Friday, September 23, 2011

Air Races, Air Shows, and Risks

The recent fatal crash of the modified World War 2 P-51 “Galloping Ghost” at the Reno air races is a horrible incident. Lots of things have been said on the news and on the internet about it, but all of this is speculation based on incomplete information.

The National Transportation Safety Board (NTSB) will investigate this and determine its cause. They will use all the available information, including stuff not yet reported or on the internet. Until they publish their findings, perhaps a year from now, all else is mere speculation.

That being said, some speculators are more informed than others. I would place more trust in the speculations of an actual aircraft engineer over the speculations of most other members of the public. Being such an engineer, here are my speculations:


“Galloping Ghost” suddenly pitched up and climbed, rolled over inverted, and dove to the ground, all in a matter of scant seconds. Impact was not directly upon, but was immediately adjacent to, spectators, many of whom were killed by pieces thrown from the wreck. There was no post crash fire.

The left trim tab was photographed departing from the airplane’s horizontal tail before impact. The pilot’s head was not visible in the canopy before impact. The retractable tail wheel was seen extended before impact.

This aircraft was modified in several ways from its World War 2 configuration to compete in the races. Most notable were “clipped wings” reducing span (and aileron size) by 5 feet each side, and removal of the belly air scoop and radiator in favor of a sacrificial coolant system. These enable higher top speed, at the cost of higher landing speed and perhaps a reduced maximum roll rate, not a loss of basic stability.

Less obvious were changes to canopy size, wing fillet size, and smoothing of protuberances, for drag reduction. The race speeds significantly exceed 500 mph, when the original level-flight top speed for the P-51 during World War 2 was 435 mph.

I do not know what the original “never exceed” speed was for the P-51, but these race speeds would be approaching or exceeding that limit. Flying too fast risks structural failures of wing and tail components by a phenomenon called flutter.

Similar Previous Incident

About a decade ago, a similarly-modified P-51 named “Voodoo-5” experienced a very similar incident: sudden high speed pitch-up (at high acceleration) into a climb, with the pilot losing consciousness briefly due to excessive gee forces. He woke up, with no memory of events, at 9000 feet altitude, and regained control, landing successfully.

“Voodo-5” was found to have lost the same left trim tab as “Galloping Ghost”. Loss of the tab at high speed caused failure of part of the elevator control linkage, leaving only the right elevator for pitch control. Aerodynamic flutter was blamed for loss of the trim tab.

At 400+ mph, P-51's exhibit a relatively unusual nose-up tendency that you fight with down trim and down stick. Other aircraft exhibit high-speed nose-down "tuck", or no trim-change tendencies at all.

In the P-51 with that nose-up tendency, sudden loss of half your elevator effectiveness at very high speed causes the aircraft to suddenly and violently pitch up, at something near 10-15 gees. The pilot passes out, or can even be killed with a broken neck, depending on helmet weight and head restraints, or the lack thereof.

Speculations Regarding “Galloping Ghost”

“Galloping Ghost” lost the same left trim tab, and pitched up similarly at high gee. Some on the internet say telemetry from the aircraft indicated 11.5 gees. The differences between “Voodoo-5” and “Galloping Ghost” are (1) “Galloping Ghost” also experienced a roll, and (2) it appears her pilot never woke up, or was perhaps already dying of a broken neck.

It also appears that the high pitch-up gee level forced deployment of her tail wheel. The roll motion on the way up caused her to peak in inverted flight, as photographed. I think I see a light-colored helmet on the dark dashboard in that internet inverted-flight photo, but I could be wrong. She then continued her pitch-roll motion into a dive-to-impact.

There has been speculation that the pilot’s seat failed in “Galloping Ghost”, which might explain why his head was not visible in the canopy. I would be surprised at seat failure in a fighter plane at only 10-15 gees, but I guess it could happen. It did not in “Voodoo-5”, though.

Waiting for the Truth

The NTSB will opine officially maybe a year from now, but I'd almost bet they say that P-51 trim tabs are vulnerable to flutter-induced departure at race speeds beyond the original design's never-exceed speed. Few designers provide aerodynamic or mass balancing, or any other anti-flutter structural treatment, to a trim tab. Maybe they should. If so, the NTSB will say so.

Inappropriate Fear-Mongering

The public safety issue raised by some reporters has less to do with any given aircraft being "pushed too far", or being modified "too radically", and more to do with simple spectator crowd placement. The wording in those reports seems deliberately chosen to inflame fears, and is a disservice to the public, much like yelling “fire” in a crowded theater when there isn’t one.

At air shows, spectators may not legally be located beneath expected aircraft flight paths. At the Reno air races, they can be (and are) located under flight paths. Perhaps they should not be, similar to the air show restrictions. While these are the first spectator deaths at Reno since the 1950's, that risk has always been there.

There was a fatal crash at an air show the day following the “Galloping Ghost” incident. No one but the pilot was killed, because no one was underneath the falling plane.

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