Turboprop lower than normal torque

[AV8trixx]

At approx FL200 in our Kingair 300 we feel a vibration, and notice a drop in the torque on both engines. The torque reads about 10% below normal now at cruising altitude since this started. The vibration is intermittent and we don’t feel it on descent. Has anyone else experienced this and what might be the possible causes? Thanks for any help. :)

[Station Zero]

Would certainly recommend reporting this to your maintenance provider so they can investigate. Any drop in torque and unusual vibrations could indicate deterioration in the engines and if that is the case its only going to get worse.

I am not familiar with this aircraft exactly, could I ask if there are any keep out zones for the engines, this could also be a possibility if there is. The FCOM should clarify that though.

If your interested check with maintenance the engines Total Time, Time Since Last Shop Visit and Time Since Last Hot Section inspection. If the issue is internal of the engine they may be approaching a defined service limit that will require a shop visit or perhaps there is some premature deterioration if you operate in a particularly harsh environment. Also check that the compressor turbine blades and power turbine blades are being replaced, on the PT6 the life limits for them are contained within a Pratt Service Bulletin rather than the engine life limits where the disk lives are.

[bingofuel]

What was the temperature and moisture conditions? Were your ice vanes deployed correctly? Possible compressor damage if not!

[AV8trixx]

What was the temperature and moisture conditions? Were your ice vanes deployed correctly? Possible compressor damage if not! Clear skies, temps normal, however we have noticed that our OAT has been reading incorrectly. It’s been showing incorrect temps, 10c+ degrees above normal at altitude.
Yesterday we flew at FL270 and the OAT was reflecting realistic temps, we noticed that our torques and fuel consumption were back to normal readings. Could that sensor cause these issues?
Another consideration is our prop stop switch, might the vibrations be related to that not working correctly?
The aircraft is going in for inspection, we’ve asked for the compressor to be checked and a hot section inspection as well.
Thanks :)

[misd-agin]

Do you know that the OAT was reading high? Or was the actual temperature ISA +10?

[AV8trixx]

The OAT display showed temps that were definitely warmer than the actual temps, where it should have read -22 it was showing -10, or warmer. We verified this with the upper winds and temps chart during our flight recently.

[Sunstrand]

I have seen this same thing in the Saab 340, Noticed that during cruise the torque was lower than normal. Couldn't figure out why, double checked cruise power charts and it was fine. looked at OAT reading and it also seemed to be reading warmer than normal. The FMS performance page showed ISA +30 which is not typical.

I was lucky to have another Saab flying same route a few thousand feet above at the same time and spoke to them to get their OAT reading for their altitude and confirmed that our OAT was reading warm. In the end there was a small blockage in the TAT probe which was giving us incorrect readings.

[misd-agin]

The OAT display showed temps that were definitely warmer than the actual temps, where it should have read -22 it was showing -10, or warmer. We verified this with the upper winds and temps chart during our flight recently.

I'd ask other aircraft nearby what their SAT is, vs a chart, for verification.

[tdracer]

One of the characteristics of Ice Crystal Icing (ICI) is that TAT will read right at 0C (give or take depending on the sensor accuracy). The ice crystals will hit the heated probe, melt, then freeze again forming an ice cover over the probe with liquid water inside due to the probe heat. It's also unpredictable - it may not affect the aircraft probe, but affect dedicated engine probes, or any combination there of.
Ice crystals are also have low radar reflectivity, and don't generally show up on weather radar.