What is the danger in the over-use of reverse thrust during ground operations when operating a turboprop powerplant?

Question

I'm new to turboprops, and everyone I've flown with seems extremely cautious about using only the bare minimum reverse thrust after touchdown and never during forward taxi.

Why might that be?

Answer 1

One concern is Foreign Object Damage (FOD). More reverse thrust means more dirt is thrown into the air, which can then be ingested by the engine:

Damage to turboprop engines is not as common as in jet engines, because the inlets are generally smaller and the propeller serves as a first line of defense. Nevertheless, first-stage impeller nicks and scratches are caused when small stones and debris are picked up from the runway during propeller reverse pitch operation after landing.

^{(Aviation Mechanics Bulletin - Foreign Object Damage)}

This is especially true when moving more slowly:

[...] sand and dirt can be ingested causing premature wear and possibly minor damage especially when using reverse thrust at lower airspeeds.

^{(kingairnation.com, emphasis mine)}

Answer 2

We used lots of reverse thrust in the C-130; it was normal to go to full reverse on landing, and also to use some reverse thrust in order to back the plane into or out of parking spots.

The two common cautions were oil temperature (on older engines) and dust ingestion / brown-out (on dirt or unimproved runways).

On early model engines, sustained reverse thrust while taxiing could limit the airflow to the oil coolers, leading to rising oil temperatures. This was avoided by using reverse briefly during taxi - much like avoiding "riding the brakes". Later engines (early 80's aircraft on, iirc) had Oil Cooler Augmentation which solved this. This used jets of bleed air to pull the necessary volume of air through the oil coolers on the ground, essentially allowing any amount of reverse thrust without concern for oil temperature.

On dirt strips, going to full or nearly full reverse for more than a couple seconds could draw enough dirt up that it would get into the engine inlet, and a few more seconds beyond that you could brown-out your view out the windows. Coming out of reverse solved the latter, and for the former, it was common to close the engine bleeds during dirt LZ (landing zone) operations, so that the air conditioning system didn't get contaminated with dirt. Some amount of dirt going through the compressor, combustion, and turbine sections was, I think, an accepted cost of operating on dirt LZ's -- especially if the performance margins were sufficiently tight that you needed reverse thrust in order to make performance numbers work out.

It sounds like the situation in the OP's aircraft is significantly different that what we had, so I suspect that there may be some aircraft-specific concerns at work there. Or maybe issues of noise (reverse thrust tends to be significantly louder than normal idle) or sucking up FOD into the props (not a concern with our props well above the ground in the Herk).

Answer 3

In addition to FOD, the use of reverse thrust, as pretty much any mechanical system in aircraft is limited to safest possible minimum.

Using reverse on high power settings imposes considerable loads on structures, and even low power use slowly but certainly wears down the propeller blade adjustment system. These considerations are not a major safety issue as such, but within larger fleets and accumulation of thousands of hours will lead to unnecessary costs. Wheel brakes are the preferred wear item as they are relatively cheap to service and replace.

Flying is and it's procedures are, after all, not only about safety, but very much about economics also.

Answer 4

The guys flying cargo in and out of our local airport in Cessna Caravans also claim that the time spent in reverse thrust has to be accounted for in the plane's log, and is counted against the engine's TBO in a special way. For this reason, they generally avoid using reverse thrust as an airbrake during steep descents- but they claim that just setting the power lever to nearly zero so the prop is turning but generating no thrust is itself an effective speed brake.

Answer 5

As Bianfable stated, FOD ingestion is a big risk when using reverse power settings. It does come down to an analysis of the runway one is operating on, and a risk assessment of the conditions. Typically on unpaved runways or ones covered in snow or ice, reverse will be discontinued below 40 kias to minimize FOD ingestion risks. Paved and well cared for runways may allow for max reverse at zero airspeed.