I'd bet on an Inversion.
Happened to mee to the opposite side. Descending with an ATR 42 on autopilot maintaining 180 kt as instructed by ATC.
Temp above a cloud layer at about 2500 ft AGL 12 deg positive. diving into the layer the speed came up to 205 kt ! and the temp dropped to -6 deg celsius within not even 100 ft altitude difference. the cloude layer also looked smooth from above.
We were already on the LOC so I noticed a change in wind correction of about 3 - 5 deg.
In climb I'v seen that thing happen on a Piper Aztec ride for fun
. expecting the speed and performance loss when climbing out (got smart and asked met office ...) once it started getting lighter i increased the speed to 140 mph. coming out of the cloud layer I hit the inversion. temp up some 10 deg C, speed down by about 10 mph.
If you know there is an inversion basic pitch mode could be best practice.
I only climb at VS in cruise for a level change and keep my hand on the glareshield to be sure not to forget setting it back to IAS once it reaches climb speed.
Miserlou,
on cold days with -10 or more you can get quite impressive climb figures out of an ATR
and his 72 wass 1 ton short of MTOM
