Q1. PFLs in Fuel Injected engines, and 'warming' the engine?

[SIMUL8D]

Hi all.

I have a quick question for all the Instructors out there. In a New C172SP with a fuel injected and 'closely cowled' engine, is there still a requirement to warm the engine on descent? Cessna suggests that it's not required.


If it was matter of a carby engine then of course this should be done.
If a matter of CHT, then at what temp should it be done? (we used 105 degrees C in the CT-4B).
If it's matter of clearing the plugs, then surely a run up to 1800rpm and straight back to idle again will achieve little, other than upsetting the students glide?
I have also heard of some suggesting that they only run up the engine periodically on descent to ensure that the engine is still running? I can understand that, but it only needs a slight throttle advance to achieve that goal.


I would welcome your experience on the matter.

Cheers

[A and C]

Why are you compleatly shutting the throttle in the descent? whould you not be better starting the descent a little sooner and not fully shutting the throttle, this would give you a slower ROD that is likely to be better for the ears of any passengers, and it would keep more heat in the cylinders to avoid any chance of shock cooling.

As for PFL's the best way to avoid shock cooling is to reduce the power by about 200 rpm just before fully shutting the throttle, this gives the cylinders chance to cool slightly and avoids shock cooling.

Some years back the British gliding association did some research into the shock cooling of cylinders and the recomended the slight reduction in RPM for a short time before descent, this resaech was conducted with a fully instrumented aircraft. The thrust of this research was to prolong the life of gilder tug cylinder heads but it makes very good reading for all who fly piston engined aircraft.

May be some one who is better with the internet search engine could find it!

[SIMUL8D]

Cheers for that.

Sorry if my thread was misleading. I am only discussing the PFL, wherein the throttle is completely closed for a simulated engine failure.

I'll see if I can find that research. Sounds intriguing.

Cheers

[bfisk]

For most of the descent phase in the C172, normal (or just slightly reduced) cruise power works a charm. It's not exactly a rocket, you know, and the added speed is usually welcome. For the final descent less power is obviously required, and the other tips here about avoiding shock cooling could be useful.

"Clearing the engine" my momentarily adding power and reducing it again, to clear or prevent spark plug fouling has always worked very well for me, in both 152s, 172s and 172RGs. Normally there would not be any fouling to clear, and the few times it was (long idle descent), it would clear within a few seconds, leaving the engine running just like a song.

[SIMUL8D]

Thanks bfisk. That was certainly the kind of info I was after. Cheers

[A and C]

I don't see plug fowling being much of an issue with PFLs and think that the "walming to clear the plugs" is a hangover from WW2.

Only two things can fowl the plugs

1 The fuel, this can have an fowling effect from not burning hot enough but the odd PFL would not be enough to result in this........50 hours of flight trainning might.

2 Engine oil going past the rings!. if this is happening the cylinders need to come off. Both my C152's burn about 1 USQT between 50 hour oil changes so I think that in a well maintaned engine oil fowling is not an issue.

As I said i think plug fowling may well have been an issue with a Tiger Moth that was being thrashed in a WW2 training enviroment but not with a modern anerican engine.

I would take a guess that BFISK has has a bit of induction ice!

[FlyingStone]

A and C has a point here: try to reduce power before completely closing throttle - to cool the engine a bit...

When regarding to CHT it's important to know that low CHT itself isn't so bad (otherwise, how would you start the engine if CHT shows zero without causing any "damage?"), it's the change of CHT that will and does hurt the engine.

Also, spark fouling during PFLs shouldn't be a problem, since spark fouling is usually caused by prolong idling on the ground (1000 RPM or even less), but during PFLs the RPM is around 1500, and they usually don't take more than a few minutes (compared by waiting for more than half an hour for IFR departure from a busy airport at 1000 rpm).

[bfisk]

A and C: the engines referred to were installed in training aircraft, doing a lot more touch and goes, landings, ups, downs, and generally more abuse than a privately owned touring aircraft. Noteworthy also that these were rather old aircraft, and occured more or less on certain invividuals than others. And you're absolutely right, it could have been carb icing, but whatever the cause, a gentle "goose" on the throttle works just fine in my experience.

[IO540]

"Shock cooling" is an old topic, with a number of religious camps, and very little real data :)

The best data I am aware of (from a glider towing operation) is that it is a risk only if the CHT is above a certain figure. If one makes an effort to cool the engine down gradually before the event which requires a sudden power reduction, then it is not a risk.

This is consistent with PPL flying school experience - because they fly full-rich most of the time so the cruise CHTs are pretty low, so they rarely crack cylinders.

The magic CHT value is unknown but it is probably about 300-350F. Much above 400F, aluminium starts to weaken and above 450F it is pretty weak.

When I was doing checkrides in my TB20, I would tell the instructor that a simulated engine failure is not to be done unless the engine was cooled first, by crusing at say only 18" and full-rich.

The PPL training practice of briefly increasing power to "warm the engine" is IMHO bollox. If one had a CHT gauge, one would see that the brief power burst do more or less nothing in CHT terms.

[BEagle]

I think of it more as 'response' check - and used to teach as such.

