Airworthiness Directive 2002-08-20
The FAA has received reports of numerous incidents of
severe airframe vibration, or limit cycle oscillation (LCO), in flight after the horizontal stabilizer had
been deiced/anti-iced on the ground. The reported incidents occurred on Boeing Model 737-600, -
700, and -800 series airplanes. The empennage structure on these, as well as Model 737-700C series
airplanes, is identical; therefore, all of these airplanes are subject to the identified unsafe condition.
These events have been attributed to an accumulation of deicing/anti-icing fluid or other residue in
the elevator balance panel cavities and on the external surfaces of the elevator tab. The accumulation
of fluid in the balance bays has been attributed to inadequate drainage provisions. Drainage
provisions on Model 737-900 series airplanes are improved over those on the airplanes affected by
this AD.
Preliminary results of the investigation of the incidents indicated that only Type I and Type II
deicing/anti-icing fluids were susceptible to this type of accumulation; however, a recent LCO event
occurred following deicing/anti-icing with Type I and Type IV fluid on one affected airplane. One
operator reported finding up to 30 liters of fluid trapped in the balance bays on one airplane. Other
operators have reported visible accumulations of foreign substances on the external surfaces of the
elevator tab. The additional weight of accumulated residue on the tab can initiate LCO. The elevator
tab is so aerodynamically sensitive that repairing and painting the subject area have been prohibited
by related existing ADs. The reported airspeeds at the onset of the incidents have ranged from 276 to
325 knots.
Fluid or residue accumulated in the balance bays, or foreign substances accumulated on the
external surfaces of the elevator tab, in combination with normally recommended maximum
operating airspeeds, can initiate LCO or flutter and result in loss of controllability of the airplane.
"After any ground deicing/anti-icing of the horizontal stabilizer, airspeed must be limited to 270 KIAS until the crew has been informed that applicable maintenance procedures have been accomplished that would allow exceedance of 270 KIAS. Once the applicable maintenance procedures have been accomplished, exceeding 270 KIAS is permissible only until the next deicing/anti-icing."