MD-82
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 10500
Confusion; Human-Machine Interface
Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors; Weather
I was the Pilot Flying; level at FL300 in cruise flight; Mach .76; autopilot and autothrottles engaged. We flew in and out of some light cirrus clouds; otherwise clear. Anti ice was not on. I noticed that the airspeed was at 220 KIAS with EPR at normal setting around 1.7-1.8. Disconnected the autothrottle and pushed throttles up; no apparent response. Captain (PM) concurrently selected engine ice on and we got almost immediate increase in thrust. Airspeed begins to increase and we accelerate back to normal speed. We discussed making a descent as we had trained in the sim; however the aircraft was accelerating fine and flying normally with the autopilot on. We maintained assigned altitude with no problem. We did not receive a speed low indication; no perceptible change in attitude; the autopilot did not disconnect; did not feel any airframe buffet; no pitch trim warning horns. The rest of the flight continued normally. After 16 years on the MD80; I had never seen anything like this on a line flight. There was no perceived need to have the engine anti ice on as we were in clear skies and the aircraft seemed to be performing normally. The auto throttles and autopilot never clicked off which is always a cue that something is amiss. It was an insidious event; closer scanning of airspeed in cruise would have prevented this along with selecting engine anti ice regardless of how short the flight through even the thinnest layer of clouds.
MD80 First Officer detects low airspeed at FL300. Autothrust is disconnected and the thrust levers pushed up with no apparent effect. When the Captain turns on the engine anti ice an immediate thrust increase is noted and the aircraft accelerates back up to normal cruise speed.