Tuesday, July 19, 2005

Escaped Comment

I started to respond to comments on the previous entry and my comment got so long I've decided it's tonight's blog entry. Back to the PT6 tomorrow.

I have seen the 'won't engage' problem actually caused by missing or broken teeth on the ring gear, just like anoynmous' friend, and also by the sticky starter arm. I have heard of the engage-at-shutdown technique, but not of the screwdriver-in-the-starter technique. I've never been that desperate. So far I've always managed to obtain a start by turning the propeller to a new position, although sometimes it takes a couple of iterations. Flying commercially there is a very strict line between work the pilot can do and work that maintenance must do. Usually you cross that line when you pull out a screwdriver, but again it's all about desperation.

I wouldn't hand prop an airplane if there were a guy or gal around in coveralls to fix it for me, nor would I attempt it on an engine too big for it to be a success. I have had training hand propping small aircraft. But even if you have had none, there are places in this country where the chance of losing limbs or worse to frostbite or polar bears are greater than the chances of losing them to a propeller. And I know the propeller would be quicker than the cold. Not sure about the polar bear. Do they play with their food?

Carb ice right at start only occurs when it's close to freezing. If it's three degrees and 100 per cent humidity (i.e. raining) this morning and you start sucking that air into the carburettor, the temperature only has to drop three degrees for ice to clog the narrow gap in the only-just-open-enough-to-idle carb throat, and there's not yet a big hot engine block to prevent that happening. In fact, the engine is probably still at minus two, from the overnight chill.

Typically with carb ice at start, the engine starts up completely normally, and then the power drops as the pilot continues with the after start checks. In cold wet weather I will apply carb heat immediately after start. The EGT won't be that high either, so sometimes it dies anyway, but that and opening the throttle more if power starts to drop should do the trick.

It can be amusing if someone is taxiing on a cold, wet day and making little power adjustments, not really paying attention as the power drops. By the time they check carburettor heat, the throttle is almost wide open, but the engine is still at idle because ice almost completely blocks it. They put on carb heat and the power roars up to a cruise setting.

Twenty-five thousand hours and no hung starts, eh? Encouraging. If people knew how many hours we spend memorizing, practicing, being tested on, and briefing each other on procedures we may never use, they'd stop worrying about whether we were fit to fly because our ties are on crooked.


Connie said...

Lol! As many AME will attest, a pilot with a screwdriver ain't nothin' but trouble ;)
But then again Aviatrix, you know your stuff - You might be able to weild the coveted screwdriver just fine...
I'm learning quite alot from you! Keep up the good work....

Avimentor said...

My policy is to not hand prop planes. I've been trained in how to do it, I've done it, but think it's just too risky. If you do hand prop, ensure the prop is indexed correctly.

With high enough relative humidity, I've had carb ice form while on the ground, right after engine start, at temperatures as high 14 degrees C.

Old Blind Dog said...

The only hung start I've ever had has been in the sim. Of course PT6 engines are notoriously reliable if maintained.

Now TSIO-520-J engines have been giving me fits lately. I may have finally figured out a reliable hot start technique.

dibabear said...

Haven't had any bad starts. Well okay, IO-320s are a bit touchy. That and needing he arms of Ahnold Schwarzneggar to pump our club plane's primer.