Four cabin attendants were also on board to look after the 181 passengers. The aircraft had been loaded with enough fuel for just over an hour's reserve. The weather was good and darkness had just fallen as 173 approached Portland. Shortly after 5:00pm, with Beebe at the controls, McBroom called Portland Approach to report descending out of 10,000ft. Approach told 173 to maintain it's heading for a visual approach to runway 28. McBroom conformed the instruction, reporting the airport in sight. As 173 continued it's approach, Beebe called for the landing gear to be extended. When McBroom moved the gear handle, there was an immediate thump and a shudder which shook the whole aircraft. Only the nose gear light went green and the main gear showed neither "down and locked" nor "in transit".

At this point, Approach told 173 to contact the tower, unaware of 173's problem. McBroom replied, saying that 172 would like to stay with Approach at 5,000ft and that they had a gear problem. Approach told 173 to turn left to 200 followed by another left turn to 100 saying "...I'll just orbit you out there until you get your problem right." For the next 23 minutes, 173 circled the outskirts of Portland trying to remedy it's problem. Mendenhall went back to the passenger compartment, checking the main gear position indicators which appeared to show that the main gear had in fact extended. McBroom brought the senior flight attendant forward, explaining the problem and telling her to be prepared if they were unable to fix the problem.

173 then called it's San Fransisco maintenance facility to get assistance with the problem, reporting that they would hold another 15 to 20 minutes before getting the passengers ready and making it's landing at about 6:05pm. United's facility confirmed that the crew had done everything possible to remedy the problem. Now having held for more than thirty minutes, McBroom had another discussion with the senior flight attendant concerning the imminent landing and how it would be announced to the passengers. Beebe, still flying the aircraft, asked Mendenhall what the remaining fuel was, to which he replied 5,000 pounds. At this point, Mendenhall pointed out the fuel pump lights, which had started to blink, indicating that the remaining usable fuel was, in fact, 5000 pounds. McBroom then asked Mendenhall to recalculate the weight for landing, saying he expected to land in approximately 15 minutes.

Mendenhall was heard to say to himself "Not enough! 15 minutes is going to really run us low on fuel here." McBroom again spoke to Mendenhall, asking him to call the United facility at Portland and inform them of the number of people on board and the fuel remaining at landing, which he guessed to be 4,000 pounds. After this was done, they completed the final checks in preparation for landing. McBroom then suggested that Mendenhall go back into the cabin and check on the passengers, which resulted in him being gone from the flight deck for the next four minutes. Mendenhall returned and the McBroom told him that when 173 was about four miles out, he should call over the PA for the passengers to assume the brace position. Mendenhall then told McBroom that they were down to 3,000 pounds of fuel remaining.

At this point, 173 had just turned south away from the airport on the last of it's orbits. Approach called and asked 173 for a status report to which Beebe replied that the gear indication was abnormal and that 173 would land on runway 28 in approximately five minutes. In addition, he requested fire and rescue crews standing by. Approach asked when 173 would like to begin it's approach and McBroom replied that they would begin in another "...three, four, five minutes."

McBroom then reported that they had 3,000 pounds of fuel remaining and 178 people on board. McBroom then began a discussion with the senior flight attendant during which Beebe began a conversation with Mendenhall, saying "I think you've just lost number four, better get some crossfeeds open there or something." As the flight attendant was leaving, Beebe said "We're going to lose an engine!" to which McBroom replied "Why?", Beebe saying "Fuel!" At this point, the crew scrambled to get control of the situation. Mendenhall succeeded in getting the number four engine back by opening the crossfeeds, but he reported that now they were showing 1,000 pounds on the totaliser with zero in the number two tank.

173 then told Approach that they were returning to the airport and it was cleared to land on runway 28. When 173 was 12 miles out, Mendenhall called out "We've lost engines guys! We just lost two engines, number one and two." McBroom then said "They're all going, we can't make Troutdale [Portland's general aviation airport]!" Beebe then replied "We can't make anything!" McBroom told him to report a Mayday and Beebe then said "Portland Tower, United 173 Mayday! We're...the engines are flaming out...we're going down! We're not going to be able to make the airport!" At 6:15pm, the aircraft, now with all four engines flamed out, crashed six miles short of the runway in a wooded suburb area. It just missed an apartment building and crashed into an unoccupied house. 156 of the passengers were able to escape with only minor injuries. Ten occupants were killed including Mendenhal and the senior flight attendant.

......Wreckage of the examination revealed that the main landing gear had been fully extended at the time of the crash. The starboard retraction mechanism showed corrosion of an eyebolt thread which had pulled away from the actuator cylinder piston rod. Investigation showed that 173 had consumed fuel at a normal rate, arriving at Portland with nearly 14,000 pounds remaining. When the landing gear was selected down, the corrosion from the thread allowed the gear to fall free, immediately swinging into down position and damaging the micro switches, which caused the lack of gear down indication on the flightdeck.

 United's policy for landing gear problems calls for verifying the position visually at which point a landing can be made. Mendenhal verified that the visual indicators showed the gear to be extended, though he said he couldn't be sure. By the time 173 contacted maintenance, nearly half an hour had elapsed. Once maintenance had confirmed that the crew had adequately handled the problem, investigators felt that McBroom should have began the approach. Instead, he spoke with the senior flight attendant about the coming landing, but did not tell her to complete preparing the passengers by a certain time.

The crew obviously was not fully aware of their fuel state or they would have calculated that they would run out of fuel not long after their proposed landing time of 6:05pm. Another indication of McBroom's lack of awareness was his statement just 16 minutes before 6:05pm that they would be landing with 4,000 pounds remaining, while he had just two minutes prior acknowledged that there was only 5,000 pounds remaining. In reality, the aircraft would have used at least 3,000 pounds. Further, McBroom was obviously unconcerned about their status as evidenced by his ordering Mendenhal to walk the cabin to check on the passengers.

 When Mendenhal returned, the aircraft was headed towards the airport and all of the checks had been done. At this point, there was no reason not to continue towards the airport and land. Instead, McBroom accepted a vector outbound for another circuit. At this point, the accident was unavoidable. Clearly the accident resulted in a breakdown of crew co-ordination. McBroom remained unaware of the aircraft's fuel state throughout the flight. Even when the first engine flamed out, he said "Why?", clearly not understanding how critical a situation they were in. His continued efforts to rectify the gear problem and prepare the flight attendants apparently distracted him from making sure the airport was within range at all times.