Ask the Expert: Propeller Restraints – The Good, the Bad, and the Ugly

Ask the Expert: Propeller Restraints – The Good, the Bad, and the Ugly

Ask the Expert: Propeller Restraints – The Good, the Bad, and the Ugly

As you have experienced, the propeller on the PT6A engine series turns very freely. Even a child using a little finger can spin it quite easily in the hangar or on the ramp due to the fact the PT6 is a “free turbine” engine in which the propeller shaft is not physically connected to the gas generator or compressor section of the power plant. Wind can also supply the force that causes prop rotation. When parked on a windy ramp, almost always one or both propellers will be rotating, sometimes in the proper clockwise direction, sometimes in the opposite way, depending on the wind’s direction. Due to the ratio of input to output shaft speed designed into the Reduction Gearbox (RGB) – N2 to NP – the Power Turbine is rotating 15 or more times faster than the prop.

The lubricating oil supply to the RGB comes from the engine’s oil pressure pump that is driven by the compressor section … the other shaft in the engine: N1, not N2. See the problem? When the propeller is allowed to turn without the engine running, no lubrication is supplied to the gears and bearings in the RGB except for residual oil that was originally there.

So that is why airplanes with PT6 engines are always supplied with some type of propeller restraint device that can be connected to the propeller when parked on a windy ramp: The engine people want to prevent propeller RGB and N2 shaft rotation when no positive lubrication is being supplied. Beech’s factory-provided restraints consist of a rubberized fabric sleeve that fits over the end of one propeller blade and two elastic straps that connect the sleeve to two rubberized fabric exhaust stack covers.

Questions have often been raised during initial and recurrent King Air pilot training classes about the importance of and use of the restraints. A common one is “How important is it to stop rotation? If I am just going to be in the FBO for 15 minutes, paying the ramp fee, should I put the restraints on?” Also, “How cool should the exhaust stacks be before I attach the restraint?” And the ever-popular one, “Should I place the propeller blade with the sleeve attached in the up or down position?”

There are not black-and-white, right-or-wrong answers to these questions. Some operators let the props spin for an hour or more while others install the restraint immediately before leaving the airplane alone. Some exhaust stack covers are more heat-resistant that others … and some pilot’s fingers are more heat sensitive than others! The up versus down question? I doubt that an answer will ever be resolved to everyone’s satisfaction. But let me provide a few of my observations, comments and “war stories.”

If you have spent time in Wichita, Kansas, you well know that it is usually windy there. Leaving the King Air on the ramp without restraints will almost always lead to aggressive propeller rotation. We at the Beechcraft Training Center always emphasized the importance of installing the restraints if the airplane would be parked for more than 30 minutes or so. Yet, when our students drove to lunch, what did they see on the factory ramp? They saw every new King Air sitting there with no restraints and the props turning like crazy! Right or wrong, the production test personnel had been directed not to worry about it … and they surely did not! Was harm caused? Probably; no one knows the exact answer, but I’ll say this: I don’t think Beech ever received a warranty claim for RGB damage in a new King Air. (At least not when I worked at the factory, 1972-1977. I wonder if the Textron production test flight department still lets them spin?)

From this experience I, personally, am not worried about restraining the props during a fuel stop or passenger drop-off or pick-up. Even if the airplane will be sitting for hours waiting for the passengers to return, I will still not install them if the wind is light and forecasted to remain that way. But overnight stops? I will always install the restraints along with the other “loose equipment” items, like pitot tube covers and intake plugs. In fact, I will do this even if I have arranged for hangaring the airplane overnight at the FBO, since I don’t know what the wind condition will be when they pull the airplane out onto the ramp an hour or so before the scheduled departure the next day.

I have observed some operators who do not always use the Beech-provided restraints, but instead have taken a simple bungee cord of the right length so that it can be looped to tie one blade to one exhaust stack. This eliminates the hot stack worry since the tight-fitting exhaust cover does not need to be installed. The simple bungee is used when it’s windy even during a relatively quick turn and the “proper” restraint only is used for the overnight situations. By the time the passengers and baggage are off-loaded, the fueling is done, the potty stop has been made, and the other covers and plugs installed, the exhaust stacks are cool enough that affixing the restraints to them is not problematic.

However, a word of caution: If you are going to use the simple bungee cord, I strongly suggest having a big, red, “Remove Before Flight” tag that can be readily seen from the cockpit attached to it. It’s embarrassing to get all situated ready to start and then realize the bungee was never removed. And that leads to a humorous war story:

One day back in the 1960s, a King Air A90 had made a trip from its Northeastern home base airport to LaGuardia airport in New York. The Chairman of the Board (COB) and his aides were the passengers and he, the COB, had a very important dinner meeting back home that evening. It was a cold and windy day, so the airplane sat at KLGA with the standard prop restraints in place.

As the day wore on, the nervous-nelly chief pilot, who was PIC that day, got more and more worried that the boss would not return in time to make his dinner engagement. He briefed the co-pilot, “Now when the boss arrives, you load the passengers and get the door and I will go straight to the cockpit to get the clearance and start the engines.”

“Is that him?” “Is that him?” was the PIC’s query as each limousine arrived. Finally, there he was! Like a bullet, our intrepid hero ran to the cockpit to get ready. The copilot, as directed, saw that everyone was safely aboard, briefcases stowed, briefing given. Before he pulled the airstair door up, he heard the sound of the right engine being started … but he hadn’t yet pulled the restraints! He quickly, temporarily (he thought), closed the door and raced up the aisle. “Mike! Mike! Wait, I haven’t untied the props!” He could also see the linemen waving at the cockpit and pointing to the still-tied down right prop that was not yet rotating but pulling strongly against its restraint. About that time, as the engine reached high idle, the exhaust burned through the stack covers and the elastic cord broke, flinging off the sleeve and freeing the propeller. “Hell, that worked. We need to get moving. Sit down and buckle up.” And with that, the PIC fired up the left engine, burned its restraint off, and proceeded with the flight!

