- UAL Flight 173 near Portland
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- UAL Flight 173 near Portland
Photo of DC-8 with extended landing gear showing retract piston
Photo copyright Chris Coduto - used with permission
(View Large Image)
History of Flight
On December 28, 1978, a McDonnell Douglas DC-8-61 turbofan powered airplane, operated by United Airlines and registered as N8082U, crashed into a wooded suburban area while on approach to Portland International Airport (PDX), Portland, Oregon.
The aircraft ran out of fuel while on approach to PDX after the flight crew had spent considerable time troubleshooting an apparent landing gear malfunction, as well as preparing the aircraft and passengers for an emergency landing.
When the landing gear was initially extended on approach into Portland, the right main landing gear retract cylinder assembly pulled apart due to corrosion on the mating threads between the rod end and the piston rod. This allowed the right gear to free fall while the left gear extended normally.
The resulting abnormal noise, vibration, and transient asymmetric drag immediately indicated a failure to the flight crew. Knowing something abnormal had happened and being unable to verify that the landing gear was down and locked, at 17:12:20, when requested by Portland Approach to contact Portland tower, the captain stated: "Negative, we'll stay with you. We'll stay at five. We'll maintain about a hundred and seventy knots. We got a gear problem. We'll let you know."
DC-8 Instrument Panel showing landing gear indicator lights
Photo copyright Capt. David O. Hill – used with permission
The DC-8 landing gear is extended and retracted by moving a handle in the cockpit. For gear extension, the handle is moved down, and the landing gear uplocks are released, allowing gear extension. When the gear is properly extended and locked down, three green lights near the landing gear handle indicate that the landing gear is safe for landing. If the gear extends improperly, or does not lock down for some reason, a red light for the specific gear will illuminate, indicating that the particular landing gear is not extended and/or locked. For retraction, when the cockpit handle is moved up, the gear retraction system is hydraulically pressurized and the retract piston retracts, pulling the associated landing gear assembly into its wheel well. When the retraction is complete, hydraulic pressurization is turned off. On the accident airplane, when the gear was extended, the retract piston became disconnected (or had been disconnected prior to extension), which resulted in unusual noises and an indication that the gear had not properly extended and locked.
The flight crew elected to delay their landing while they investigated the landing gear problems. At this time there was approximately 13,332 pounds of fuel remaining on board.
Throughout the subsequent delay, Flight 173 remained at 5,000 ft. with the landing gear down and flaps set at 15 degrees.
Under these conditions the NTSB estimated that the airplane would have been burning fuel at the rate of about 13,209 pounds per hour (220 lbs./min.).
The DC-8 flight deck has both engine fuel flow and fuel tank quantity indications available to the flight crew. These instruments would have provided enough information for the crew of Flight 173 to determine that they had approximately one hour of flying time left under those conditions.
For approximately the next 54 minutes, the flight crew's attention was focused primarily on the landing gear problem, as well as in the preparation of the passengers and cabin for the possibility of a landing gear collapse during landing.
While the remaining fuel quantity was occasionally noted by the flight engineer during the subsequent 54 minutes, at no time did any of the crew relate the fuel remaining and fuel use rate to the flying time required to reach the airport. In addition, the crew did not discuss the potential of having to perform a missed approach or a go-around and return to land.
The only indication given by a crewmember on the fuel state was at 17:50:34, some 38 minutes into the hold, when in response to the captain's request: "Give us a current card on weight, figure about another fifteen minutes," the flight engineer told the captain: "Fifteen minutes is gonna --- really run us low on fuel here."
Photo of wreckage
Photo copyright Jeff Schroeder - used with permission
Over the period between 18:06:40 and 18:13:21 all four engines failed. Number four engine was the first to fail. Review of the discussion in the flight deck made it clear to the investigators that at least the captain and flight engineer were surprised by the fuel exhaustion. The captain repeatedly asked "Why?" when informed that they were losing an engine. The flight engineer stated, "Five thousand in there ... but we lost it." Number 3 was the next to fail, reportedly followed by the near simultaneous failures of both numbers 1 and 2.
At approximately 18:15:00, the aircraft crashed into a wooded section of a populated area of suburban Portland about six nautical miles east southeast of the airport. After striking several trees, an unoccupied house, and high tension power lines, the fuselage of the airplane came to rest between some trees and on top of another unoccupied house. There was no post crash fire. The wreckage path was about 1,554 ft. long and about 130 ft. wide.
