Boeing 747-151 accident at Honolulu, Hawaii, 13 May 1971 (N607US)

Accident narrative

**Northwest Airlines Flight 9**

On May 13, 1971, Northwest Airlines Flight 9, a Boeing 747-151 (N607US), experienced an in-flight separation of a turbine disk shortly after departing Honolulu, Hawaii. The aircraft sustained structural damage but returned safely to Honolulu with no injuries to the 31 revenue passengers and 11 crewmembers on board.

**The flight** Flight 9 was a scheduled passenger service from San Francisco, California, to Tokyo, Japan, with an intermediate stop at Honolulu. During takeoff from Honolulu at 1330 Hawaii standard time, the crew had to reduce power on the No. 3 and No. 4 engines from a predicted 1.39 to a 1.33 engine pressure ratio to keep exhaust gas temperatures within operating limits. Power was also adjusted during the initial climb for the same reason.

At 1333, three minutes after takeoff, the aircraft was climbing in a right turn at 1,300 feet m.s.l. when a loud explosion occurred. The airplane yawed to the right, experienced moderate vibration, and a fire warning sounded for the No. 3 engine. The combined effect of the explosion and the yaw led the crew to believe they had collided with another airplane.

The crew initiated emergency fire procedures and shut down the No. 3 engine. To reduce the aircraft's gross weight to maximum allowable landing limits, they jettisoned approximately 54,000 pounds of fuel. The flight returned to Honolulu International Airport and landed safely at 1351.

**What the investigation found** The No. 3 engine, a Pratt & Whitney JT9D-3A, had accumulated 660 hours and 201 operating cycles since new. The investigation found that a segment of the second-stage turbine disk had separated, rupturing the high-pressure turbine case and causing varied degrees of secondary damage to the cowling, wing, and pylon. A section of the outer rim measuring approximately 4 inches deep and 17 inches long, along with 23 blades, was missing. The separated fragments were not recovered.

The failure caused structural damage to the No. 3 strut, the engine cowl, and the lower wing surface outboard of the engine. The No. 4 engine sustained minor foreign object damage to its fan blades.

Records showed the failed disk was an early production configuration that had been reworked to the JT9D-3A configuration. This rework included building up the air seal land and its blend radius with nickel plating. The disk had previously been installed in a Boeing 747 flight test aircraft, during which it experienced overtemperatures. A 1969 fluorescent penetrant inspection revealed a crack indication in the nickel-plated area, but it was judged by engineering personnel to be oil entrapment, blended out, and returned to service. It passed subsequent overhaul inspections with no indications found.

Laboratory examination confirmed the disk material and hardness were proper. The fracture originated from a 5.75-inch circumferential fatigue crack in the nickel-plated area of the turbine air seal contact land. The Board noted that the nickel plate was applied in two layers and was more than twice as thick as normally used in turbine disk repair. Fatigue cracks originated in a severely galled area of the surface and from a separation between the plating layers.

Manufacturer tests demonstrated that the fatigue strength of the nickel-plated disk material was appreciably reduced compared to unplated material. The Board concluded that the nickel plating contributed to the reduced fatigue life of the disk, and that the effects of the thicker plate were not sufficiently evaluated before the incident.

**Probable cause** The National Transportation Safety Board determined that the probable cause of the incident was "the in-flight separation of the second-stage turbine disk of the No. 3 engine. The separation of the disk was the result of a fatigue crack which originated in a nickel-plated area of the reworked turbine air seal land and progressed into the base material under operating loads."

***

**Colorado Aviation Aero Commander 680E**

On May 28, 1971, a twin-engine Aero Commander 680E owned by Colorado Aviation, Inc., crashed into a mountain 14 nautical miles northwest of Roanoke Municipal Airport in Virginia. The private pilot and five passengers—including Audie Murphy, described in the report as America's most decorated World War I hero—were killed. The aircraft was destroyed by impact and fire.

**The flight** The aircraft departed DeKalb Peachtree Airport in Atlanta, Georgia, at 0910 under visual flight rules, bound for Martinsville, Virginia. Prior to takeoff, the pilot received a telephone weather briefing covering the route and winds aloft. No flight plan was filed.

There was no record of communication with en route facilities until 1149, when the pilot contacted the Roanoke Flight Service Station. He did not provide a position report. The station gave the pilot the latest Roanoke weather observation: a measured broken ceiling at 1,000 feet, an overcast at 2,600 feet, and three miles visibility in light rain and fog, with mountain ridges obscured. The pilot advised he intended to land at Roanoke and was given the altimeter setting of 30.11 inches. This was the last known communication.

About 19 minutes before this radio call, ground witnesses in Galax, Virginia—approximately 45 miles west of Martinsville—saw the aircraft circling in and out of the cloud base at 100 to 200 feet. Witnesses reported it was foggy and raining. The aircraft circled for about 20 minutes, and the pilot made an unsuccessful attempt to land on a four-lane highway bypass northwest of Galax.

When the flight failed to arrive at Martinsville, a communication search revealed the destination change to Roanoke. Search activities were delayed by adverse weather and visibility, and the wreckage was located on May 31.

**What the investigation found** The aircraft crashed at the 2,700-foot level on the western side of Brushy Mountain at about 1208 e.d.t. Flying in a high-speed level attitude on a 100-degree heading toward the Roanoke VORTAC, the airplane severed two large trees on a steep, heavily wooded slope before ground impact. The Board noted that the area would have been obscured by low overcast clouds at the time.

The investigation found no evidence of preimpact failure or malfunction in the aircraft's structure, systems, or powerplants. All navigational aids at Roanoke Municipal Airport were operating properly.

The pilot held a private pilot certificate for single and multi-engine land airplanes, but did not hold an instrument rating. He had extensive flying time, but only about six hours in Aero Commander type aircraft. The Board found that the pilot lacked experience in the aircraft type and in the Roanoke-Martinsville area, and that he attempted to fly in weather conditions for which he was not properly certificated and lacked the requisite ability.

**Probable cause** The Safety Board determined the probable cause was "the pilot's attempt to continue visual flight into adverse weather conditions at an altitude too low to clear the mountainous terrain. The Board also finds that the pilot attempted to continue flight into instrument weather conditions which were beyond his operational capabilities."