# NTSB SEA99IA045: Fokker F.28 MK 4000, N477AU, Spokane, WA, April 1, 1999 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: March 31, 1999, 19:43 local (Apr 1, 1999, 03:43 UTC) - Place: Spokane, WA - Coordinates: 47.650497, -117.309181 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=47.6505&lon=-117.3092&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 44 uninjured - Status: Completed, report June 21, 2000 - Report: Approved by Office Director - NTSB case key (mKey): 45998 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA99IA045 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/45998/pdf - Page: https://openairdocs.org/ntsb/SEA99IA045 ## Probable cause A chafed hydraulic line. ## Analysis The flight crew reported that, while in cruise flight, the number 1 hydraulic (utility) fluid quantity dropped below the low red mark on the gauge. The flight crew switched to the secondary hydraulic system and diverted to a nearby airport, where a landing was made without incident. Maintenance personnel inspected the aircraft and found a hydraulic line to the flap drive motor was chafed through and leaking. The flight crew reported that this was the second time that day that the number 1 hydraulic system fluid level had dropped. Prior to the last departure, maintenance personnel had serviced the system with hydraulic fluid. On that occasion, the flight crew ran the engines and cycled the flight controls while maintenance personnel looked for leaks. No leaks were found and the aircraft departed on the next leg of the scheduled flight. ## Factual narrative On March 31, 1999, about 1943 Pacific standard time, a Fokker F.28 MK 4000, N477AU, registered to Atlantic Credit Corporation, and operated by Horizon Air as a 14 CFR Part 121 scheduled passenger flight was en route to Seattle, Washington, when the primary hydraulic system lost pressure. The flight diverted to Spokane, Washington, where a landing was made without further incident. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft was not damaged. The two airline transport pilots, two cabin crew and 40 passengers were not injured. The flight had departed from Billings, Montana, as Horizon Flight 2419, about one hour and 15 minutes prior to the incident. The flight crew reported that, while in cruise flight, the number 1 hydraulic (utility) fluid quantity dropped below the low red mark on the gauge. The flight crew notified maintenance control, and a decision was made to divert to Spokane. The secondary hydraulic system was utilized for the remainder of the flight to Spokane. The flight crew reported that this was the second time that day that the number 1 hydraulic system fluid level had dropped. While in Billings, after the previous occurrence, maintenance personnel serviced the system with hydraulic fluid. The flight crew then ran the engines and cycled the flight controls while maintenance personnel looked for leaks. No leaks were found and flight 2419 departed for Seattle. The aircraft was inspected by Horizon Air maintenance personnel in Spokane, who reported that a line for the flap drive motor was found chafed through and leaking in the left side wheel well. Maintenance personnel removed the chafed line and installed a flex line in accordance with the F28 maintenance manual instructions. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, cruise - hydraulic system,line - chafed (cause) ## Aircraft - Registration: [N477AU](https://openairdocs.org/deregistered/N477AU-2006-02-03), registration cancelled 2006-02-03, S/N 11226 (OpenAirDocs) - Reported make and model: Fokker F.28 MK 4000 (F.28 MK 40) - Registry model: [Fokker F.28 MK4000](https://openairdocs.org/model/fokker-f-28-mk4000) (registry code 4990810) (OpenAirDocs) - Category: Airplane - Serial number: 11226 - Engines: unknown - Damage: No damage - Flight: Part 121 - Operator: Horizon Air - Registered owner: Atlantic Credit Corp. - Airworthiness certificate: Standard, Transport - Departed: BIL, BILLINGS, MT - Destination: SEA, SEATTLE, WA - Flight plan: IFR ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 42; commercial, airline transport, flight instructor; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, valid medical--w/ waivers/lim; seat: left; flight hours: 11,000 total, 1,100 in this make and model ## Weather - Conditions: Visual (VMC), Night/dark - Wind: 340° at 7 knots - Visibility: 10 statute miles - Lowest ceiling: 18,000 ft broken - Temperature: 37 °F, dew point 21 °F - Altimeter: 29.00 inHg - Observed at: GEG, 40 nm away ## 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 19990331020959I](https://openairdocs.org/incident/19990331020959I), Mar 31, 1999: Fokker F28MK0100, Spokane, WA. (-5) Flap motor hydraulic frayed. Ridgid line chaffed. Line replaced with temporary hose. Checked okay. Deferred temporary repair. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date; 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.