The fuselage, from about the fifth row of passenger seats forward, sustained severe impact damage. The cockpit upper structure, which included the cockpit forward windows, had separated and was found to the right of the fuselage just forward of the inboard end of the right wing. The cockpit floor structure, which included portions of the crew seats, sections of the instrument panel, and the nose tunnel structure with the nose gear assembly partially attached, had separated and rotated to the right and aft. This structure was in a partially inverted position.
Photo of wreckage
Photo copyright Jeff Schroeder - used with permission
Of the 181 passengers and eight crew members aboard, eight passengers, the flight engineer, and a flight attendant were killed. There were 21 passengers and two crew members who were seriously injured. Although there were many occupied houses and apartment complexes in the immediate vicinity of the accident, there were no casualties outside the airplane.
All of those fatally injured during the crash were seated on the right side of the airplane between the flight engineer's station in the cockpit and row 5 in the passenger cabin. That section of the aircraft was destroyed during the accident sequence. The most seriously injured passengers were seated in the right forward portion of the cabin near an area of the fuselage which appeared to have been penetrated by a large tree. These persons were seated near those passengers who were injured fatally. Some seriously injured passengers were seated in the rear cabin near the trailing edge of the wings.
The fuselage in this area had been penetrated and the floor and seats had been disrupted.
DC-8 Flight Engineer's panel
Photo Copyright Robert Hockemeijer - used with permission
Example of fuel management panel on DC-8 flight engineer's panel
Photo of DC-8 digital fuel gauges
DC-8 Fuel System and Unusable Fuel
The DC-8-61 can be configured with a total of eight, nine, or ten fuel tanks. The total number of fuel tanks each aircraft has depends upon whether center auxiliary tank and/or wing leading edge auxiliary tanks are installed. The basic aircraft consists of eight tanks and each set of tanks (1-4) consists of a main and an alternate fuel tank. The normal operation of the aircraft would be that each set of main/alternate tanks feeds a specific engine. View a United Airlines training document containing a description of the fuel system.
Fuel quantity remaining in each tank is provided to the flight crew by a set of gauges installed in the flight engineers panel.
Representative DC-8 Fuel system schematic (left side of airplane - right side identical)
A new type of fuel quantity indicating system was retrofitted to this aircraft on May 12, 1978. With the new system installed, there are eight individual tank quantity gauges. Each of these gauges has three digits. On these individual tank gauges, the digital reading is multiplied by 100 to obtain the total amount of fuel in the tank.
The totalizer gauge receives input from each individual tank gauge and displays the total fuel available on three digital readouts. However, this digital reading must be multiplied by 1,000 to obtain the value of the total amount of fuel on board. The smallest increment of change that can be indicated on the individual tank gages is 100 pounds. The smallest increment of change on the totalizer is 1,000 pounds.
The FAA certification standards required that aircraft fuel quantity gauges in the aircraft to be calibrated so that each gauge reads zero when all the useable fuel has been used from that tank. The amount of unusable fuel is established by flight tests to determine how much fuel is left in the tanks when the ports for the fuel pumps first become uncovered. The amount of fuel left in the tanks at this point is referred to as the unusable fuel and the fuel quantity gauges in the cockpit are calibrated to read zero, in level flight, when only the unusable fuel is left in a given fuel tank.
Flight Crew Coordination
The hierarchical "chain of command" system in place at the time of the accident did not always provide for effective flight crew resource management. This was evident to the investigation, as the second officer failed to adequately express the appropriate level of concern about fuel state of the aircraft. It was also clear in the accident report that the captain was not aware of the fuel remaining on board as he repeatedly asked "Why?" when informed that the first of the engines had flamed out.
There was anecdotal information following the accident that some of the surviving passengers were able to find ground transportation (taxis) from the crash site to the airport, and were later found waiting in the baggage claim area of PDX. As a result, their absence at the crash site reportedly made it difficult for emergency first responders to determine how many passengers had been on the airplane and how many had survived the accident.
As a result of this accident, the NTSB issued a recommendation to the FAA to "Include in the anticipated new rule a requirement for domestic and flag air carriers to maintain passenger lists with the provision that both ticketed and non-ticketed passengers' names be provided. This recommendation was subsequently adopted by the FAA.