Sunday, June 21, 2009

The mystery of N10TM

So, I'm out driving to get groceries here in the south-end of Grand Forks, ND, and down the avenue comes this heavy pickup truck with a kingpin thingy to haul this semi-sized flatbed trailer. What's on the trailer immediately gets my attention, I mean, you don't just see stuff like this every day. It's a wrecked airplane! And by the looks, it had been a really nice medium GA airplane.

I see the trailer turn into the parking lot of the Super One strip mall area. It appears the driver's going for a break or pizza at the Pizza Hut or something. I cross over the lanes and follow him into the parking lot and then come to a standoff distance alongside, gawking over the awesomeness of the man's cargo.

He jumps out of the heavy pickup, and heads for the Hut. I want a picture, but I don't have a camera on me, but it appears I'll have some time, so I complete my grocery shopping and return 30 minutes later with a camera.

I arrive just in time to meet the driver as he's emerging from the Hut. He proceeds to check the straps securing the load of wrecked airplane: mangled engine pods forward, mostly intact fuselage in the middle, and wings, bit of gear, and empennage at the back of the trailer.

I quickly go for my camera and approach, snapping a few shots on what's about the end of my memory card (still nearly full after a wedding shoot).

The driver notes my interest and I engage him with questions. He's hauling salvage. This airplane wrecked in 2007. Ran out of fuel and crashed into a truck in a parking lot almost exactly like the one we were in now. I didn't recognize the model immediately. The driver tells me it's a King Air, and I mentally note from the three rows of cabin windows that it must be a C90 King Air. I note the tail number. Since it wrecked in 2007, the NTSB probably has an accident report up on it by now.

The driver shows me pictures of the accident scene. I note to him how the cabin is squished a bit under the nose, but is otherwise fully intact and ask if there were serious injuries. "Yes," he replied, "pilot and three passengers got thrown around pretty bad."

"He ran 'er out of fuel," the driver explained.

"Wow, do you know how high he was when he ran out, I mean, was he on approach to an airport?", I asked.

"Oh God no."

I gave a grimace in acknowledgment. The driver offered all he knew, which was that he appeared to run out of fuel at a bad moment, and didn't seem to have the height necessary to execute any better a forced landing, like on an airport or away from people and cars and buildings. Considering this, some unoccupied wrecked cars and no fatalities seemed to be a decent outcome to a bad situation.

He explains that the stalling speed of this airplane is something like 90 mph, so coming into the parking lot would be like wrecking your car at full interstate speed.

He's preparing to depart, so I quickly walk around a shoot a couple more pictures, filling the memory card and hoping for the best, and then thank him and allow him to be on his way. This was a privilege. The airplane crashed in Chattanooga, TN and sat around there, and now was sold for scrap and though I didn't catch the final destination, I got a little insight into the life cycle of downed airplanes.

So, curious to know more about the circumstances, once home again I plug the tail number into the NTSB's database and get the accident report.

If you're reading this, skim the report and come back. Okay? Now it felt to me that the NTSB didn't care all that much about this incident, doing a phone interview of the pilot. It doesn't appear anyone else was too much involved. An FAA inspector confirmed the absence of fuel in the wings, but the pilot's story begs some questions in my mind:

He reported gauge readings between FULL and 3/4 and estimated by this he had fuel for 3 hours at least, more than enough for the 1h20m flight. Now...I don't know the preflight procedures for this type of airplane, but I do know that aviators generally regard fuel quantity gauges in GA airplanes to be liars. I guess certification standards are such that they must read accurately when the fuel tanks are full, and when they are empty, but the middle indications that come during operation may mean only that the tanks are neither full, nor empty, but by how much...? Are you timing your flight? Do you have any sort of totalizer measuring fuel burned?

Well, I was almost right on my guess as to aircraft model. It was a B90, the late 1960s forerunner of the C90 which I had guessed. The changes amounted mainly to perhaps a slightly buffed engine model and longer wingspan, so I mostly nailed it.

Pulling some B90 performance specs off the internet, I see that in cruise the airplane ought to burn about 64 gallons/hour. Now if the gauge indications are anything like my car's, when the gauge reads 3/4, the actual level is more like 1/2. And having half-full tanks in this airplane at that cruise burn rate would yield something close to 3 hours cruising time. So the pilot's estimate of flying time available seems to check here.

To my total surprise, still had the accident flight's history in its database! Humorously, it listed the destination as Chattanooga (it was really Georgetown, KY), and that the airplane had "arrived." Yeah, I'll say that's true. One way or another, they always arrive. This data features prominently later.

According to the report, the airplane had reached its cruise altitude of FL210 when the pilot noticed that two of the four gauges suddenly read practically empty. Reassessing his situation, he reported to the NTSB he estimated having about 50 gallons of fuel aboard at this point, and opted to make a diversion to Chattanooga. emergency. It's quite strange that the left side gauges went from nearly FULL to empty in just 22 minutes, but...maybe there's some sort of electrical fault with the gauges. Who knows?

Do I dare to call shenanigans on the pilot, and on the NTSB for not making this clear in its report? Not being a real-world pilot myself, just an enthusiast (for now?), I'm about to get pretty presumptuous. But, this is the internet after all.

Again, according to the data on the B90 from the internet, in cruise power the airplane will burn approximately 64 gallons per hour. So that means with 50 gallons estimated remaining he ought to be able to continue up there in cruise for another 45 minutes at about 200 knots.

According to the report, it's with this estimate in mind that our dear pilot elects to prudently divert to Chattanooga, about 45 nautical miles away. If he stays at altitude and cruise power, he'll get there inside of 14 minutes, leaving 30 minutes to descend and execute an approach (at cruise power, which he wouldn't use of course, so he may have even more absolute reserve).

