Boeing 747-128 accident at US, 17 Aug 1970

Accident narrative

On August 17, 1970, Air France Flight 030, a Boeing 747-128, experienced an uncontained engine failure and fire shortly after takeoff from Montreal, Canada. The flightcrew extinguished the fire and diverted to New York, landing safely. None of the 174 revenue passengers, two infants, or 17 crewmembers were injured.

### The flight

Flight 030 was a scheduled passenger flight from Chicago's O'Hare International Airport to Orly Airport in Paris, France, with an intermediate stop at Montreal's Dorval Airport. The aircraft, registration F-BPVD, was nearly new, having been certificated on July 14, 1970. The flightcrew consisted of Captain Jacques Nits, First Officer Louis Chativeau, and Flight Engineer Rene Cobut.

The first leg from Chicago departed at 1820 eastern daylight time and was normal in every respect. The aircraft departed Montreal at 2226.

### The incident

At 2235, nine minutes after takeoff, the aircraft was climbing through 5,600 feet m.s.l. over the St. Jean, Quebec VOR. The flightcrew heard an explosive sound, followed immediately by a fire warning for the No. 3 (right inboard) engine. A cabin attendant reported seeing fire in the area of the No. 3 pylon.

The crew initiated emergency procedures, shut down the No. 3 engine, and discharged both available fire extinguishing bottles. The fire warning terminated. The flight engineer went to the passenger cabin to assess the damage through the windows, but darkness and the outboard location of the damage precluded a valid in-flight assessment.

Although there was no physical evidence of further fire on the inboard side of the engine, the crew was apprehensive about jettisoning fuel. As an alternate means of reducing the airplane's weight to the maximum allowable for landing, the flight diverted to John F. Kennedy International Airport in New York. The aircraft proceeded at an assigned cruising altitude of 13,000 feet and 230 knots indicated airspeed, landing safely at 0004 on August 18.

### What the investigation found

The aircraft was equipped with four Pratt & Whitney JT9D-3A turbofan engines. The No. 3 engine had accumulated 267 operating hours and 58 operating cycles.

Examination revealed a total separation of the second-stage turbine disk outer rim. The outer two inches of the rim and the turbine blades had separated around the full circumference of the disk assembly. These fragments ruptured the high-pressure turbine case and caused massive deformation of the adjacent engine structure.

Investigators found that the high-pressure turbine module had been incorrectly assembled. The second-stage turbine outer shroud assembly in the upper outboard quadrant was not properly engaged over the lip in the high-pressure turbine outer case. This improper engagement caused the second-stage stator inner shroud to move rearward and contact the second-stage turbine disk web. In the affected quadrant, only one thread or less of the vane retaining bolts protruded, whereas three threads were observed in properly engaged quadrants.

The Board noted that the module had been reworked at Pratt & Whitney. A review of assembly procedures disclosed that while there were instructions for torqueing the 90 bolts securing the 30 vane segments, they did not provide a specific sequence for the operation. Furthermore, there were no provisions for making dimensional checks that could have detected the improper positioning of mating parts.

The Board concluded that the incorrect assembly resulted in a transient condition where the second-stage turbine disk rubbed against the second-stage turbine vanes during the application of takeoff thrust, when maximum rearward gas velocities existed. This rubbing grooved the disk in the rim area, progressively weakening it until it separated.

The separated fragments caused extensive secondary damage. The No. 3 strut assembly sustained tears, holes, and fractured spars, and the main generator cables were severed. Fragments also damaged the wing's leading edge high-lift devices, flap track fairings, and the No. 4 engine cowling. The No. 4 engine ingested fragments from the No. 3 engine, damaging 23 of its 46 fan blades and its low-pressure compressor, which necessitated the engine's replacement. The No. 3 engine also exhibited molten aluminum spray and sooting from a localized fire in its upper forward portions.

Following the incident, the Safety Board recommended that the FAA review installation records and inspect JT9D engines. The FAA subsequently directed inspections using a new X-ray technique, and Pratt & Whitney revised its assembly procedures to include sequence tightening of vane retaining bolts and measurements of critical axial dimensions.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this incident was "the in-flight separation of the second-stage turbine disk rim of the No. 3 engine. The separation of the disk rim was the result of incorrect assembly of the high-pressure turbine module."