The ugly truth about crashworthiness

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image: © Civil Aviation Safety Authority

Thomas P. Turner on the plain truth behind aircraft crashworthiness

Three aeroplanes crashed under similar circumstances. Two accidents were fatal; six people aboard the third were virtually untouched. Read these reports and consider what might have made the difference.

The pilot of a Cessna 182Q, operating under visual flight rules, departed on a flight the route of which encompassed the Alpine National Park (in Victoria). The forecast and actual weather included extensive thick cloud and severe turbulence. The aircraft crashed on the side of a mountain at about 5000 ft above mean sea level. The pilot was killed and the aircraft was destroyed.

The ATSB found that the pilot departed with less than visual meteorological conditions forecast along the planned route. From the evidence available it was likely that the pilot encountered reduced visibility to the extent that terrain avoidance could not be assured, resulting in the aircraft colliding with terrain in controlled flight.

Here’s the second report:

The pilot of a Piper PA28-235 planned to take off under visual flight rules and fly through the mountains along a highway. The Piper took off to the west, turned south, and then climbed. Once the aeroplane reached the highway it turned west toward the mountains. About 25 minutes after taking off, radar contact with the aeroplane was lost. Witnesses near the accident site were consistent in indicating that the aeroplane was flying 200 to 300 feet above the ground with the engine producing power. It was in a nose-high attitude when it started turning left away from rising terrain. The aeroplane turned about 180 degrees, rolled over to the left, and entered a steep dive before impacting trees and terrain.

One of the witnesses indicated that, before the left turn, the aeroplane’s path seemed pretty flat with little gain in altitude. It is likely that, as the pilot attempted to cross over the mountainous terrain, he raised the aeroplane’s nose such that the [wing] was beyond its critical angle-of-attack, which, combined with the aeroplane’s decreased climb performance led to an aerodynamic stall and loss of control.

Contrast those two reports with this one:

The pilot of a Gippsland Aeronautics GA-8 Airvan was conducting a charter flight over Tasmania with six passengers on board. The aircraft entered instrument meteorological conditions (IMC) several minutes after departure while climbing to the intended cruising altitude of about 1500 ft. The pilot did not hold a command instrument rating and the aircraft was not equipped for flight in IMC. He attempted to turn the aircraft to return to the departure aerodrome but became lost, steering instead towards high ground in the Strzelecki National Park in the southeast of Flinders Island.

The aircraft exited cloud in the Strzelecki National Park, very close to the ground. The pilot turned to the left, entering a small valley in which he could neither turn the aircraft nor out climb the terrain. He elected to slow the aircraft to its stalling speed for a forced landing and, moments later; it hit the treetops and then the ground. The first passenger to exit the aircraft used the aircraft fire extinguisher to put out a small fire that had begun beneath the engine. The other passengers and the pilot then exited the aircraft safely. One passenger was slightly injured during the impact; the pilot and other passengers were uninjured.

Ignore the similarities

All three of these horrible events involved attempted visual flight into instrument meteorological conditions in areas of rising terrain. All three pilots’ experience and ratings fell short of what was required to attempt flight in the weather known to exist along the intended route of flight. Two of the three crashes, including the one that resulted in almost no injury, meet the definition of CFIT—controlled flight into terrain.

For purposes of this article, none of that is important. Although the similar circumstances most assuredly precipitated each crash, the outcomes are independent of the specific accident causes. Ignore the similarities of these reports—what are the differences between the three events? Let’s look at the moment of impact for each.

The Cessna 182Q’s pilot was manoeuvring in reduced visibility. The Cessna hit trees and terrain wings-level, controlled flight at cruising speed.

The Piper Dakota’s pilot was also manoeuvring in reduced visibility, and was attempting a turn away from terrain. Partly because of reduced engine and aeroplane performance at the mountainous elevation, and perhaps compounded by aggressive manoeuvring when the pilot detected the proximity of terrain, the aeroplane could not sustain the rate of attempted turn in level flight and the wing stalled. The aeroplane departed controlled flight and crashed.

The Gipps Airvan’s pilot found himself in a similar tough spot. He chose, however, to slow the aeroplane to just above stalling speed and make contact with trees and terrain with the aeroplane’s wings level, and under control.

