Saturday, July 30, 2005

I love these questions....

Anyway- the lesson.. remember I had had no prior of flying or flight and did not know better about implications of things that might have happened. I think it was my 3rd or 4th lesson. We were doing the engine run-ups when the C152 (ZK-ELV, known as ELVis) ran very roughly on the left mag. We increased the rpms to see if that would clear it but it still ran rough. Idled for a few more seconds, increased the rpms and it seemed to clear. Continued thru the DVA's then taxiid for takeoff. It seemed to take a looong time for the plane to lift off and actually settled back down very briefly, so we would have used more runway than normal. Finally lifted off, then at about 200 ft the engine sounded real funny and the instructor said "I'll just take control for a bit" a couple of seconds later everything sounded fine and we continued on- doing stalls and he even did a spin to lose height on the way back down. When we landed and taxiid in, the other instructors at the aero club had heard the rough running of the engine (we were maybe at the far threshold of the runway at 200 ft by then)and ran out to have a look. I also found out we had taken off with a quartering tailwind, that was why it took so long to lift off. A couple of people I talked to after (remember I was totally ignorant of what quartering tailwinds would mean to performance etc)said that with the engine running rough at engine run-ups, it was foolish to take off with the tailwind, as had the engine continued to run roughly, we would not have been able to make a safe landing (houses etc in the way). Also, to continue on to do stalls etc was foolish too. Now, possibly if it was 1 mag sticking, we could have switched to the other mag and made it back OK to land - but I also know there are other reasons for engines to run rough. What are your thoughts on this? Is it a big deal or as my instructor said, it wasn't a problem? I think it was just the reactions of those other people (1 a high hour glider pilot and senior instructor and the other a high hour private pilot)who unsettled me on this.
Ruth




First of all, that is a great nickname for an airplane.

A couple thoughts on the questions. First, magnetos could run rough for several reasons. The one the instructor was checking for would have been fouling of the spark plugs. If a bit of fouling was the problem, then running the engine hotter could solve that by burning off deposits. If the mag check is clear after that, generally you have no further issues, and learn to lean the mixture control better, or get the mixture cable adjusted by a mechanic. Clearing the plugs may take a couple tries. If they sound clear after that, you generally don't have any problem continuing. If not, then stop the flight right there.

Departing with a tailwind can be a very bad idea. Or it can be no big deal. It sort've depends. The toughest part of learning to fly is acquiring the desicion-making skills. I forget the V speeds and performance numbers on the 152. It has been a very long time since I flew one. Whether or not to depart with a tailwind would come down to a couple factors though: 1) expected takeoff and obstacle clearance distances (which increase dramatically with a tailwind) versus what is available. 2) the "cost" of going to the headwind. (meaning, extra time spent, or whatever, though being in a hurry there is never a great idea). 3) potential for unexpected situation.

So to sum up, it is the margin of safety built into the situation, compared to whatever might be lost by the other option, plus the emergency you might be about to have, but don't know about yet. How much margin do you have, and how much are you willing to give up?

Think of it this way (I'll use round numbers to make it easy). Kinetic energy is the big factor. (energy coming from the velocity of the aircraft). with a 50 knot stall speed, and a 10 knot wind, the difference is this: requiring a groundspeed of either 40 knots or 60, depending on which runway you use. In that scenario, using the tailwind forces you to come up with 56% more energy than the other option, before you have a chance to leave the ground. A C152 is rather underpowered, and that would increase your runway needs dramatically. Unless you have huge amounts of extra runway, it is risky at best.

Eight days before my crash, I was forced to land the CT with a 10 knot tailwind component & gusty winds, or seriously risk being stuck in a severe thunderstorm, with no means of safely getting out. If I'd had time to get to the other runway, that would have been the best option, and I'd have done that under any normal scenario. My kinetic energy upon touchdown was almost three times as much as the other option. Seriousness of potential injury also goes up about that much. The only reason I did that, is because the other option was more dangerous.

In any case, landings are mandatory. Takeoffs are not. Unless you have far more runway than you need, with a plane that you don't suspect of any possible problems, and the tailwind component is very small, then the risk is generally not acceptable.

Settling back to the runway might be a couple things. Was it a soft-field takeoff? If so, the instructor might have simply tried to get too far from ground effect before getting enough airspeed. That one, everybody does at some point. But knowing the plane was taking too much runway, that would be a sign to abort. Always have an abort point in mind before getting on the runway in the first place. Always.

Sometimes, I've gotten a feeling in flight that some system wasn't quite right, and done some troubleshooting. When it seems ok, I continue the flight, always having at least a couple options for the emergency that can develop. If not, I return to the airport and prepare for the possibility of the problem getting worse. I mentally prepare for an emergency on every flight. Every flight. The few times something has gone wrong, I reacted the way I prepared myself to react. But you should always give yourself room to have at least one safe option. They say flying is 99% boredom and 1% sheer terror. You just never know when that 1% is going to happen.

Not having heard the sounds, or felt the sluggishness of the actual flight in question, I'm not sure I could be adamant about that instructor being unknowledgeable, lazy or negligent. (How much wind was it, how bad was the noise, did it really sound like fouling or something else, how much runway was there, etc....).

My guess would be, if multiple other more experienced pilots were concerned, then I wouldn't make light of it.

2 Comments:

At 3:44 AM, Anonymous Ruth Holman said...

Hi

Thanks SO much for your comments. The take off speed for C152 is 55 knots. We share the airfield with the local gliding club, and they were still using the "wrong" (parallel) grass runway (we were using the seal), they hadn't shifted the control caravan, although they did straight after- so I think that was the reason why we had to use the runway with the tailwind.

Today, for the first time in 6 months, I went back down to the aero club and got one of the instructors to do some "ground school" with me - going thru all the preflight, engine runup checks and DVA's, also circuit planning. I felt that I was in danger of forgetting too much otherwise. It was good to be doing something again, even if I didn't leave the ground! It's still likely to be awhile before I have money to fly.
Oh, as well as an ELVis, we have a "Fishpie"- ZK-FPI. There's also a JEN, KAS and JAZ!
Ruth

 
At 8:30 AM, Blogger also-known-as said...

Hey, thats great. Even being near airplanes when you can't fly is fun.

Wind shift at the last moment is a common and often reasonable cause for using the wrong runway. It could be a mess trying to get all the airplanes turned around and headed the other way. As long as the wind isn't too strong, that's justifiable. (Still have to reserve judgement on the rest of it though).

In the U.S., more often than not, aircraft tail numbers have mostly numbers, and it is rare to have an all-alphabetic tail ID. So we are probably missing out on a bunch of interesting aircraft nicknames.

 

Post a Comment

<< Home