Boeing 747-121 accident at US, 30 Jul 1971 (N747PA)

Accident narrative

On July 30, 1971, at 1529 Pacific daylight time, a Pan American World Airways Boeing 747-121, N747PA, operating as Flight 845, struck the Approach Light System structure at the departure end of Runway 01R while taking off from the San Francisco International Airport. The flightcrew continued the takeoff, dumped fuel, and returned for a landing. Two passengers were seriously injured during the takeoff impact, and 27 passengers were injured during the evacuation after landing.

### The flight

Flight 845 was a scheduled international passenger and cargo operation from Los Angeles, California, to Tokyo, Japan, with an intermediate stop at San Francisco. The aircraft departed the gate at 1501 with 199 passengers and 19 crewmembers aboard. The crew was properly certificated and qualified for the flight.

### Sequence of events

About 1230, the Pan American flight controller planned a takeoff on Runway 28L based on aircraft weight and forecast weather. The flight controller did not check the airport conditions prior to the release of the flight and was unaware that Runway 28L had been closed at 0830. In the dispatch office, the crew extracted takeoff reference speeds for a takeoff at 708,000 pounds with a 10-degree flap setting: a V1 speed of 156 knots, a VR (rotation) speed of 164 knots, and a V2 speed of 171 knots.

While completing the prestart checklist, the first officer monitored the Automatic Terminal Information Service (ATIS) broadcast and learned that Runway 28L was closed, and that the first 1,000 feet of Runway 01R was also closed. At 1511, the flight was cleared to taxi to Runway 01R. The first officer requested Runway 28R. The captain directed him to request 28L, and was then informed it was closed.

At 1512:44, the crew contacted Pan American Operations/Dispatch to check limitations for Runway 28R. The flight controller informed them that Runway 01R with a clearway could be used with a zero wind component. When the crew asked about the closed 1,000 feet on 01R, the flight controller called the tower. The tower controller voluntarily informed him that the closure would not affect the runway distance available for a Boeing 747 takeoff, referring to a standing jet blast restriction. The flight controller relayed to the crew that they had 9,500 feet plus a clearway ahead of them from the painted threshold.

To take off from Runway 01R at their planned gross weight, the crew reset the flaps to 20 degrees. The takeoff reference speeds for this configuration were V1 149 knots, VR 157 knots, and V2 162 knots. The crew did not recheck the computations, and the reference speed bugs were left at the settings for a 10-degree flap configuration.

The takeoff clearance was issued at 1528, with the wind reported from 270 degrees at 22 knots. The first officer called V1 at the bug setting of 156 knots. He then called VR at 160 or 161 knots because the end of the runway was coming up rapidly, not because the aircraft had reached the calculated 164-knot rotation speed. As the aircraft rotated at 1529, the first officer saw the airspeed passing 165 knots and felt a bump or jolt.

The aircraft's main body gear struck the Approach Light System (ALS) structure. Three pieces of angle iron penetrated the passenger cabin, seriously injuring two passengers.

The flightcrew continued the takeoff. The flight engineer observed the loss of hydraulic fluid from systems 1, 3, and 4, and executed emergency shutdown procedures. The captain checked flight control effectiveness and climbed to between 2,500 and 3,000 feet. A Coast Guard aircraft visually checked the damage, reporting that the right body gear was missing and the left body gear was hanging down.

The crew dumped approximately 180,000 pounds of fuel over the ocean to reach a landing weight of about 430,000 pounds. The flight returned to San Francisco for a visual approach to Runway 28L, which had been reopened for the landing. Flaps were extended by the alternate electrical system to 30 degrees inboard and 28 degrees outboard. A threshold speed of 123 knots was selected. As the aircraft slowed to about 133 knots at an altitude of 200 feet, the captain noted a loss of elevator control effectiveness.

The aircraft touched down hard, bounced, and veered off the right side of the runway, stopping in the unpaved area approximately 5,300 feet from the approach end.

The first officer attempted to order an evacuation but transmitted over the radio instead of the passenger address system. The announcement was not completed because the battery power was turned off, shutting down all systems except those with self-contained power. About 30 seconds after the aircraft stopped, the second officer and second flight engineer entered the cabin and shouted to start the evacuation. Four of the 10 passenger slides failed to function properly. During the evacuation, the aircraft tilted slowly back onto the rear section of the fuselage. Twenty-seven passengers were injured during the evacuation, eight of them suffering serious back injuries.

### What the investigation found

The Board found that the aircraft sustained major structural damage aft of fuselage Body Station 1380. The right main body gear was forced back and up through the fuselage, displacing the main passenger cabin floor. The right horizontal stabilizer received severe structural damage, severing hydraulic lines to the elevator actuators. The remaining flight controls, powered by the No. 2 hydraulic system, were found to be operational.

The investigation revealed confusion regarding runway lengths and restrictions. The airport authority had issued a restriction prohibiting Boeing 747s from using takeoff power until reaching the displaced threshold on Runway 01R, leaving 8,400 feet of runway available. This information had been transmitted by the FAA as an AIRAD rather than a NOTAM. Pan American did not receive this AIRAD, and their route manual erroneously listed 9,500 feet available for Runway 01R.

The Board calculated that with 8,400 feet of runway available, a takeoff at 708,000 pounds with 20 degrees of flaps required approximately 8,675 feet with a clearway. The Board noted that had the actual runway length been known to the flight controller, he would not have dispatched the flight to Runway 01R.

A primary factor was the crew's failure to calculate and utilize the reference speeds appropriate for a 20-degree flap configuration. Using the 10-degree flap speeds required an estimated 8,430 feet to liftoff. The Board believed that the aircraft crossed the departure end of the runway with the main gear in firm contact with the ground, rotating to a climb attitude as it passed over the first 300 feet of the ALS.

Regarding the landing, the Board noted that degradation of longitudinal control did not manifest until the captain attempted to slow the descent rate near 200 feet. There was no information in the manuals pertaining to control effectiveness when only one of the four elevator sections was operable.

The evacuation was complicated by slide failures. One slide failed because its trigger mechanism was damaged by impact, another was dislodged, a third failed due to a shifted gas generator bottle, and a fourth inflated but was blown out of position by the wind. The aircraft tilted backward because passenger movement shifted the center of gravity while the body gear were incapable of bearing weight.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the pilot's use of incorrect takeoff reference speeds. This resulted from a series of irregularities involving: (1) the collection and dissemination of airport information; (2) aircraft dispatching; and (3) crew management and discipline; which collectively rendered ineffective the air carrier's operational control system."