Ice Detector Needed

Air Safety Week, May 24, 2004

After some 15 minutes of flying through icing conditions while cruising at 33,000 feet, the pilots of Spirit Airlines Flight 970 heard the stall warning and felt the stickshaker activate. They snapped off the autopilot, activated engine anti-ice and descended. They shut down the right engine of the MD-82 twinjet when its temperature climbed, but they were able to restart it on the second attempt when the aircraft was at 17,000 feet. The airplane subsequently landed at Wichita, Kan., cutting short the planned flight with 105 passengers aboard from Ft. Lauderdale, Fla., to Denver, Colo.

In its April 29 letter to the Federal Aviation Administration (FAA), the National Transportation Safety Board (NTSB) noted that the pilots were flying in ice-prone conditions when, according to the airplane flight manual (AFM), they should have activated engine anti-ice (i.e., visible moisture and outside air temperature less than 6[degrees] C).

According to Eugene Hill, the FAA's national resource specialist on icing, high altitude ice crystals can adhere to engine inlet pressure probes, partially blocking the opening and causing a false high indication of engine power, when in fact engine power may be decreasing.

Such was the case in this Spirit stall. The airplane lost speed and power while at the same time the engine pressure ratio (EPR) was increasing. As a result of this incident, the NTSB urged the FAA to pursue research and "develop an ice detector that would alert pilots of inlet pressure probe icing and require that it be installed on new production turbojet airplanes, as well as retrofitting to existing airplanes." The full text of the NTSB missive may be viewed at http://www.ntsb.gov/Recs/letters/2004/A04_34_35.pdf

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