BILLINGS, MT, USA
N336PH
Dornier 328
The crew lost elevator trim after leveling at flight level (FL) 310, and they had to hold continuous control pressure to maintain level flight. Total air temperature was -50 degrees C at altitude. The captain elected to continue flight to the destination. Normal elevator trim operation was regained as the aircraft descended through 16,000 feet on its approach (total air temperature -7 degrees C); the flight subsequently landed without further incident. Postflight inspections of the elevator trim actuators by FAA inspectors revealed paint overspray on the elevator trim actuator push rods, scoring and corrosion on the push rods, and white desiccant windows on the actuators. The airline's FAA principal maintenance inspector (PMI) reported that there had been 10 unscheduled removals of DO-328 elevator trim actuators in the last 2-1/2 years for moisture and/or freezing in flight; that maintenance check cards did not address checking desiccant windows for moisture contamination or color criteria for checking the desiccant windows; that teardown reports from the vendor, AVIAC Technologies of France, indicated the paint overspray on the push rods was damaging the actuator quad rings during operation; and that he believed that damage to the quad rings was allowing moisture to enter the actuator housings.
On May 29, 1996, approximately 1645 mountain daylight time, N336PH, a Dornier 328 on a scheduled 14 CFR 121 domestic passenger service flight from Billings, Montana to Spokane, Washington, operating as Horizon Air flight 2573, experienced an elevator trim system malfunction approximately 20 minutes after takeoff, upon leveling at its cruise altitude of flight level (FL) 310. The malfunction required the flight crew to hold continuous manual control pressure to maintain level flight. The captain elected to continue the flight to the planned destination. Normal elevator trim operation was subsequently regained as the aircraft descended through 16,000 feet approaching Spokane. The aircraft landed uneventfully at Spokane without damage or injuries to the airline transport pilot-in-command, first officer, two cabin attendants, and 19 passengers. The captain reported that visual meteorological conditions prevailed in the area of the occurrence. An instrument flight rules flight plan had been filed. In an irregularity report filed with Horizon Air, the captain stated: Leveling FL 310 (TAT -50 [degrees] C) discovered [elevator] trim inop[erative.] Accelerated to cruise flight experiencing incr[easing] back pressure on yoke. Pressure remained tolerable and flight continued to dest[ination.] Regained control of trim in descent through 16,000 ft. TAT -7 [degrees] C. Discussed slowing to trim speed used for our climb to reduce pressure if necessary....During descent, as speed approached 195 KIAS, CAS [message] 'EL DISC LOAD HI' appeared. We slowed to 190 and CAS [message] disappeared. After regaining elev[ator] trim, [flight] proceeded normally. An FAA inspector examined the aircraft at Spokane. His examination noted scoring and corrosion on the elevator trim actuator (part number 8409) push rods. Function checks on the units were normal. Horizon's FAA principal maintenance inspector (PMI) subsequently conducted a follow-up examination, which he reported to the manager of the Portland FAA Flight Standards District Office (FSDO) as follows: ...Reliability records show that January 1994 through May 1996, there have been 13 unscheduled removals of the elevator trim actuators, and ten (10) of those were for moisture and or [sic] freezes in flight.... The Dornier 328 maintenance check cards call out for function check of the elevator trim at C check intervals. However, the cards do not address checking the desiccant window for moisture contamination or the criteria for the color of that window. Our office investigation has revealed the following findings... A. Actuators show signs of scoring on Push Rods[.] B. Desiccant windows are white in color. C. White paint overspray is on push rods. D. Rust residue appears on Push Rod scoring areas. It appears...that the paint overspray may be causing the scoring of the Push Rods...destroying the actuators['] quad rings, allowing moisture to enter the housing. Review of [teardown reports from] AVIAC Technologies [the French vendor for the actuators] confirms that the paint overspray is indeed causing this problem. These findings indicate that there is a trend of freezing in flight of the actuators.... The PMI made three safety recommendations to the Portland FSDO manager in his report as follows: "A. Airworthiness directive be initiated immediately to inspect for scoring of actuator push rods at full travel, and desiccant window check for moisture. Remove and replace actuator if moisture contamination is evident. Note: This should include the Aileron Actuator also." "B. The desiccant window should be added to the Dornier 328 maintenance program C check cards and the color criteria of the window." "C. The Dornier 328 manual should reflect that the elevator and aileron trim actuators should have a warning to keep actuators [sic] Push Rods free from paint overspray." In a telephone conversation with the NTSB investigator-in-charge on October 7, 1996, the FAA PMI stated that actuators he had examined which had come directly from AVIAC contained no paint overspray, and that the overspray was being deposited on the actuators during aircraft painting at the factory. (NOTE: Horizon's aircraft are painted white.) Aircraft maintenance records supplied by Horizon indicated that the incident was the third elevator trim occurrence on the aircraft within a 90-day period. A discrepancy report dated March 20 indicated: "TRIM SPEED CAS MESSAGE ON BELOW 200 KTS WHEN CAPT PITCH TRIM ACTUATED NOSE DOWN ABOVE 200 KTS AMBER CAS MESSAGE TRIM SPEED FST ELV TRM PPS CB". The corrective action for this discrepancy was listed as: "OPERATES NORMAL ON F/O SIDE AND STBY POSITIONS SLAVED IN NEW ACTUATOR ON LH SIDE N/H SLVED IN RELAY 42CC N/H ORG PARTS STILL INSL. OK FOR FE". A related entry on that date stated: "...'TRIM SPEED FAST' CAS MESSAGE ON BELOW 200KTS". The corrective action was: "REPLACED TRIM MASTER RELAY 45CC. ELEVATOR TRIM CKS GOOD." A discrepancy entry dated April 19 stated: "'EL MISTRIM NOSE UP' CAS MESSAGE. 'EL TRIM FAIL' CAS MESSAGE. NORMAL ELEVATOR TRIM INOP EITHER YOKE. STANDBY TRIM OPERATED NORMALLY." The corrective action for this discrepancy was given as: "VISUALLY INSPECTED, NO DEFECTS NOTED. OPS CHECKED GOOD IAW DAC MM 27-32-00-520. OK FOR SERVICE." The aircraft maintenance records indicated that an "ops check of elevator trim norm/stby (over the full range)" (task number 27-30-00-08, work card 727M203) was accomplished during the aircraft's last inspection, a continuous airworthiness (A5) inspection performed on April 25, 1996.
paint overspray on the elevator trim actuator push rods, which damaged the actuator quad rings during normal operation and allowed water to enter the actuators; and the resultant freezing of the actuators. Factors relating to the incident included the aircraft manufacturer's failure to adequately protect the actuators from overspray during aircraft painting, insufficiently defined maintenance procedures by the aircraft manufacturer, and low temperatures at cruising altitude.
Source: NTSB Aviation Accident Database
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