Prop overspeed is an ugly thing!

mrivc211

mrivc211

Well-Known Member
Today we went over many systems in the brasilia. Most of it is review but when time goes by, some of the minute details are forgotten very easily. Of the many systems we went over throughout this whole week, the prop really stuck out at me.

We saw a recreation made by Embraer of a brasilia going into a prop overspeed condition. It was just plain old scary to put it mildly. The pilots were on approach and doing 265knots descending thru 2500' with the GPS warnings going off. At 1000' AGL they leveled the airplane to drop the gear. (200 kias limitation) And at 203 KIAS they promptly dropped the gear and INSTANTLY the number two prop went into overspeed. I beleive I saw over 128% Np on the right side. The airspeed instantly dropped about 100 knots, and they lost 800' in about 9 seconds. The captain then tried to compensate quickly by adding power to the number one engine which only worsened the problem creating it to veer even more towards the right. At certain points both props were overspeeding. I'll see if I can get the video file of it somehow. It will definetly give you more respect for the airplane you are flying and take away the typical complacency we pilots tend to develop flying day in and day out without a hitch.

The outcome was pretty bad. They eventually could not keep control of the aircraft and the right wing hit a building flipping the plane over onto its back. You could hear both pilots scream just before impact.

Studying the systems over again, you tend to have a better overall idea of how things work. So when systems are explained to you in depth a second or third time, they make more sense. You find out what the weak points in the system is. Such as when pilots used to use the props as speed brakes above 200 knots which would create too much force on the prop, sending it into an overspeed condition, or tearing a drive shaft.

Careful out there guys!
 
mrivc211 said:
We saw a recreation made by Embraer of a brasilia going into a prop overspeed condition.

They eventually could not keep control of the aircraft and the right wing hit a building flipping the plane over onto its back. You could hear both pilots scream just before impact.
Click to expand...


What kind of a sick recreation was this? Hopefully it was cartoon or something like that, or did this really happen?

Pretty interesting though, I didn't realize the prop overspeed could get out of hand that quickly... Something to really pay attention to.
 
It really happend. It was kinda like the ones the FAA puts out to get a better idea of what happend. A visial mock up.
 
That's kind of a worst case scenario. Statsictically you stand a better chance of it happening in cruise (you spend 80+% of your time there) and then most checklists call for immediate shutdown.
 
Burned into my memory from Saab 340 training: "If you get a prop overspeed, DON'T TOUCH ANYTHING. Just go right to the QRH." I had these twice in the sim.
 
Someone care to explain the physics that makes the prop overspeed cause you to lose lift? In dumbed down English, please.
 
Chris_Ford said:
Someone care to explain the physics that makes the prop overspeed cause you to lose lift? In dumbed down English, please.
Click to expand...

I'd venture to guess the same reason why a windmilling prop creates more drag than a feathered prop. Faster rotation=more drag.

~wheelsup
 
Quick Reference Handbook?


This is of interest to me. On descent in the Twin Otter, I somtimes push the prop levers towards max governing to increase the drag on the descent. Some pilots (one or two that trained me) used this on the entire descent.

That never sat well with me but I do use it when I need to drop some alt quick, say for a high approach or maybe a short field with an obstacle.


Interesting. I wonder what the prop manf. says about it. I'll have to ask.


Id love a link to that video if you have one.
 
Chris_Ford said:
Someone care to explain the physics that makes the prop overspeed cause you to lose lift? In dumbed down English, please.
Click to expand...
I'm not 100%, but I'm hypothesizing that it's because you get a greater flat-plate drag effect (a windmilling prop gives you drag equivilant to a flat plate the size of the prop), and as the props go supersonic or close to it they become massively inefficient and lose lift (remember that props are airfoils).
 
Overspeed is when the blades go to a low angle, prop speed goes into overdrive (Well over 100%.) with little or no torque from the engine. Lots and lots of drag. Like you guys mentioned, well over a windmilling prop's drag.

The Capt I am flying with this month had recurrent this week and saw the same video. It must be relatively new, because I missed it when I did recurrent. Anyway he said it was sobering to say the least. Thinking about that story definately keeps me from feeling complacent. The prop overspeed checklist is one that we have to have commited to memory. (At SkyWest.) When they give it to you in the sim it takes all of the flying pilot's strength and skill to keep it marginally controlable. The non flying pilot has to be "Jonny on the spot" with the checklist. In this scenario it would cost valuable time to fumble through a written checklist. Gotta have that one fresh in your head, always. If it happened close to the ground you would be in a load of trouble.

I don't remember for sure but I believe that was the problem that Howard Hughes had when he crashed that twin engine plane, as was depicted in "The Aviator."
 
"When they give it to you in the sim it takes all of the flying pilot's strength and skill to keep it marginally controlable. The non flying pilot has to be "Jonny on the spot" with the checklist. In this scenario it would cost valuable time to fumble through a written checklist. Gotta have that one fresh in your head, always. If it happened close to the ground you would be in a load of trouble."

Straight from the horses mouth. Just like Lee said, there ain't no time to do any kinda checklist. This problem is from memory because things are happening so fast that you have no time to pull out a checklist. I beleive in the recreation the pilots only lasted about 15-20 seconds before impact. Given they were only about 1000' AGL when it occured, they still had two good running engines. Which shows you how powerful a prop overspeed can be. You will have more than enough trouble trying to keep the thing from flipping over or figging out on you. All Skywest pilots have had prop overspeeds in the sim and anyone of them can attest to the fact that it is almost uncontrollable.

The corrective action in the bro is to :

1. Power Lever(thats for props jet guys :) flight idle
2. Condition Lever.............................feather then check.
3. Reduce speed to slow Np below 120%
4. Flaps as necassary.

5. Electric Feather switch....if necassary.....On.

Almost 99% of the time, in a prop overspeed situation your propeller has so much drag on it that the condition lever does not have enough force to put it into feather. That force is approx 780 psi. Thus you have to initiate the electric feather switch which uses slightly more juice at 820 psi. However the whole problem is that Np number needs to be below 120% for you to even be remotely close to have enough force to put it into feather.

Often times, the electic feather switch will still not do the trick. Embraer thus put out a limitation that you can cycle the pump a maximum of 6 times.

There was a crew that was at 500' AGL going into LAX that had the right engine Np rise to 109%. Thats considered an overspeed. Upon recognizing this, they immediatly feathered the prop via the condition lever, however it did nothing. They then hit the electric feather switch and it did the trick.

I'm back home in OC right now but when I get back to Salt Lake on monday I'll try and somehow grab that video.
 
Well the propeller is hydraulically controlled via the Propeller Control Unit (PCU). One of the components of the PCU is the prop speed governor section. At 104% Np oil pressure to the prop is kicked off line. In better terms, the metered oil is drained via the Overspeed Governor/P3 drain. If that doesn't work fuel flow is cutoff at 109% Np. Additonally, your condition lever allows you to feather the prop via the manual feather valve. However, like before, its limited to 780 psi. If the prop is not feathered, your back up is the electric feather switch which activates the electric feather solenoid and attempts to dump the metered oil again. Allowing supply oil to transfer via the transfer tube and feather the prop.

On one occasion a brasilia pre-mod went into overspeed because the quill shaft sheared. Turns out the two metals that were joined were of different types causing them to deteriorate. The item was previously supposed to be replaced at 400 hour intervals, and because of that accident they changed it to a 100 hour replacement and inspections at sooner intervals.

Makes you feel safe that the same material is being used isn't it!
 
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