One day one of my students was out solo and was practising a PFL. Final 'response check' at about 1500ft agl - nothing happened. All knobs and tits in appropriate positions and nothing untoward on the gauges, so he made a quick call and landed without incident in the field.

The problem was later found to be a carburettor fault, I was told. Which the engineers fixed a few days later, then I flew the aircraft out.

ALWAYS treat a PFL as a 'real' forced landing - and DO ensure that the engine WILL respond when you need it!

Another time I was conducting a PPL Skill Test in a PA28-180. The applicant had flown a good pattern; on the go-around there were a few long seconds of farting and grumbling fom the engine before it cleared its throat and produced the requested power......

[Big Pistons Forever]

The main reason the engine is warmed on PFL's is to detect the formation of carb ice before it is too late. If the engine has iced up you will know as soon as the engine starts to gag when the power is brought up. Therefore it is important that the engine RPM is raised to at least a cruise RPM value and held for long enough to ensure the engine will maintain its power. Obviously any sign of a reluctance to accelerate and/or maintain power should result in
terminating the exercise and positioning the aircraft for a real precautionary or forced approach.

Since fuel injected engines are much less susceptable to icing this is less important for aircraft with these engines. I still like to see the engine cleared just for my own piece of mind that the engine is still there.

Finally IMO the only acceptable way to simulate an engine failure is to slowly close the throttle.

[FlyingStone]

Big Pistons Forever: So you are suggesting that carb ice can form after extensive before takeoff check (which includes checking for proper operation of carb heat), after the engine has been running smooth under cruise power and even with applied carb heat (which is common procedure) during PFL? And let's say it's a normal summer day (not 0°C with > 95% humidity)...

If this is true, I don't know why we even have carb heat if it doesn't prevent carb ice formation... :hmm:

[Big Pistons Forever]

flyingstone

Carb ice is most likely under idle power settings (closed throttle valve) and high airflow through the carb (flying speed). It is least likely at cruise or higher power settings (throttle valave almost fully open). A cruise ice check only confirms that ice has not formed under those conditions, not the conditions of idle power. If you start a PFL at 2000 ft the engine could be at idle for as long as 3 miniutes which is plenty of time for ice to form. Furhter more since carb heat comes from the heat of the exhaust stack, you will get an instant blast of very hot air if carb heat is applied in cruise, but much less hot air if the engine has been at idle for an appreciable lengh of time, which is why the engine should get a good shot of high RPM operation when clearing the engine.

[DFC]

I agree with BEagle, it is a response check.

Big Pistons Forever also correctly points out that the source of hot air is the exhaust and some form of shroud or scoop. Therefore increaqsing the heat of the exhaust slightly (or preventing it from cooling too much) will assist the operation of Carb Heat.

If it was more than a response check, one would be doing it on twins as they climb away simulating one engine failed!! or even during the whole arival, approach, missed approach and subsequent landing which is far longer than any PFL takes.

BEagle makes a very good point also - choose the area for doing PFLs carefully and allow for the posibility that the engine will stop at the worst possible moment. In other words if the student is making a really bad choice of landing area, stop it there.

[pasir]

Hi folks - Just an observation on f.injection - Years ago when
I switched from a 172 to a Rheims 172J Rocket I found it a great relief no longer having to bother with carb heating with an injection engine. Regarding engine warming I cannot recall ever having to be too bothered on this aspect on the occassions I may have flown for a few minutes with partially closed throttle - but then I may have been either just lucky, ignorant or plain stupid. However having enjoyed flying with fuel injection - I could never go back to an a/c having carb heating.

[Pull what]

If this is true, I don't know why we even have carb heat if it doesn't prevent carb ice formation..

Carb heat does not prevent carb icing it REMOVES it--I would advise you to revise engine icing

[Roller44]

Hi all,

To answer the Q, is it a good idea to warm the engine for fuel injected types during a PFL, I can help by stating the current practice as taught at the ADF BFTS, in the IO-360 HP equipped CT4B’s.
We teach a variety of simulated engine emergencies, however the simplest “flame out” case has us initially select idle power, and configure for the 80KIAS glide, followed by an evaluation, which simple looks for the presence of fire/ smoke.
After this, we select the “practice” case power setting of 10-11” MAP, and half flap to mimic the handling ROD’s and attitudes expected from a real failure, and then would follow a continuation of our IA’s. The slight increase in power slows the cooling and helps to stabilise engine performance.

During the descent, part of the work cycle is to monitor CHT, and this is reinforced in our Student Training guide so at specific points in the pattern the CHT is called and, if it is close to or at the specified minimum CHT value of 115C, the procedure is to suspend the PFL and conduct a “Warming orbit” at 20”MAP for as long as it takes to raise the CHT to a reasonable working value, such as 140-150C. We then recommence the pattern from the knock off point until either the PFL is terminated or it’s time for the next warming orbit. From 7000’ it may be we would need to repeat this 3 or 4 times.
This saves the engine, but does have the result of a fairly dislocated procedure, especially in the dead of winter, when we may need to perform the first warming orbit as little as 60 seconds after the initiation of the PFL. So far, our failure rate of engines due to cracked heads/ valve seats are negligible. Food for thought?