# NTSB ATL91IA017: Fokker F28 MK0100, N850US, Atlanta, GA, November 11, 1990

- Type: NTSB incident investigation
- Classification: Incident · No injuries · Incident investigation · Final report
- Date: November 11, 1990, 18:10 local (23:10 UTC)
- Place: Atlanta, GA
- Location: 33.7490° N, 84.3880° W: Placed by OpenAirDocs: center of Atlanta, GA, within about 7 miles (OpenAirDocs)
- Nearby events: https://openairdocs.org/map.md?lat=33.7490&lon=-84.3880&mi=10
- Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping)
- Highest injury: No injuries
- Injuries: 75 uninjured
- Status: Completed, report May 10, 1993
- Report: Approved by Office Director
- NTSB case key (mKey): 8341
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ATL91IA017
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/8341/pdf
- Page: https://openairdocs.org/ntsb/ATL91IA017

## Probable cause

FROZEN MOISTURE IN THE STABILIZER ACTUATOR HYDRAULIC SERVO VALVE WHICH RESULTED IN REDUCED MOVEMENT OF THE SERVO VALVE AND LIMITED ITS ABILITY TO RESPOND TO FLIGHT CONTROL INPUTS. THE LACK OF RESPONSE FROM THE SERVO VALVE RESULTED IN THE JAMMED STABILIZER MESSAGE. A FACTOR WAS THE INADEQUATE DESIGN OF THE SERVO VALVE WHICH PRECLUDED MOISTURE DRAINAGE.

## Sequence of events

In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf).

- Airframe/component/system failure/malfunction, descent - normal
  - flight control,stabilator - jammed (cause)
  - flt control syst,horiz stab drive - frozen (cause)
  - acft/equip,inadequate design, manufacturer (contributing factor)

## Analysis

DURING DESCENT THE AFCAS STABILIZER TRIM & AUTOPILOT DISENGAGED. BOTH WERE RESET WITH A 2ND DISENGAGEMENT. NOSE DOWN CONTROL PRESSURES WERE ENCOUNTERED & THE FLT CREW ATTEMPTED TO USE MANUAL TRIM, FOLLOWED BY A JAMMED STABILIZER MASTER WARNING. AFCAS STABILIZER TRIM WAS LEFT OFF IAW ABNORMAL PROCEDURES. FDR DATA SHOWED THE STAB THEN CREEPED TO A FULL ACFT NOSE DOWN POSITION & HYD PRES TO THE STAB ACTUATOR WAS AUTOMATICALLY REMOVED, PER DESIGN. SUBSEQUENT TESTING OF THE HORIZONTAL STABILIZER CONTROL UNIT ASSEMBLY SHOWED THAT, AFTER SPRAYING THE SERVO VALVE WITH WATER AND COLD SOAKING, ICE WOULD FORM INSIDE THE END CAP AND A JAMMED SERVO VALVE CONDITION WOULD OCCUR. THE END CAP AND SPRING EXHIBITED SUBSTANTIAL RUST CORROSION. CREW MOVEMENT OF THE MANUAL TRIM RESULTED IN THE JAMMED STAB WARNING.

## Aircraft

- Registration: [N850US](https://openairdocs.org/deregistered/N850US-2003-05-28), registration cancelled 2003-05-28, S/N 11276, built 1989 (OpenAirDocs)
- Reported make and model: FOKKER F28 MK0100
- Registry model: [Fokker F.28 MK 0100](https://openairdocs.org/model/fokker-f-28-mk-0100) (registry code 4990807) (OpenAirDocs)
- Category: Airplane
- Serial number: 11276
- Airframe hours: 2674
- Engines: Rolls-Royce TAY650715 15000 lbs thrust turbo jet
- Damage: No damage
- Flight: Part 121
- Operator: Usair
- Airworthiness certificate: Standard, Transport
- Last inspection: Continuous airworthiness, October 10, 1990, 147 hours before
- Departed: PHL, PHILADELPHIA, PA
- Destination: ATL, ATLANTA, GA
- Flight plan: IFR

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 41; airline transport; airplane ratings: multi-engine land; instrument ratings: airplane; medical: class 1, valid medical--no waivers/lim.; seat: left; flight hours: 15,127 total

## Weather

- Conditions: Visual (VMC), Dusk
- Wind: 300° at 4 knots
- Visibility: 20 statute miles
- Lowest ceiling: none
- Temperature: 61 °F, dew point 39 °F
- Altimeter: 30.00 inHg
- Observed at: ATL

## 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 19901111057209I](https://openairdocs.org/incident/19901111057209I), Nov 11, 1990: Fokker F28MK4000, Atlanta, GA. On descent computer fault light and stabilizer trim fault lights came on. Autopilot disengaged.

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.

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