But instead, he's out of gas and crash-landing on some guy's pickup in the middle of a strip mall parking lot! How could this be?

Well now I turn to the Flightaware data for N10TM on the 19SEP07 incident flight. Flightaware gets its data on aircraft position from the same data network air traffic controllers use to monitor the skies. Radar sites get controllers the raw data. Their terminals process it for their needs. After that it goes into a network to which other entities may acquire special access for fleet monitoring, ground service planning, traveller updating, etc.

From this data, I noted that the airplane never reached its cruising altitude, and entered a turn toward Chattanooga near the apex of its flight, around 19600 feet. From this point the flight proceeded more or less direct to the field in a continuous descent. The descent rate and airspeed appeared to be not always stabilized, but averaged 1300 feet/minute at 180 knots groundspeed. That's enough height and speed to go 45 nm, and the straight line distance between reported radar points was in fact 45.6 nm. The field was about .6 miles further along.

It appears from internet sources that a best glide speed isn't published in the POH for the B90, but one source inferred one from data published for a C90 and listed it as about 125 knots. I don't know what sort of descent rate that would translate into, but 1300 fpm doesn't seem out of the ballpark to me.

Now in his report to investigators, he'd estimated having 50 gallons aboard about the time he noticed the two empty gauges and elected to divert. In descent, the power is normally pulled back somewhat, in some cases (certainly for jets, but maybe less so for turboprops like this) all the way to idle. This allows essentially a gliding descent and initial approach, making up for the excess fuel used on the climb to altitude by now using very little on the descent back down, certainly much less than that used on cruise. So the picture should still be okay.

Somewhere in this descent he reported he ran out, and too late to do anything but strap down tight and pancake on the nice pickup truck, like it was a last minute happenstance. I don't buy it. If his estimate had been right, he ought to have ample fuel to make a normal approach and landing. Maybe even enough to afford one go-around if he messed up flying his approach path. How could this be?

In my view, simple: he's shading the truth to investigators. He doesn't appear to have made it to cruising altitude at all (about 1400 feet under it). At the time he decided to divert he was probably already out of gas or very close to it. He didn't methodically consider and then execute a diversion while still carrying at least some reserve of fuel. He hastily and with OK, but somewhat less than perfect form beat a hasty retreat to the nearest suitable airport that came to his mind.

He turned direct from his climb heading, to the airport at Chattanooga, and didn't even try to line up for an approach to the closest runway end. He appeared to be trying to make a B-line for the field and I think he hoped to kick the airplane 'round at the last second to line up and touchdown on the runway there.

Only, he didn't even make it that far. His groundspeed and descent rate suggest a path that might not be unreasonable to expect from a gliding aircraft of this type. Surely if he were still powered, as he suggests when he elected to divert, I would expect he'd want to keep his altitude until he was certain of making his diversion airport. This would mean a delayed descent by some amount.

Even in normal circumstances, one wouldn't choose to make a continuous descent from the point he had, as obviously it's still too far out, and the data doesn't suggest any level flying segment. I think under normal circumstances one might plan to be in the airport vicinity at around 3000 feet, so as to have some flexibility to set up a normal approach.

In my armchair cockpit, I think I'd keep at cruise altitude to benefit from the fact that my fuel burn would be more efficient up there. I'd start down only if an approach and landing was assured, and for utmost margin, I might even fly until overhead the field at cruise altitude, and then enter a descending holding pattern above the field and inbound to a holding fix lined up with the landing runway. You can be certain of gliding in, in that situation.

None of this happened, and I believe that's because he'd already lost power. And while the outcome was a fair one for he and his passengers, he got lucky that no one was injured or killed on the ground. The track data had him near a golf course just before the parking lot. That might have been a safer forced landing site. I think he was fixated on just trying desperately to make it to that field.

The NTSB might have been wise to this pilot though. They didn't seem to care about the obvious possibility that there might have been a gauge problem when the gauges seem to be showing close to FULL, or maybe some sort of fuel leak. A conservative assumption of 1/2 full tanks at this stage would, as the pilot mentions, rightly given him at least 3 hours of cruise flight. Yet only 22 minutes after takeoff, he's noticed two gauges reading about zero (it's not reported what the other gauges read). And fifteen minutes and 45.6 nm later, he's glided that bird to the deck. I'd say his gauges were all probably reading closer to empty all along.


Keith K. said...

I'd have to agree that your theory fits the facts better than the pilot's story.

steve said...

And now we have a non-pilot, non-accident investigator analyzing this accident. Go get a game of golf.

Andrew said...

Conjecture? Yes. Could I be wrong? Yes.

I think it's helpful to allow one's curiosity some freedom to deconstruct and lay out some possible explanations. It's part of the way we try to understand things.

Maybe my scenario got it all wrong, but then again, maybe it's exactly right.

If you got all the way to this comment and you're a pilot, this discussion could help you avoid a similar situation (heck, the great Sean Tucker just squeaked out unharmed and unbent from a similar situation a few weeks back).

As for my own credentials, some financing and determination will improve them, and until that point, well, I did say I was about to get presumptuous!

Thank you so much for commenting, I very much appreciate it.

Saili said...

Hmmm, that was interesting. Looks like somethings will always remain mystery.

I myself has been trying to solve the mystery of the legend that forces you to have "earn it before

having it", for a wile now. Could not understand much though.

Let me know in case you get to understand the mystery of the Old Hound and the Legend

By the way, good writing style. I'd love to read more on similar topics