Boeing 747-151 accident at Florida, 15 Dec 1972

Accident narrative

On December 15, 1972, at 1714 eastern standard time, a Northwest Airlines Boeing 747-151, operating as Flight 723, ran off the end of Runway 27L while landing at Miami International Airport in Florida. There were 149 passengers and 11 crewmembers aboard; no one was seriously injured. The aircraft sustained substantial damage when its nose landing gear collapsed.

### The flight

The flight crew consisted of Captain William F. Burnett, who had 23,774 flight hours including 825 in the Boeing 747; First Officer Harry L. Camm, with 4,934 total hours and 1,478 in the 747; and Second Officer Rodney D. Hold, who had 6,044 pilot hours and 457 hours in the 747.

At 1655, the aircraft was cleared for takeoff on Runway 27R. While rotating, the aircraft encountered a flock of sea gulls. The crew heard a noise that sounded like a bird strike on the right side and simultaneously detected an aircraft vibration. The second officer reported that the No. 4 engine turbine section vibration meter indicated maximum deflection. The captain retarded the No. 4 thrust lever and shut down the engine, at which point the aircraft vibration ceased.

At 1658, the crew advised departure control of the bird strike and stated they would return to land at Miami. The flight was vectored for an Instrument Landing System (ILS) approach to Runway 27L. At 1711:55, the tower reported radar contact at the outer marker and advised the crew that the wind was 120 degrees at 10 knots. Flight 723 acknowledged. The tower subsequently advised other flights that the wind was 160 degrees at 10 knots.

According to the flight crew, the ILS approach was flown normally in light rain. Below 1,000 feet, they continued the approach clear of clouds and rain. With flaps set at 25 degrees and a computed threshold speed of 140 knots, all crewmembers stated that touchdown was made at the normal point.

The captain stated that at touchdown, brakes were applied and engine reverse was attempted on the Nos. 1, 2, and 3 engines. However, the No. 3 reverser lever failed to go beyond the interlock position. According to the captain, the Nos. 1 and 2 engines were reversing, but yawing of the aircraft precluded their use to any extent. The aircraft was hydroplaning, and groundspeed was not being reduced sufficiently despite full braking and continual attempts to accomplish full reversing. The first officer noted that the No. 2 turbine section reversing had been previously blocked out.

Approximately one minute after the tower's wind advisory, Flight 723 reported, "723 is going off the end." The aircraft rolled past the end of the runway and stopped in a sodded area 611 feet beyond the load-bearing surface, 78 feet left of the extended runway centerline. The nose landing gear struck a concrete slab that had been a building foundation and collapsed rearward, damaging the lower fuselage, nose wheel well structure, and cabin flooring. All occupants evacuated successfully via the emergency slides.

### What the investigation found

Witnesses indicated the aircraft landed approximately 2,475 feet from the threshold. Surface weather records showed heavy rainshowers at the airport between 1707 and 1728. Runway 27L, an asphaltic surface with no gradient, had been resurfaced and opened to traffic on November 7, 1972.

The aircraft's landing weight was 507,776 pounds, below the maximum allowable 546,000 pounds. According to the flight manual, the runway length required for landing in wet conditions with 25-degree flaps, a 3-knot tailwind component, and operative spoilers and antiskid devices was 8,250 feet.

Flight recorder data showed touchdown occurred at 145 knots. The aircraft decelerated at approximately 2.7 feet per second squared, a rate characteristic of a free-rolling airplane. Deceleration forces only showed a marked increase from a point approximately 1,400 feet from the end of the overrun area. Scrub marks on the runway indicated the aircraft experienced dynamic hydroplaning for approximately 3,000 feet during the initial ground roll, followed by varying degrees of viscous hydroplaning.

Friction tests of the runway conducted by the Federal Aviation Administration and NASA revealed a stopping distance ratio of 3.22, compared to the regulatory requirement of 1.92. The Board noted that when applied to the aircraft's dry braking distance, this ratio indicated a required braking distance of 8,694 feet if engine reverse thrust was not used.

Examination of the engines showed no evidence of bird strike damage to the Nos. 1, 2, and 4 engines. The No. 3 engine, however, had extensively damaged fan blades, punctured cowls, and bird remains impinged on the fan section. The No. 4 engine turbine vibration monitor pickup accelerometer was found to have a broken wire. The Board noted that the flight crew would logically assume the excessive vibration indicated on the malfunctioning No. 4 monitor resulted from the bird strike, as the reduced vibration in the damaged No. 3 engine coincided with it.

During functional testing, the thrust lever-sequencing actuator for the No. 3 engine was found locked, preventing the lever from advancing. The antiskid and spoiler systems were tested and found to function normally.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the ineffective braking capability of the aircraft on the wet runway because of the low coefficient of friction of the new runway surface, and insufficient engine reverse thrust to decelerate the aircraft. The combined effects of the lack of the No. 4 engine reverse thrust and malfunction of the No. 3 engine reverser resulted in a directional control problem and restricted the use of Nos. 1 and 2 engine reversers."