A few words about Colgan 3407

My story related to the JungleBus' FMS will have to wait for the next post. I've been debating whether I should write about Colgan 3407; I generally take the "let's wait for the investigators to do their jobs" approach to airline accidents. Unfortunately the media doesn't share this sentiment in the least, and in their drive to solve the accident before the wreckage is cold they've put out a tremendous amount of disinformation in the last week. Few of the media's sources appear to have any experience flying turboprop airliners, much less the Q400. While my knowledge of the airplane isn't perfect and has faded a bit in the year and a half since I last flew it, I do have a few thousand hours in it.

I'm not going to speculate on what caused the crash. All that I know about the circumstances are what's been reported by the NTSB thus far and repeated in the media. The morning after the crash, enough was already known that there were only a few likely culprits. I myself suspected it was one of two scenarios. The first known facts made one seem most likely, and subsequent information is now shifting the investigation towards the second possibility. The media hasn't reported accurately on either scenario, with a few exceptions. There's a decent chance that more information will come to light that will take the investigation in a completely different direction before it's all over. To say I have any idea what really caused this accident would be a farce. I will, however, give my take on some of the ways the known information has been interpreted and reported to the general public.

"Significant" versus Severe Icing

Because the air traffic controller was prudent enough to collect icing PIREPs from other pilots immediately following the accident and the audio of those interactions was immediately available on LiveATC.net, speculation that this was an icing accident reached a fever pitch before the fire was even out. The investigation now seems to be proceeding in a different direction, but it could came back to icing as a contributing factor.

The media seized upon the NTSB's statement that the crew noted "significant" icing on the descent. They've treated this term as the equivalent of severe icing, even though the NTSB has specifically said they have no reports or evidence of severe icing in the area. There were a number of other airlines that landed just prior to and after the accident aircraft. One that landed a half hour later was another Colgan Q400. There were also several light aircraft in the area at the time. The very worst reports of icing were for what would normally be considered moderate. None of the airliners requested routing or altitude changes to get out of the ice, and nobody even bothered giving a PIREP until the controller started soliciting them after the accident. That's not what a bad ice night in the northeast sounds like. Although icing conditions can vary significantly over small changes of distance and time, it seems rather unlikely that one crew could encounter severe ice when multiple pilots around them were barely noting it.

Ice and the Q400

Horizon has been flying the Q400 in the Pacific Northwest, Montana, and southwestern Canada since 2000. This area gets its share of bad icing conditions every year, and the Q400 has shown itself to be up to the challenge. It does have deice boots, but they generally do an excellent job of keeping the leading edges of the wing and tail clean in moderate icing. The one problem spot is that the deice valves sometimes freeze closed, but this is immediately annunciated in the cockpit with a "Deice Pressure" caution message and nine times out of ten the crew can cycle the system off and on and get the valve to unstick. The NTSB has indicated that the deice system was turned on before the aircraft entered icing conditions (which was Horizon's procedure, too), and there were no obvious, annunciated malfunctions of the system.

It's easy to tell when you're getting a lot of ice on the Q400. The windshield wipers are excellent collectors of it, and there's a little plastic pin on the top of them that accumulates ice before any other part of the airframe. The cockpit even has a built in light to illuminate the pin at night. All of the wings outboard from the engines are visible from the cockpit, as is the unprotected propeller hub. Both are well illuminated by ice inspection lights.

As soon as the ice detection probes detect ice, a message starts flashing on the EICAS and won't stop flashing until the pilots select the ice speeds switch on. This makes the stall protection system speeds 20 knots faster, forcing the pilots to use adjusted ice speeds for landing.

I've had decent ice loads on the Q400 several times, including one thankfully short encounter with icing possibly falling into the "severe" category. The airplane has so much excess power, especially down low, that performance wasn't even an issue. I never felt that controlability was an issue either, although I suppose it's pretty easy to get to the edge of controlability in ice without realizing you're at the edge (more on that later).

Turboprops under Fire

Given that the Q400 is a turboprop with deice boots, there have been (premature) parallels drawn between this accident and others involving turboprops with boots. There has been a fair amount of insinuation that turboprops are inherently dangerous in ice. Today, former NTSB chairman Jim Hall, who now partners in an aviation litigation firm, carried this idea to it's ultimate, idiotic conclusion: all twin turboprops ought to be immediately grounded.

It's true that jets are superior to turboprops in ice. The reason has a lot less to do with equipment than with performance. Yes, hot leading edges are nice and do a better job of keeping the wing perfectly clean in "normal" conditions. In severe ice, though, hot wings are just as susceptible as boots are to runback (ice forming behind the protected area). A jet aircraft's main advantage is that its superior performance and greater altitude capabilities allow it to get out of ice quicker and stay out for longer.

That said, I believe turboprops can be safely operated in icing conditions so long as their pilots monitor the situation carefully, know the limits of their equipment, and always have an out if things get nasty. Overall, turboprop pilots have done a great job of doing just that. Two icing accidents out of 30 years and millions of hours of flying small turboprop airliners does not make them inherently unsafe, as some would have it - especially when you look at what actually happened in those accidents. One involved prolonged flight through supercooled water droplets (SLD), which wasn't widely understood but we now know is the worst kind of severe ice, because it runs behind the protected areas before freezing, with drastic implications for controlability. The other involved getting too slow in an iced up airplane on which the deice system had not been activated. Both of these had little to do with the systems or limitations of a turboprop aircraft, and could've as easily happened in a jet. To use these accidents, plus a currently unsolved accident in which ice may have played a factor, to call for the grounding of all turboprops is the height of insanity.

