delight to fly.
It was also very much an electric airplane, the landing gear operation, for example, was electrically driven.

[Edited 2006-07-09 05:15:44]

User currently offlineAR385 From Mexico, joined Nov 2003, 6593 posts, RR: 35
Reply 16, posted (8 years 5 months 2 weeks 1 day ago) and read 9729 times:
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Thanks evryone for their answers, specially 411A. Now, I have a basic understanding of what is a runaway propeller. I relly like to come to this forum because I get the answers I want from nice people experienced on the field.

[Edited 2006-07-09 09:25:46]

User currently offline57AZ From United States of America, joined Nov 2004, 2556 posts, RR: 2
Reply 17, posted (8 years 5 months 2 weeks 22 hours ago) and read 9716 times:

Boeing certainly did have a problem with runaway props throughout the 377's operational lifespan. Pan Am lost something like three 377s-one over the South American jungles and one over the Pacific with a loss of all onboard. A third 377 was forced to ditch near a Coast Guard ocean station on a flight between HI and the mainland when a prop ran away and the effects of high drag eliminated any hope of making the West Coast or returning to HI. About a year after the last Pan Am incident, a USAF C-97 on the ATC Travis-Hickam run suffered a runaway prop and catestrophic failure of both No. 1 and 2 motor after passing the point of no return and Coast Guard ocean station. No. 1 prop ran away and eventually sheared off, wrecking No. 2 motor in the process. Fortunately, the aircraft commander shut down No. 2 prior to the prop shaft failure and had entered a steep turn to allow the No. 1 prop to hurtle over the fuselage when the shaft failed. Minus the drag of a windmilling prop, the C-97 was able to limp along in ground effect at reduced speed for several hours, making an emergency landing at Hilo. In order to keep the C-97 aloft, both pilots had to literally stand on the right rudder and bank the aircraft into the good Nos. 3 and 4 motors to counteract the drag from the lack of thrust from the damaged motors.

While the 377s and C-97s were great aircraft, they certainly had to be finessed and even a slight mistake on the handling of the motors could be really bad news. NW lost a 377 at Seattle when the FE forgot to close the gills on the engines. This affected the aircraft's performance so greatly that it crashed in the sound with a loss of ten lives. All because the engine gills were improperly set.



"When a man runs on railroads over half of his lifetime he is fit for nothing else-and at times he don't know that."
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