What’s the difference?

What’s the very important difference? In one fatal crash the pilot crashed at very high speed, in the other the pilot lost control. In the third mishap the pilot managed both airspeed and flight path—and everyone on board survived. That is the vital difference when it comes to off-airport touchdowns.

The reason for an off-airport landing isn’t important. It could be the result of poor planning—such as attempted visual flight into IMC—or something entirely beyond your control—like an engine failure due to a mechanical defect that could not be detected before flight. Regardless of the reason, as legendary airshow pilot Bob Hoover said, you must ‘fly the thing as far into the crash as possible.’

The 1982 movie, Fast Times at Ridgemont High, introduced a term to the vernacular (I’ll apologise on behalf of all Americans now). That word is ‘wuss’, used to describe a ‘weak or ineffectual person’ or someone who ‘fails to do or complete something as a result of fear or lack of confidence’. The word wuss, however, is an excellent mnemonic for what it takes to survive an off-airport landing regardless of its cause: touch down Wings level, Under control, at the Slowest Safe speed. Throwing the derisive term into a positive, you must ‘land like a wuss’ to survive.

Why wings level? Aircraft structure is designed to sustain forces in the direction it is expected to encounter those forces. Going in wings level means the structure and the passenger restraint systems are best able to protect the aircraft’s occupants. Any other direction and it will likely be less so. Further, in single-engine aeroplanes the engine and its mounts may absorb impact into ground obstacles and protect the occupants.

Why is speed at touchdown so important? The faster you stop, the greater the G force you must absorb … and the aircraft must sustain, to protect you. Importantly, most aeroplane seat-belt restraint systems are designed to withstand about 9Gs of sustained force (some newer aeroplanes may have stronger restraints). The table shows that doubling the impact speed, from 50 knots to 100 knots, nearly quadruples the distance the aeroplane must slide without stopping to avoid exceeding the 9G limit. It also shows that a sudden stop, travelling only one metre before coming to rest, far exceeds the survival limit.

Impact G forces

Speed (kts) Minimum distance (m) for 9Gs Gs per 1m of travel
50 4 34
65 6 57
80 10 71
100 15 134
Source: U.S. Federal Aviation Administration (converted to metric)

One more thing

One more thing will often make the difference between a survivable and a fatal crash—the use of shoulder harnesses. Many otherwise survivable impacts result in death from head injuries when front seat occupants are not restrained against forward motion into the panel. Take a look at these two photos (from the U.S. National Transportation Safety Board). Figure 1 shows a Beechcraft Debonair after an engine failure shortly after take-off. Figure 2 is a Beech Baron that suffered a dual engine failure on takeoff as a result of misfuelling. Which was the fatal mishap?

You’ve probably guessed, since it if was obvious I wouldn’t have asked. The two aboard the Debonair survived with minor injuries. They were wearing shoulder harnesses. The pilot of the Baron died from blunt force trauma to his head. He was not.

Figure 1: Beechcraft Debonair
Figure 1: Beechcraft Debonair
Figure 2: Beech Baron
Figure 2: Beech Baron

The ugly truth

There’s always an element of luck—good or bad—that contributes to the outcome of an off-airport landing. If there’s nothing but steep, rocky terrain even ‘landing like a wuss’ will not have a good result. But history shows that when luck is on your side and there’s a good spot to land, survival still depends on the pilot controlling the aircraft all the way to impact and for as long as they can until the aircraft comes to a stop.

There is a major difference between automobiles and aeroplanes where surviving an impact is concerned. Automobiles are designed with airbags and crush zones and other survivability features so that, if an impact is survivable at all, you’ll likely live through it as long as you’re buckled in. But in aeroplanes, surviving is almost entirely dependent on the pilot’s actions up to and including at the point of impact. Wear your shoulder harness at all times—you might not have time to put it on if you need it—and touch down wings level, under control, at the slowest safe speed.

It’s not the aeroplane; it’s your actions that determine the outcome of an off-airport landing. That’s the ugly truth about aircraft crashworthiness.

2 COMMENTS

  1. Nah, the “selling point” is a controlled crash is more survivable at lower speeds than at higher speeds, and nearly always more survivable that one that is entered out of control. Physics applies to cars, airplanes, and ships. ;)

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