# NTSB CHI95IA045: Fokker F-28-4000, N479AU, Evansville, IN, November 23, 1994 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: November 23, 1994, 12:34 local (18:34 UTC) - Place: Evansville, IN - Location: 37.9748° N, 87.5558° W: Placed by OpenAirDocs: center of Evansville, IN, within about 6 miles (OpenAirDocs) - Nearby events: https://openairdocs.org/map.md?lat=37.9748&lon=-87.5558&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 53 uninjured - Status: Completed, report May 16, 1995 - Report: Approved by Office Director - NTSB case key (mKey): 9755 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CHI95IA045 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/9755/pdf - Page: https://openairdocs.org/ntsb/CHI95IA045 ## Probable cause the failure of company maintenance personnel to adequately service (fill) the horizontal stabilizer drive actuator with hydraulic fluid. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, descent - normal - fluid,hydraulic - lack of (cause) - flt control syst,horiz stab drive - inoperative - maintenance,service of aircraft/equipment - improper, company maintenance personnel (cause) ## Analysis THE FLIGHT CREW REPORTED THAT WHILE DESCENDING FROM CRUISE IN PREPARATION FOR LANDING, THEY NOTICED THAT THE HORIZONTAL STABILIZER WAS NOT RESPONDING TO TRIM INPUTS. THE AUTOPILOT WAS DISENGAGED, AND BOTH NORMAL AND ALTERNATE MODES OF STABILIZER TRIM WAS ATTEMPTED WITHOUT SUCCESS. THE CREW FOLLOWED THE JAMMED STABILIZER APPROACH CHECKLIST, AND WAS ABLE TO EFFECT PITCH CHANGES BY BOTH CREW MEMBERS EXERTING FORCE ON THEIR CONTROL YOKES. POSTINCIDENT INSPECTION OF THE STABILIZER DRIVE ACTUATOR DISCOVERED NO HYDRAULIC LEAKS. THE ACTUATOR'S GEAR BOX WAS DRAINED, AND FOUND TO CONTAIN APPROXIMATELY ONE OUNCE OF MURKY, SLUDGE-LIKE HYDRAULIC FLUID. THE GEAR BOX IS REQUIRED TO BE FILLED WITH SEVEN OUNCES OF HYDRAULIC FLUID. THE ACTUATORS ARE SHIPPED FROM THE MANUFACTURER ESSENTIALLY DEVOID OF HYDRAULIC FLUID, WITH INSTRUCTIONS THAT THEY MUST BE FILLED PRIOR TO SERVICE. ## Factual narrative HISTORY OF FLIGHT On November 23, 1994, about 1234 central standard time, a Fokker F-28-4000 airplane, N479AU, operated by USAIR as Flight 689, experienced a sticking horizontal stabilizer while on approach to the Evansville Airport, Evansville, Indiana. The flight made an uneventful landing at Evansville. None of the 49 passengers or four crew members aboard were injured. The 14 CFR Part 121 flight originated in Pittsburgh, Pennsylvania, and operated in visual meteorological conditions. The captain reported the first officer was flying the airplane. During the descent from cruise into Evansville, with the autopilot engaged, the first officer told the captain that the horizontal stabilizer was not responding to trim inputs. The autopilot was disengaged, and both master caution yellow lights on the hydraulic system flight control panel illuminated. Neither manual or alternate (electric) trim modes had any effect on the horizontal stabilizer. Abnormal checklists were reviewed and followed. Both the captain and first officer found it necessary to exert simultaneous forces on their control yokes in order to accomplish elevator movement and pitch changes for landing. An after landing functional test of the horizontal trim mechanism discovered no obvious anomalies. Both the hydraulic and electric back-up trim systems were able to move the horizontal stabilizer in the absence of flight loads. At the request of the NTSB, an FAA Inspector assisted USAIR maintenance personnel at Evansville during the trouble shooting of the sticking stabilizer. During the inspection of the hydraulic horizontal stabilizer trim actuator, the gear box was drained of fluid. According to the FAA Inspector, the fluid drained was dark brown in color, and had the consistency of "murky sludge." Approximately one ounce of fluid was recovered from the system. The gear box capacity is approximately seven ounces. No hydraulic leaks were observed emanating from the actuator. Several flushes of the gear box was necessary before a clean fluid sample was obtained. The actuator was serviced with the appropriate quantity of fresh hydraulic fluid, and the airplane was ferried to Pittsburgh. The airplane's hydraulic stabilizer trim actuator was removed from the airplane and shipped to the manufacturer, Menasco Aerospace, in Oakville, Canada, for inspection and disassembly. AIRCRAFT INFORMATION N479AU is a Fokker F-28-4000 airplane with approximately 18679 service hours on the airframe. The hydraulic horizontal stabilizer actuator, part number 11100-39AA, serial number MC375, was manufactured (and subsequently overhauled) by Menasco Aerospace. It was a replacement unit installed in the airplane on August 4, 1994. It had accumulated approximately 734 service hours since overhaul at the time of the incident. TESTS AND RESEARCH On December 15, 1994, a functional test and disassembly of the actuator was performed at the Menasco facility at Oakville, Canada, under the direction of the NTSB and the Transportation Safety Board of Canada (TSB). The actuator was placed on a test stand and pressurized to operating pressure. The actuator functioned within allowable parameters in both the primary and alternate modes. Following the functional test, the gear box was drained of hydraulic fluid. The fluid, which was the new hydraulic fluid placed in the actuator at Evansville, was now a deep, reddish-brown in color. Disassembly of the gear box disclosed that the worm screw, bevel gear, clutch plates and bearings were visually corroded. ADDITIONAL INFORMATION The hydraulic horizontal stabilizer actuator is manufactured (or overhauled) and shipped without hydraulic fluid from the manufacturer. Servicing and fill instructions are shipped with the units. The incident operator also has a "job card" which details fill instructions and quantity of the gear box. ## Aircraft - Registration: [N479AU](https://openairdocs.org/deregistered/N479AU-2004-02-27), registration cancelled 2004-02-27, S/N 11228, built 1985 (OpenAirDocs) - Reported make and model: FOKKER F-28-4000 - Registry model: [Fokker F.28 MK4000](https://openairdocs.org/model/fokker-f-28-mk4000) (registry code 4990810) (OpenAirDocs) - Category: Airplane - Serial number: 11228 - Airframe hours: 18679 - Engines: Rolls-Royce turbo prop - Damage: No damage - Flight: Part 121 - Registered owner: Usair - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, August 4, 1994, 734 hours before - Departed: PIT, PITTSBURGH, PA - Flight plan: IFR - Airport: EVV, Evansville ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 36; airline transport, flight engineer; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 1, valid medical--no waivers/lim.; seat: left; flight hours: 7,434 total, 256 in this make and model ## Weather - Conditions: Visual (VMC), Day - Wind: 310° at 13 knots - Visibility: 15 statute miles - Lowest ceiling: none - Temperature: 48 °F, dew point 21 °F - Altimeter: 30.00 inHg - Observed at: EVV ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 19941123048029I](https://openairdocs.org/incident/19941123048029I), Nov 23, 1994: Fokker F28MK4000, Evansville, IN. Lost horizontal stabilizer servo landing. Actuator gear box low on fluid and contaminated. Used manual control. ## Other investigations on N479AU (1) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB NYC95IA065](https://openairdocs.org/ntsb/NYC95IA065), Mar 4, 1995: Fokker F28-4000 (N479AU), Huntington, WV, incident, minor damage. Probable cause: THE FATIGUE FRACTURE OF THE LEFT MAIN LANDING GEAR UPPER TORQUE LINK. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.