Tailplane Stall

One subset of the icing scenario which attracted the most attention among pilots but received fairly little coverage from the media was the possibility of a tailplane stall. The reason it caught so many pilots attention was the NTSB's announcement that Colgan 3407 suffered an upset immediately after the pilots selected Flaps 15, and NASA's previous research has shown that flap extension can cause a nearly-stalled tailplane to stall. Subsequent information from the flight data recorder, however, appears to contradict the tailplane stall sceneario.

On all conventional aircraft, the wing is positioned so that the center of lift is behind the center of gravity (which is essentially the pivot point of the aircraft during maneuvering). This causes a nose-down, tail-up pitching moment whenever the wing is producing lift. To compensate, the horizontal stabilizer is designed to generate tail-down force. It does so with an airfoil much like an upside-down wing. Like a wing, the horizontal stabilizer can only generate lift up to a certain angle of attack. Beyond that critical AoA, it stalls, or ceases to generate lift. When that happens, the aircraft rapidly pitches down thanks to its natural pitching moment.

Under normal conditions the tail is pretty hard to stall. At slow speeds where the boundary layer might tend to separate, the aircraft is usually flying at a higher AoA, which is actually a low AoA for the tail. Lowering flaps decreases the aircraft's AoA, making it greater for the horizontal stabilizer, generally at slower speeds where the boundary layer can detatch more easily. High-wing aircraft with conventional tails, like the Twin Otter, also generate quite a bit of downwash on the tail with flap extension, which further increases the A0A. Throw in some ice contamination and you have the potention for real trouble: an unexpected, rapid pitch down at presumably low altitude. It looks a lot like a conventional stall, but the recovery is exactly opposite: pull up, retract flaps, and go easy on the power. Aircraft with unpowered elevators can be very difficult to recover from a tailplane stall, with stick forces of well over 100 pounds required.

The Q400 has a hydraulic-powered elevator, which would make recovery from a tailplane stall much easier, assuming you know it's a tailplane stall and take the appropriate recovery steps. I'd be surprised if this accident had anything to do with a tailplane stall due to more recent information that's come to light: the initial upset was a pitch up, not down, and the autopilot disconnect was precipitated by the stick shaker. A stick shaker indicates critically high aircraft angle of attack, which would be a low AoA for the horizontal stabilizer.

If you're interested in learning more about tailplane stalls in icing - and if you're a pilot who flies in ice, you should be - there's a very interesting NASA video for you to watch here. Of particular note is the inadvertent tailplane stall they experience in a Twin Otter.

On Autopilot Usage

For a few days there was an absolute uproar over the fact that the aircraft was on autopilot just prior to the upset. If anything indicates the media's cluelessness about how we operate airliners, this is it. I'm a big proponent of turning off the automation and hand flying the airplane at times. A dark, snowy night when I'm about to shoot an approach is not one of those times. That's when you use the automation to keep your workload low. Yes, if really iced up, I'll turn off the autopilot early to get a feel for the plane. But there's nothing in the Q400 manual (or Colgan's procedures, apparently) that says you have to hand-fly the airplane except in severe icing. The media acted as though Captain Renslow was being negligent merely by having the autopilot on in fairly normal icing conditions. That's baloney.

Now, automation does pose its own hazards. You need to make sure its doing what you want it to do, and you have to do your own part. The Q400 has a very capable autopilot but it doesn't have autothrottles. You need to pay attention and bring up the power when leveling off from descents or its possible to get into a low-airspeed situation very quicky; those 13 foot props produce a lot of drag at flight idle.

A Big Upset

The most recent information the NTSB has released is that the aircraft was approaching the marker and was at 134 knots at the time gear was selected down and flaps selected to 15. If that number turns out to be correct, that is a very, very low speed in the Q400 without being in the landing configuration. Shortly after the flaps were selected to 15, the stick shaker and then the stick pusher activated, which automatically turns the autopilot off. An upset occured at that time, with pitch angles as high as 31 degrees nose up and 45 degrees nose down, and bank angles as high as 105 degrees.

That's a pretty huge upset, and one difficult to recover from at 1500 feet even if done perfectly with a clean, undamaged airplane. Although it's only been about a day since the media started letting go of their ice obsession and began reporting on the low speed upset, there's already been a fair amount of finger-pointing that the pilot flying let the aircraft speed get so slow, or that he supposedly pulled up and fought the stick pusher. Suffice it to say that we know very little about what was going on other than those basic numbers that the NTSB has released. It'll come out soon enough; this investigation is unusual in that the NTSB has been releasing information more or less as they find it out rather than waiting to put together a final report in a year or two. The point is, though, that until a lot more is known, about all we can say is that the aircraft appeared so suffer from a low-speed upset. We don't know why, we don't know whether icing was a contributing factor, we don't know whether recovery was possible. All those answers will come with time; in the meantime, any certitude on the part of the media, most of their sources, bloggers, or web board participants is mere affectation.