Once while preparing for a flight test in a simulator, I flipped a C-17 over its tail onto its back. When something like that happens–especially in a high fidelity simulator–it is very eye-opening, to say the least.
Modeling and simulation, or M & S, is just one of several ways we employ the build up approach and prepare for elevated risk testing. Because it is such a fundamental part of the incremental approach to flight test, the flight test community prides itself on the fidelity of its models and simulations.
We had gone to the sim as we were preparing to airdrop the Ares JDTV from the C-17, pictured above. Seventy-thousand pounds of rocket was going to get ripped out the back of the airplane at 25,000 ft by massive parachutes, and this had never been done before.
The sim mission helped us prepare for contingencies, but since I’ve already given away the punchline in the title, let me say it again here:
Modeling and simulation have limitations.
Here are 3 specific obstacles that we encountered on the fateful day we did the back-flip.
1. No model for an airdrop load that weighed 70,000 lbs
The cleared envelope for the C-17 was 60,000 lbs, so models only existed up to that limit.
2. No model for 70,000lbs/1g extraction parachutes
Normally, the force generated by the extraction parachutes was much less than the weight of the aidrop load. But how much less?
3. No model for airdrop malfunctions
Airdrop malfunctions were simulated verbally, by an instructor who would say, “malfunction.” There was no way to make the aircraft simulation respond to any malfunction, of the several that we envisioned.
That represents uncertainty in the model.
Our vision was quite clear and focused, however. We were there, in the simulator, to develop a response to emergency situations.
For example, what if only one of the extraction chutes inflated–how much would the aircraft pitch up if the extraction was slower than normal? What if the airdrop load did not exit and we were dragging parachutes through the thin air at 25,000 feet? How much would it decelerate the aircraft before the line could be cut?
We had countless questions, but we had no way to quantify the uncertainty in our highest fidelity model. Well…almost no way…
| ATOMs are applied tools for transforming uncertainty and risk into decisive action. |
How would you overcome these limitations? What analytical tools would you use?
(I’ll explain how the clarity of our focus helped us develop a solution next time.)
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Where Do We Go From Here
How do we find our way then, when we are exploring the unknown, blazing a trail into uncharted territory? How do we apply elementary statistical principles to transform uncertainty into decisive action? What is to prevent us from making a preposterous application of ATOMs when we deal with very complex situations, those in which our intuition fails?
These questions are not much different from those faced by Chuck Yeager before he ever broke the sound barrier or Neil Armstrong as he took that first step on the moon. Neither of these men, nor anyone around them–with hundreds or thousands of highly educated, very scientific people on these teams–knew what to expect. Or did they…?
ATOMs is a monthly column that introduces analytical tools of mathematics and statistics and illustrates their application. To read more about ATOMs, you can read Where Do We Go From Here, or view the online workbook here.
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