- Braniff L-188 (Electra) in Texas
Northwest L-188 (Electra) in Indiana
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Braniff L-188 (Electra) in Texas
Historic photo artists rendition of a Lockheed Electra Airplane
Braniff Airways Flight 542 (September 29, 1959)
Braniff Airways Flight 542, a Lockheed Model L-188A (Electra) airplane, was scheduled between Houston, TX, and New York International Airport, with stops in Dallas, TX, and Washington, DC.
Flight 542 departed the ramp in Houston at 2237. There were 28 passengers and a crew of six onboard. At 2240, the flight was cleared for takeoff and was airborne at 2244.
Houston air traffic control (ATC) tower had Flight 542 in radar contact after takeoff and subsequently cleared it to 9,000 feet, and advised the flight to contact San Antonio Air Route Traffic Control Center (ARTCC) (hereafter referred to as "Center") upon passing the "Gulf Coast" intersection.
Flight 542 reported to company radio at 2251, and at 2252, reported to San Antonio Center as being over the "Gulf Coast" intersection at 9,000 feet. San Antonio Center cleared Flight 542 to the Dallas airport and directed it to maintain 15,000 feet cruising altitude and to change to, and monitor, the Fort Worth frequency.
Shortly thereafter, Flight 542 contacted company radio with a message for maintenance. This was the last transmission from Flight 542, which was logged at 2307.
Photo of an Electra Engine
Photo copyright Colin Abbott - used with permission
Structural failure of the wing occurred at approximately 2309. The main wreckage was located 3.19 miles east-southeast of Buffalo, TX. The time correlates with impact-stopped watches recovered at the scene and from witnesses to the accident.
There were no radar or radio contacts with Flight 542, nor were any emergency calls received after 2307. There was no known traffic, missiles, or unmanned aircraft in the region at the time of the accident.
The recovered flight engineer's logbook recorded altitude at 15,000; airspeed of 275 knots; 15 degrees F outside air temperature; anti-icing off; and time of 2300.
Weather reports for the late evening of September 29, 1959, and the early morning hours of September 30, 1959, show a weak pressure gradient in southwestern Texas. A pilot en route from Dallas to Houston later reported encountering intermittent light rain and moderate turbulence at 7,000 feet, northwest of Buffalo, TX.
According to ground witnesses in Buffalo, TX, and the immediate area at the time of the accident, skies were partly cloudy, the visibility was good, and no lightning was observed.
Company meteorologists provided the flightcrew of Flight 542 with current and forecast weather information for the route and terminals concerned.
Photo of an Electra Wing
Photo copyright Peter M. Garwood - used with permission
Witnesses reported hearing various noises of different intensities and pitches (clapping of two boards together, thunder, roar of a jet breaking the sound barrier, etc.). The majority observed a large fire in the sky.
Wreckage was located within a total distance of 13,900 feet from the first recovered item to the nose crater, and it was distributed within a long narrow ellipse. At the main area, there were three basic concentrations of wreckage: around the nose crater, the center section crater, and the tail section. Light material was found a half mile to the north and northeast. The nose crater, about four feet deep, was at the easternmost end of the investigation area. The center section crater was approximately 200 feet northeast of the nose crater. The tail section was located 250 feet northwest of the center section.
After studying the wreckage distribution pattern, it became evident that the airplane had experienced airborne disintegration, which broke the airplane up into a number of major sections. Damage to the airplane was extensive, with only a few parts and system components recognizable. In addition, fire had destroyed or damaged individual components making functional checks impossible. As part of the investigation, considerable time was spent identifying, listing, and describing damage to parts and system components. No indication of operational distress, fire, or overheating was found through examinations. The Civil Aeronautics Board Aircraft Accident Report concluded that the probable cause of this accident was structural failure of the left wing resulting from forces generated by undampened propeller whirl mode.
Photo of flight 710 impact site
Photos of an Electra L-188 landing
Photo copyright (top) Ralf Manteufel,
(bottom) Fred Seggie - used with permission
Northwest Airlines Flight 710 (March 17, 1960)
Northwest Airlines Flight 710, Lockheed Model L-188C (Electra) airplane was scheduled between Minneapolis, MN and Miami, FL, with a stop at Midway Airport, Chicago, IL.
Flight 710 departed Minneapolis at 1251 and arrived at Midway at 1355. Some of the passengers said the landing was very hard. The airplane was refueled and prepared for its flight to Miami.
The hard landing described by the passengers became significant in the determination of the accident cause. The Lockheed Electra exhibited landing characteristics such that if power were reduced at too high an altitude above touchdown, the resulting sink rate would cause a hard touchdown. The investigation concluded that a series of hard landings, or a single landing of sufficiently high sink rate could have caused the failure of the aft engine mount, and reduced the stiffness of the nacelle to the point that it coupled with the natural frequency of the wing, leading to the described whirl mode event.
