Over the past several weeks, I’ve been setting the stage for a story that illustrates two of my favorite things, some very important flight test techniques and ATOMs, applications of mathematics and statistics to flight test. Some of the most rewarding and dangerous flight test that I ever participated in was airdrop flight test with NASA’s Ares and Orion teams. On one such test flight, something started to go seriously wrong at 25,000 feet, high above the Yuma, Arizona desert.
|If you haven’t been following along, you might want to check out these 5 pieces of background information about C-17 airdrop flight test.
1. What’s it like at 25,000 feet? (a narrative of the airdrop countdown sequence from the test pilot’s perspective)
2. C-17 and NASA Orion Photos
3. Links to C-17 and NASA Orion airdrop flight test reports and videos
4. Photos of three C-17 airdrop design characteristics and the NASA Ares JDTV
5. Recovery parachute test photos and NASA Ares website
What could go wrong?
It was the second time that we, the test team, were airdropping the NASA Ares Jumbo Drop Test Vehicle (JDTV). The first one had gone off without a hitch. This drop was at 60,000 lbs–the heaviest amount allowed by the C-17 flight manual. This airdrop was part of our build up approach: We were planning to eventually perform an airdrop at 90,000 lbs–that meant envelope expansion. The data we gathered would be used to validate modeling and simulation used to plan for successively higher airdrop weights.
We had an enormous team that included two aerospace physiologists, two loadmasters, an airdrop systems engineer, two instrumentation operators, two test pilots, and a flight test engineer.
When the cargo door is open (as pictured above), it gets noisy, and there was a lot of coordination that had to take place between the ten of us, the drop zone, and as many people on the ground in mission control.
The actual airdrop sequence, starting at ten seconds, is very scripted (I describe it in more detail here). When the clock counts down to zero, the copilot presses the green light and the load gets extracted from the cargo compartment. Within a second or two, the loadmaster checks in with “load clear”–that is, if everything is successful. Between those two moments, marked by the callouts “green light” and “load clear”, it seems like an eternity.
There’s a clock in the heads up display, so it is possible to keep track of time accurately. On this particular day, the loadmaster hadn’t said “load clear” yet, and at least four seconds had gone by. There’s an alternative to “load clear,” if something goes wrong. In that case, the loadmaster should say “malfunction,” and when he does, everyone on the crew begins to run through their steps of the malfunction checklist. He usually says at about the two second point as well.
He hadn’t said anything yet.
Three one thousand…
Four one thousand…
The copilot had been turned around looking out the back, and he abruptly turned to face forward. Something was wrong.
Suddenly, the airplane pitched up violently and then back down–it felt like we had hit a speed bump the size of a VW bug.
Then it came, “load clear.”
I slammed the throttles to max power, rolled the airplane into sixty degrees of bank–paused momentarily to let the nose drop–and pulled swiftly into a diving turn of almost two Gs. The area boundary was very close to a major airway, and the delay in the airdrop had put us very close to the area boundary.
We flew home to find out what went wrong.
3 Things That Could Go Wrong
There are three stages of an airdrop release sequence, and these stages loosely correlate with the kinds of airdrop emergencies we could see in a flight test.
1. Unexpected movement of airdrop platform before drop time.
This would mean 60,000 lbs of havoc rolling around in the cargo compartment and causing a lot of damage.
2. Failure to extract once released.
In other words, the platform didn’t move when it was supposed to. It could be worse.
3. Slow or otherwise anomalous release sequence.
The platform doesn’t go out like it is expected too–a parachute might malfunction or a line breaks. Have you considered how to stop 60,000 lbs of 1g extraction force?
Which situation happened that day?
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.