One night in September 1959, a Lockheed Electra L-188A turboprop aircraft with 28 passengers and 6 crew apparently disintegrated mid-flight over Buffalo, Texas.

The Electra was new at the time, the first large turboprop airliner built by a United States aircraft company. It represented the pinnacle of aviation technology and boasted the highest thrust-to-weight ratio of any commercial aircraft then in service, as well as large “Fowler Flaps” that significantly increased the wing area at low speed. These features made its aerodynamic performance even better than current commercial aircraft on short runways or at high altitudes.

Yet, only six and a half short months after the Texas crash, the unthinkable happened again: another Electra disintegrated at 18,000 feet over Indiana. What was wrong with these airplanes?

 

The Incidents

Braniff Airways Flight 542. On 29 September 1959, Braniff Airways Flight 542 departed from Houston, Texas, in the evening on its way to Dallas. The weather report showed scattered clouds with no lightning and only light turbulence. As it neared its destination, however, it made a radio call to its operations center with a request for maintenance after its landing in Dallas. It was never heard from again.

The wreckage of the airplane was found about three miles southeast of Buffalo, Texas, and was scattered across two and a half miles. Witnesses nearby reported hearing loud noises resembling the “clapping of two boards together,” “thunder,” and the “roar of a jet breaking the sound barrier,” and most reported seeing a fireball in the sky. Notably, the weather report was correct in that there was no lightning reported in the area on that day.

Northwest Airlines Flight 710. On 17 March 1960, Northwest Airlines Flight 710 departed from Minneapolis shortly after noon and landed at Chicago Midway airport. Passengers on this flight described the landing as “very hard.” However, as there was no apparent damage to the aircraft, it was refueled for a subsequent flight to Miami. Expected weather was clear skies with no turbulence reported.

Flight 710 took off from Chicago without incident, and reported its position at about 3:15 pm, flying at 18,000 feet above Scotland, Indiana. Shortly after that, crewmembers from a flight of six US Air Force airplanes performing an inflight refueling exercise nearby reported seeing several puffs of smoke followed by audible explosions. The Air Force witnesses then reported seeing a large object falling from the smoke puff to the ground.

The main section of the aircraft disintegrated upon impact with the ground, creating a crater 30 feet wide and 12 feet deep. The remainder of the wreckage was scattered over 1,500 feet on a line stretching to the southwest.

 

What caused this disaster?

Engine Mount Failure due to Hard Landings. The Electra aircraft flight characteristics were such that reducing power for landing at too much height above the runway caused a high sink rate to develop and resulted in a harder-than-normal landing. The Lockheed Electra Achievement Program (LEAP) concluded that enough such events, or a single significantly hard landing, could cause the aft engine mounts (on the rear of the wings) to weaken or fail mechanically. Without the stiffness of the aft engine mounts, the phenomena of “whirl-mode flutter” described below could be catastrophic.

Significant Additional Loads on Intermediate Wing Ribs. The first issue that may have caused the airplanes to break up was significant additional loads beyond the design calculation on the wings’ intermediate bracing ribs caused by shell distortion, which is material distortion (in this case, the surface of the wing) due to uneven expansion and contraction due to heat or other factors. These additional loads on the intermediate ribs were not known to the designers and not identified in the development of the airplane, so they were not accounted for in the aircraft design. Also, these additional loads accelerated the progress of fatigue on the wing members, potentially to the point of failure.

Whirl-Mode Flutter. The phenomenon known as “whirl-mode flutter” occurs when the stiffness of the engine mounts interacts with the circular force (gyroscopic torque) of the propeller drive shaft, which can cause resonant frequencies and vibrations in the wing structure that grow in amplitude and intensity. Under normal circumstances, this flutter is constrained by the maximum amount of flex in the engine mounting, but in cases where damage makes the whole structure of the aircraft–including wings and engine mounts–less stiff, flutter can occur more easily and be more destructive. LEAP theorized that the “hard landing” experienced by Flight 710 before the crash might have weakened the wing and engine mounts and been a causal factor to the in-flight breakup.

 

Effects of the disasters

An investigation into the crash of Flight 542 by the Civil Aeronautics Board concluded that the aircraft had disintegrated because of left wing structural failure (i.e., it broke off) due to whirl-mode flutter.

After the crash of Flight 710, the FAA ordered Lockheed to conduct a design review of it’s Electra Aircraft in order to determine what could possibly cause an aircraft to break up in flight, particularly any potential cause of structural wing failure. The design review was called the Lockheed Electra Action Program (LEAP), and it identified the main causal factors of both disasters.

A study of the wreckage of Flight 710, both on-site and in Lockheed’s laboratories, by both Lockheed and the Civil Aeronautics Board, also concluded that the aircraft had broken apart in flight due to oscillations of the outboard two engine nacelles violent enough to cause the right wing to fail structurally. The presence of whirl-mode flutter in the Electra airplane was confirmed in a NASA wind tunnel, where Lockheed and NASA engineers tested a scale model of the airplane at various speeds and in various simulated flight regimes. The as-designed scale model showed no flutter, but after the engineers weakened the wing structures, the vibrations and torque of the engines became resonant with the wing and caused catastrophic flutter.

The conclusion of LEAP was that in Electra aircraft where the engine/wing structure had been weakened, whether caused by accelerated fatigue, additional force loading of the intermediate wing ribs, or by an event such as a hard landing, whirl-mode flutter became likely and caused the catastrophic failure of the airframe of Flights 542 and 710. The probability of whirl-mode flutter increased with external forces such as turbulence.

In response to the findings of LEAP, Lockheed retrofitted their Electra aircraft with engine mountings and nacelles that were stronger and less susceptible to fatigue and damage from hard landings. Lockheed also increased the strength of the wings in order to increase the strength and stiffness of the airframe. Since those improvements were made, no Electra ever experienced whirl-mode flutter again. Yet, the two high-profile crashes destroyed public confidence in the airframe, and no additional Electras were ordered.