Flight 710 departed Chicago at 1438 for a nonstop flight to Miami. There were 57 passengers and a crew of six onboard. Prior to departure the crew was briefed by a company meteorologist on present and expected weather conditions along the route. There was no mention of clear air turbulence along the route. At 1445, the flight reported to Indianapolis Center at 18,000 feet. At 1513, Flight 710 reported over Scotland, IN maintaining 18,000 feet. This was the last known radio communication with Flight 710.
At approximately 1640, reports were received by Northwest Airlines at Minneapolis that Flight 710 had crashed near Cannelton, IN. The time of the crash was estimated at 1525.
Flight 710 was flying in an approximate north to south direction in level flight. Six United States Air Force (USAF) airplanes were on a refueling mission in the area at an altitude of 31,000 to 32,000 feet. Airmen manning the USAF airplane said they first saw a smoke trail at 1532. Ground witnesses reported seeing two puffs of white smoke followed seconds later by a large cloud of dark smoke. Two loud explosions were then heard and a large object was seen to emerge from the smoke cloud and fall nearly vertically, trailing smoke and flame. The fuselage struck the ground, spreading debris nearly 250 feet into the air.
The major portion of the airplane struck the ground in a nearly vertical attitude. Impact forces formed a crater measuring 30 feet across and 12 feet deep. Most of the airplane forming the crater disintegrated and buried. A shallow depression that contained fragments of the left wing extended southward from the crater for a distance of 16 feet. Portions of the vertical tail were imbedded in the west rim of the crater. The impact scattered wreckage up to 1,500 feet to the east and southwest.
Diagram of Lockheed Electra L-188 wireframe
(View Large Image)
The south end of the crater contained the No. 2 engine and propeller, parts of the left main gear, and parts of the wing structure. The north end of the crater contained the fuselage structure. The main portion of the right wing, the outboard engines, and many small pieces of wreckage were widely scattered in north and northeast from the crater. Study of the wreckage and wreckage distribution at the scene disclosed that the outboard engines and engine support structures, the complete right wing, and the outer portions of the left wing and aileron separated from the rest of the airplane in flight. A detailed study of the main wreckage was conducted at the Lockheed factory. Various reconstructions were done to facilitate the study.
The Civil Aeronautics Board Aircraft Accident Report concluded that flutter was induced by oscillations of the outboard engine nacelles (undampened propeller whirl mode) and that it reached a magnitude sufficient to fail the right wing. Reduced stiffness of the nacelle structure, possibly due to the hard landing damage at Chicago and the entry of the airplane into an area of severe clear air turbulence, were contributing factors.
Lockheed Electra Action Program (LEAP)
Following several meetings, the FAA Administrator requested Lockheed Aircraft Corporation to conduct an engineering reevaluation of Electra model airplanes. The objective was to reveal any design or operational characteristics of the airplane that may cause structural wing failure in flight. Assistance was provided by NASA (Langley), Boeing, Douglas Aircraft Company, and other organizations. This reevaluation effort was named Lockheed Electra Action Program (LEAP).
Photos of Electra being tested in the NASA Langley 19-foot tunnel
The program disclosed two discrepancies in the design of the airplane. One, significant loads imposed on the wing's intermediate ribs between the fuselage and outboard nacelles by shell distortion had not been included in the design loads. Two, the dynamic response of the outboard nacelles in turbulence was different from the design assumptions.
An eighth-scale powered model of the Electra was tested in the NASA Langley 19-foot tunnel. Lockheed and Langley flutter experts were concerned that the propeller-driven Electra may have exhibited the phenomenon known as whirl mode, in which the stiffness of the engine mounts interacts with the gyroscopic torques produced by the engine and propeller combination. This interaction results in an unstable wobbling motion that could resonate with natural frequencies of the wing structure and could cause catastrophic flutter of the wing. The Lockheed and Langley team reduced the stiffness of the outboard engine mounts on the model and found that the propeller whirl mode motion coupled with the natural flapping frequency of the wing. The interaction resulted in an unstable wobbling motion.
Normally, whirl-mode flutter can operate only within the flexibility limits of the engine mounting structure, and is quickly dampened. If the stiffness of the supporting system is reduced through failed or damaged engine structure, mounts (as was suspected in the Northwest Airlines Flight 710 due to hard landing), or nacelle structure, the damping of whirl-mode flutter is reduced. An unstable wobbling motion plus an external force, such as clear air turbulence, might result in structural disintegration of the wing.
Lockheed redesigned the engine mounts, nacelles, and cowlings, and modified the wing to improve strength capabilities. No Electra accidents have resulted from whirl-mode flutter since.