|The outboard nacelle- the Achilles heel of the Lockheed Electra|
In my previous blog entry, I had discussed how subcontracting was a way of building goodwill between aerospace competitors, the example from I talked about was in reference to the Space Shuttle program. However, even aside from financial incentives, there have been times in aviation history that corporate rivals have cooperated beyond that of a joint venture and one unheralded example of such cooperation was the extensive investigation into the loss of two Lockheed L-188 Electras shortly after the type went into service. The first loss was on 29 September 1959 when Braniff International Airways Flight 542 went down near Buffalo, Texas, on a flight from Houston Hobby Airport to Dallas Love Field. The second loss was on 17 March 1960 when Northwest Airlines Flight 710 went down near Tell City, Indiana, on a flight from Chicago Midway to Miami.
The cooperation of Lockheed's competitors began after the Braniff crash. A Dallas warehouse was used to reassemble the wreckage as part of the investigation. Using a chicken wire frame, pieces were added in a gigantic jigsaw puzzle from October into November. In January 1960, the investigators with the Civil Aeronautics Board (CAB) were no closer to determining the cause of the loss of Braniff 542 and invited representatives from Eastern Airlines and American Airlines, both operators of large Electra fleets, along with NASA to join the investigation. The following month Lockheed invited engineers from Boeing, Convair, and Douglas to review what had been done so far in the investigation. But the loss of the second Electra when Northwest 710 went down shook the airline and aviation industry as aircraft like the Electra were the leading edge of the jet turbine age that would revolutionize air travel. There was near unanimous sentiment in American aviation that the cause of the loss of two Electras in passenger service had to be found and rectified for the good of the entire industry.
NASA immediately put its resources at Lockheed's disposal (the decision to ground versus speed restrict the Electra will be subject of a future post for this blog). Allison, the maker of the Electra's engines, initiated its own flight test program with its own company Electra. Boeing and Douglas made their resources also available to Lockheed- with both companies fielding their own advanced jetliner designs for the day, helping Lockheed determine the cause of the Electra crashes was vital to confidence in both the Boeing 707 and the Douglas DC-8. With Lockheed engineering staff getting pulled from various projects to the Electra investigation, Boeing's chief Bill Allen dispatched his own engineers and his best aerodynamicists from Seattle to assist Lockheed in Burbank. The investigation and any fix needed had a name- Lockheed Electra Achievement Program, or LEAP. To everyone involved, it became Operation LEAP.
Part of Operation LEAP was the use of a specially instrumented Electra that was flown by Lockheed and government test pilots over the Sierra Nevada range looking for mountain wave turbulence that would shake the test aircraft as no airliner had ever before been punished in-flight. Lockheed had actually pioneering the methods for testing inflight loads on structures back in 1949 that became the industry standard in flight testing. Sixty nine flights into the rough air offered a clue- the load instruments noted that the outboard nacelles were taking a rougher beating than expected. Flutter of some sort became the suspect cause- every aircraft has some degree of flutter, the air moving over various parts and imposing loads can cause them to vibrate and if unchecked, those vibrations increase in amplitude until structures fail. Aircraft structures are designed in part to dampen these oscillations.
With flutter as the suspect, Douglas had sent over to Lockheed a vane exciter that was basically an actuated vane mounted on the wingtip that could move quickly to induce flutter in the wings. Even in smooth air, the vane exciters could really dole out some punishment to the wing structure. Douglas had used the device in the DC-8 flight test program and put them at Lockheed's disposal. The devices in combination with the rough air flight testing showed the outboard engine nacelles were the source of the flutter and a review of the wreckage from the Braniff and Northwest crashes showed that the outboard nacelle structure in combination with the wing structure allowed an obscure type of flutter called "whirl mode"- in short, gyroscopic movements of the propeller caused oscillations in the nacelle which were then transferred to the wing which caused the wing to fail.
Though commercial rivals, Boeing and Douglas were only keenly aware that Lockheed was well respected in the business and its engineering and design methods were top notch. Much of how Lockheed went about the design and testing of the Electra wasn't all that dissimilar to how Boeing and Douglas went about the design and testing of their jetliners. If there was something Lockheed had missed, then it was something that left their designs and the processes used open to question as well.
On 12 May 1960, Lockheed chairman Robert Gross announced the cause of the loss of Braniff 542 and Northwest 710 as unstable whirl mode. Think about the technology of that day as this was well before computer modeling was available. Now think about the time frame- Braniff 542 went down at the end of September 1959. Northwest 710 went down in the middle of March 1960. And by the middle of May 1960 the cause had been identified. It's one of the great herculean efforts of American aviation that an obscure flutter mode was found to be the cause in just seven months. That amazing time frame wouldn't have been possible if Lockheed didn't have the cooperation of its commercial rivals as well as NASA, Allison, and the airline operators of the Electra.
Source: The Electra Story: Aviation's Greatest Mystery (Bantam Air & Space Series No. 9) by Robert Serling. Bantam Publishing, 1962, 1991. Illustration: Flight Simulator X screenshot.