One MD-80 Operator has reported several instances in which aircraft equipped with the TDG Overwing Heater Installation (heater blankets) Service Bulletin experienced Engine Foreign Object Damage (FOD) believed to have originated from ice ingested during takeoff. Investigation revealed that a possible source of damage was from ice which may have formed on the flap trailing edge and  the spoiler cavities and was not removed during a previous airframe de-icing.

The circumstances surrounding this type of ice formation are believed to be as follows:

a) The heater blankets are left ON, in an operating condition, for several hours when the aircraft is on the ground and

b) A snow and/or ice storm occurs during this period, covering the aircraft with frozen precipitation. Ambient temperature drops or remains below freezing. Precipitation over the wing heated area melts and the runoff flows aft into the flap trailing edge/spoiler cavity areas. This runoff refreezes in these areas to a depth that may be considerably greater than that normally formed on the surrounding areas, and more resistant to removal. De-icing crews clear the remainder of the airframe, but fail to detect the ice remaining in the flap/spoiler areas. If the aircraft is dispatched in that condition, the ice may be ingested during the next takeoff.

Ground and Flight Crews should be made aware of this potential hazard. If the heater blankets have been left ON during periods of freezing precipitation, then a hands-on inspection is required in the flap and spoiler areas for ice. In addition, flight crews should be reminded that wing heater blankets are designed and certified as an anti-icing system only, and the crews should be cautioned not to activate the system if the airframe is covered with frozen precipitation. A regular ground de-icing of the entire airframe, including flap and spoiler trailing edge cavities should be accomplished. The heaters should remain OFF until after completion of that de-icing.

Douglas believes that all Overwing Heater Systems may be susceptible to these phenomenon, and that the precautions contained herein may apply to all such systems.