Thursday, June 16, 2005

More thoughts on PCL 3701

The more I find out about this accident, the more it bothers me. My thinking on it has shifted as I've read more information on it.

First off, it turns out that the crew was slightly above the maximum altitude for their weight and air temperature. They were at 37,600 lbs, and the air temp was ISA + 9.4d C. The performance charts for 38k and ISA+10 show 40,400 feet as the maximum climb ceiling. Furthermore, at Pinnacle the climb profile called for Mach .70 or 250 kts minimum, whichever is slower (since Mach changes with air temp). As the crew approached FL410, they were at Mach .57 and a mere 180 knots. Given all this, the media's contention that the crew was acting recklessly is looking uncomfortably close to the mark.

In the comments section of my previous post, Glenn mentioned that there seemed to be very little CRM practiced - it was very chaotic, with both pilots trying to do too many things. Mind you, they didn't have a lot of time to burn, and it's easy to say "practice better CRM!" in the comfort of one's own living room. Still - if the dual flameout checklist would've been followed better, the thrust levers would've been moved to shutoff; they apparently remained at idle, which caused fuel to remain flowing to the engines. This, together with continuous ignition being selected to "ON," apparently caused significant melting of the turbine blades in the #2 engine. Thus, the number two engine never could be restarted.

Glenn mentioned in his blog that they attemped a windmilling airstart below the recommended speed of 300 kts, causing a hot start. Actually, they started to run that checklist, but discovered that the high pressure turbine (N2) was not windmilling, so they elected to instead try an APU-assisted start at 13,000 feet. As to why the #1 engine would not turn when they attempted the start, General Electric has admitted that the CF34 engine is subsceptible to "core lock" when flamed out at low speed. You can find out more about core lock from ALPA.

Many of the recent airline accidents are completely unforseen things - such as TWA800's fuel tank explosion or the stabilizer jackscrew snapping on Alaska 261. For this accident to happen, then, when we have 50 years of experience with high altitude jet flying, is pretty freaking senseless. Did nobody teach the young captain about these things? Many Pinnacle pilots have decried their company's poor training department and their practice of hiring very low-time applicants. Of course, they're not going to attract a lot of high-quality applicants with some of the lowest pay in the industry.

I'm a political conservative. Still, if free-market competition among the regionals results in a body count, I don't see what alternative the government will have to re-regulating the industry.


GC said...

Don't forget, Sam, that I wrote my comments before I read the CVR transcripts, which I didn't know were available.

According to the transcripts, they mention moving the thrust levers to shutoff at about 2156:45 (start levers and thrust levers are one-in-the-same), but without FDR info, we don't know if that actually occurred. Earlier, at 2156:12, they stated that they had a "little bit of engine (windmill) in one of them." That leads me to believe that there was fuel flowing to the engines when they turned continuous ignition on at 2156:42 prior to shutting the thrust levers off three seconds later.

Is three seconds long enough to torch an engine to the point that it won't run? I don't know. I kinda doubt it. The theory of core-lock is interesting, and it's a theory I hadn't heard before.

At any rate, it will be interesting to hear the NTSB ruling in a few months/years.

Sam said...

Glenn, I knew you hadn't seen the CVR when you wrote your post; I figured somebody gave you bad info regarding possible hot starts. My contention that the throttles were never moved to shutoff may be more of the same. I could've sworn I saw it in the powerplant report (FDR data) but cannot find it now.

I kinda doubt that three seconds worth of fuel would be enough to cause the extent of thermal damage they're talking about...and assuming both engines were left at idle, why only the #2 engine? On the other hand, if they didn't hot start it, what *did* cause the thermal damage? If several starts were attempted on a corelocked engine, I'm curious what sort of damage would result.

This should be one interesting final NTSB report.

Old Blind Dog said...

By "corelocked" do you mean shaft bow (where the drive shaft bows due to uneven cooling thereby preventing the engine from turning)? If so, I'm curious/confused about how this could happen to an engine that is windmilling. The airflow through the engine would prevent this. Now, I could be all wrong about these engines, but in my experience with Garrett TPE-331's the engine would have to be stopped to cause shaft bow which typically occurs between 15 and 45 min after shutdown. Are these somehow different?

Sam said...

OBD, I don't know if core lock in the CF34s is quite like the Garretts, but it *is* due to parts of the different spools cooling at different rates. Now, the situation you describe is after a shutdown on the ground, presumably with a cool down limitation after landing observed. When PCL3701's engines flamed out, they would've presumably been near max operating temps, and the CVR reflects that the crew observed 0% N2 rather soon. Under these circumstances, the onset of core lock might've been rather swift.

Mind you, this accident is the first time I've heard of this phenomenon. I'd be interested to learn more about it.