Changing propellers for skydiving plane isn’t likely

Dear Johnnie: I am one of the many who suffer all through the weekend while Mile-Hi Skydiving lifts their skydivers to their jumps every 28 minutes.

It occurred to me that their “jump ships” could be modified to help mitigate the noise their planes produce. I contacted the manufacturer of the de Havilland Twin Otter (Viking Air), and they suggested a prop modification provided by Raisbeck Engineering. Raisbeck’s website claims that the “Raisbeck Quiet Turbofans pay for themselves in fuel savings alone, and make you a better neighbor at your airport.” The Raisbeck prop has four blades. The Twin Otter shown on Mile-Hi’s website shows only three blades on their airplane.

Could it be that Mile-Hi has not taken this cost-effective option on their “jump ships” (they have three!) for reducing noise above our fair town? Perhaps an arrangement could be made where the change-out for the new props is funded by the city, or a local tax district, then the funds reimbursed over time as the savings claimed by the manufacturer are realized. Sounds like a win-win to me.

I am proud that our local airport serves as a home base to the skydiving community, but I feel they need to do their part to be a good citizen and neighbor in our community. — Otter B. Prop-erly Done

Dear Otter B.: I don’t know the first thing about airplane props, except that I would like them to be engaged if I am flying in an airplane that has them.

So I called two sources — the Federal Aviation Administration and Mile-Hi Skydiving.

Allen Kenitzer, manager of Communications & Media Relations with the FAA’s Northwest Mountain & Alaska Regions, checked with FAA Flight Standards experts about the modification you suggested.

“I’m told that it is true that some aircraft are approved to have various propellers and other modifications accomplished that can reduce noise and improve efficiency, including

Mile Hi-Skydiving's planes would be quieter with four-bladed propellers, but Mile-Hi says that the modification would be enormously expensive and would not be approved by the FAA. (Courtesy, Mile-Hi Skydiving Center)

certain four-bladed propellers,” he replied to Johnnie. “The idea is that a four-bladed propeller can produce equivalent levels of torque at a reduced RPM compared to that of a three-bladed propeller, therefore reducing the noise produced by the propeller.”

All of this jibes with what you learned from Raisbeck.

However, Kenitzer added that “changing the make and/or model of the propellers installed on an aircraft is considered a major change to type design and would require an STC if they are not listed on the Type Certificate Data Sheet for the particular aircraft.”

Any modification from an airplane’s original design requires what is a called a supplemental type certificate, or STC. “The STC … approves not only the modification but also how that modification affects the original design,” according to the FAA website.

Kenitzer said that there is no record of Mile-Hi requesting an STC for its Twin Otter. Mile-Hi owner Frank Casares explained why.

“When a request is made to the FAA for the modification,” Casares wrote, “they would see that existing STC’s prohibit the addition of the Raisbeck STC, thus denying the request.”

What would modifying Mile-Hi’s Twin Otter props require?

“This would include replacing existing engines, modifying the nacelles (engine enclosure), engine instrumentation, cables and wiring. This would have a cost of about $500,000 to $800,000 in engines, props and cost of STC’s themselves,” Casares wrote to Johnnie. “Also, when dealing with major STC’s as these are, it’s not as simple as a mechanic making the decision and the modification. There are manufacturer-approved installation facilities for this kind of major work.”

Casares said that Mile-Hi researched the Raisbeck option years ago and found that “real-world performance with the Raisbeck STC was not quite as advertised, especially in the skydiving community. Fuel savings occurred in cruise flight, not necessarily in climb configuration. … In asking other operations that have four-bladed props, there was degraded performance in climb at higher altitudes. We are already at a disadvantage starting at 5,000 feet.”

So, Otter B., it comes down to this: Mile-Hi cannot be required to make this modification to their planes, and they say that the modification would be prohibitively expensive.

The idea of a special-taxing district to pay for modifications to Mile-Hi’s airplanes is, to say the least, unique. I suppose that if taxpayers fronted the money for modifications, Mile-Hi might reconsider. But using taxpayer money would involve a different kind of STC — a Staggeringly Tiny Chance.

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