(This story was related to me by the copilot. Names have been changed to protect the not-so-innocent. I never found out if the boss made his appointment!)

I learned from that incident that (1) You need some visual reminder that the restraints are installed. (2) The engine starts satisfactorily with the fabric exhaust covers on, not reaching excessive ITT. (3) Never be in such a hurry!

Now on to the blade up versus blade down argument. In the days of nothing but three-blade propellers, there was – and still is for those with three-blades – a “correct” answer: Down. This is true for two reasons. First, the shape of the exhaust covers and the angle at which the elastic straps connect to them permit the stack cover to fit perfectly when the strap comes upward to the cover. If the strap angles downward the exhaust cover can be forced to fit, but it is obviously not designed with that position in mind. Second, why was it designed this way? So that rain could drain out of the spinner through the hole for the down blade. There were incidences reported of noticeable propeller vibration and it was traced to water that had collected in the spinner, turned into ice as the temperature dropped, and left that ice slug inside the spinner, upsetting the balance.

The main reason that a lot of pilots prefer the blade up orientation is to make the restraint more obvious from the cockpit – less of a chance of making the “Start with ’em on” mistake. With the four-blade propellers, of course there will be a hole for rain to exit the spinner no matter whether the sleeve connects to the down or up blade. Thus, putting the sleeve on the up blade does not increase the risk of propeller imbalance due to ice inside the spinner. However, the fit of the strap and cover to the exhaust stub still favors the blade down profile … leading me to still use the original orientation. But I have no complaint whatsoever for those who choose to go blade up position for the increased visibility reason.

Ready for another war story? Because of the delay needed to wait for the exhaust stack to cool enough for comfortably installing the stub covers, this particular operator had made a second set of prop restraints that replaced the elastic cords and the stub covers with simple bungees with hooks on both ends … one connected to the sleeve and the other available to hook to the exhaust stub lip. Easy peasy, right?

The early model 200 landed and was parked for the day at an airport in Kentucky that had a King Air maintenance facility associated with the FBO. It was a windy, cold and rainy day, so the pilot installed the homemade restraints. The temperature kept falling as hours passed until the rain had turned to sleet. When the passengers arrived later in the day, the pilot found a major starting problem – no N1 rotation when he activated the start switch. The large drop in voltage led him to believe that the starter was receiving power, but the compressor shaft never showed any speed. Into the maintenance facility he goes, reports the problem, and is advised that they will pull the plane into the shop. “We had this same thing happen about a month ago on another 200. The number two bearing – the forward support of the compressor shaft – had seized and we had to replace that bearing. I bet yours is the same. Probably a bad batch of bearings.”

The passengers were put on another plane and into the shop our 200 goes. The shop personnel start by removing the starter-generator and inserting a socket wrench into the accessory case splines, trying to rotate the compressor by hand. “Yep, she’s frozen solid.” The pilot authorized the shop to proceed to replace the suspected seized bearing.

This shop, being well-versed on King Airs, had the cowling off and the power section separated from the compressor section at the C Flange in a jiffy, to gain access to the bearing. Wait! What is this we see?!

We see the bottom quarter or so of the Compressor Turbine (CT) solidly encased in a hunk of ice. What the …?!

Seems like the rain and wind angle had conspired to blow lots of water into the engine via the uncovered exhaust stacks. As the temperature dropped, the water inside of the engine had now frozen, locking the CT in its icy grip. This was the reason for the lack of rotation; the bearing was fine.

I had – and still have – a question that has never been satisfactorily answered. PT6 engines have two drains, forward and aft, at the bottom of the compressor case. These are designed to spring open when the pressure equalizes on both sides of the drain but they close when the compressor’s rotation creates higher internal pressure, to avoid an undesirable P3 leak. Their primary purpose is to provide an exit path for fuel following a no-light-off starting attempt. Why didn’t they permit the water to drain from the case in this situation? Were they installed incorrectly? Were they defective? Or did the ice freeze over them rapidly enough that they became plugged before much water could exit? I don’t think we will ever know the exact reason but we do know that this incident did indeed happen and led to unnecessary delay and expense.

Bottom line? Do not use a simple bungee cord setup under these conditions. Cover up those exhaust stubs with the propeller restraint when it’s rainy and cold.

My last comment has to do with storing the restraints when not in use. There may be room in a wing locker – if you have them – simply toss them into the locker along with the pitot tube covers, engine intake plugs and heat exchanger inlet plugs. But if no locker is available or if it is full of baggage, golf clubs or skis, now is the time to fit all the items into the small canvas bag that Beech provides with the new airplane. That can be a tight fit.

What I have found that works well is to mesh or insert one exhaust cover into the other one, then roll the combination down the elastic straps until they’re three inches or so away from the sleeve. Now insert the wad of covers and straps into the sleeve. Voila! A rather compact package easy to stow and easy to unroll and use the next time. There are probably other methods that work just as well or even better, but I like this one.

The good of the restraints? They protect the RGB from possible damage. The bad? They are difficult to install until the exhaust stacks cool. The ugly? When you start the engine with them still installed, you’ll need to buy a new set … not to mention the ridicule you will receive from the line people and other pilots!

About the Author

Leave a Reply