PA 28-161 Carb heat on the approach

[mad_jock]

And some of the commercial checklist are actually wrong.

For example the the AFE PA38 checklist states that you set the rpm to 1800rpm for power checks the POH says 2000rpm.

[Say again s l o w l y]

A very valid point. Can anyone explain why there would be such a difference? Surely the AFE checklist used the POH as a guide?

[CaptAirProx]

Say again,

I don't disagree with anything you have said. If we take the original post, it asks, why do we seem to all teach something at odds with the POH. And then the suspected conjecture as to why this maybe.

Yes there are I am sure many instructors out there that may teach only one scenario with carb heat without thought that the stude may not appreciate the limitations of a POH.

But I fail to see why we should not use carb heat on approach because it does'nt say so in the POH. We have to teach the system being used (old fashioned carb etc)....through Tech exam, and instructor input. This generally implies that in pretty much any engine you will every come across that as a rule of thumb, carb heat ON whilst at low power (approach) is a wise thing to do. BUT also to teach the stude that as always, check with the aircraft POH before operating such a device.

We as instructors have to also be realistic and realise that quite often most things we tell our students get lost in the haze. So I find that its best to drum at least some form of discipline into that person so that they have some method that 'should' work in most cases. Balance of risk.

Lets remember, there are cowboys and charletens (spelling?). Cowboys are easy to spot and generally get back on track. The latter pull the wool over your eyes and bugger off into the wild blue yonder of PPL and never listen a word that was advised before.......Manuals? Who needs Manuals? I can fly? ITs a piece of P**S! Maybe just maybe that stupid fellow, will remember, as it was drummed into him, that carb heat (might) be a good idea on approach. He has a higher chance of getting it right than he has of getting it wrong as most types as said favour this technique.

We all, I'm afraid, are not perfect! I can vouch for that.

[Say again s l o w l y]

There are a great many muppets out there who shouldn't be in charge of wheelbarrow, let alone an a/c.

FI's aren't to blame for this, but we must all try and see and quosh this sort of blase attitude early on. I don't want to take any of the fun away, just get people to think about the consequences of their actions.

I have seen a large number of gash pilots, one or two of whom have passed through my hands at some point and despite my best efforts, when they are away from supervising (prying really!) they get up to all sorts of idiocy. Unfortunately thses people aren't members of our club so I'm limited in my power over them and only usually get to 'have a go' on the 2 year flight. Not really enough and anyway they are on their best behaviour then anyway.

[unfazed]

I'm limited in my power over them and only usually get to 'have a go' on the 2 year flight.


I think that the above comment is very telling ! It would be great to hear the "rebel" PPL side of the coin.

[Genghis the Engineer]

Interesting poll I started!

So far, of the PPLs who have responded, just under 50% are saying that either they're not in any way encouraged to read the POH before flying, or the document is actually kept out of their reach.

I'd like to say that I found that shocking, but frankly it's about what I expected. What it is in my view, is bloody dangerous!

Is there an FI here who will admit to teaching or releasing to fly solo pilots who have not been encouraged to read the POH? If so, what is your justification for this?

G

[homeguard]

POH

The possesion of the POH is of course a requirement of the C of A and invalidates the C of A if it is lost. So, it is treated like gold. Part of every pilots training must include reference to the POH and where to find info as with the AIP, ANO, AIC's and Notams etc.

Some manuals on older types are not a bound item but an assortment of papers found in a binder that requires particular care and protection from careless marauding hands - but just the same MUST always be available to every pilot.

All of our checklists are produced by ourselves for a particular aeroplane, none are generic. The checklists contain from the manual - actual copies where possible of performance data; TO run/TO distance tables, climb, cruise, descent and landing data, W & B and emergency procedures - nothing may be included that is at odds with the manual. Where the POH makes no specific recommendation then the CAA safety factor figures are used.

Carb icing is of course IMPORTANT but that is not to ignore the dangers of an engine running rich! Over use of carb heat with many installations may lead to rough running (so lack of power)and even to a rich cut when aggravated by a hot summers day. Knowledgable use of the mixture is also part of the engine management scenario. Carb icing should not be briefed in isolation to the full story

[FlyingForFun]

I've never yet come across a school where the POH is not available, and if I did I would refuse to use the school. I've sometimes met with confused looks and "I'll find someone who knows" when I've asked for the POH, but I've always managed to get a copy of it.

At my school, the POH for all our aircraft are in a drawer behind the counter. Anyone can ask for them at any time, and I frequently pull them out and show students around them, but I can only think of one PPL who has ever asked for the manuals. We don't allow them to be taken home, and I wouldn't expect to be able to take a manual home with me, but we do sell copies of our types' manuals although these won't have all the supplements for the specific aircraft concerned.

FFF
----------------

[Charlie Foxtrot India]

Hmm mad_jock, my PA38 POHs say to do the run-ups at 1800 RPM....?

One thing that hasn't been mentioned here is the fact that the hot air is unfiltered so to have it hot on the ground in our dusty climate causes wear and tear on the engine.

If you "check for ice" downwind, then put it cold and leave it there, you are avoiding the possibility of trying to go around with it on "hot", also the possiblity of forgetting to put it back to cold prior to landing, and filling your engine up with dust on our Aussie runways.

Because I have found any small distraction on final and the stude almost always forgets to put it to "cold".

Agree one size should not fit all, and I have had many debates about "generic" checklists and still can't see one good reason for them; here in Aus we have a "Single engine below 5700kg" endorsement but obviously poeple need a check ride before getting into something they haven't flown before. Also, the POH has to be kept inside the aircraft and they need to read it before they fly the aircraft!

[2R]

When all else fails read the instructions POH.
Questions that this has raised ;

What is carb icing ?

How do you recognize indications of Carb icing ?

When would expect to get Carb-icing ?

How do you get the most power from the engine if using the carb -heat during those conditions most conducive to carb-icing ?

If you cannot answer these questions do not fly a piston banger stick to turbine engine (s)

[mad_jock]

Unfortunatly i don't have acess to the POH any more.

The difference was noted in a Instructor renewal check ride by the examiner.

Is yours for a mark 1 or mark 2

MJ

[shortstripper]

Hmmmm

All interesting stuff! However, (PPL here) aren't you supposed to teach us why we apply carb heat, why we should select cold before power and why we need to understand the principals and not to simply refer to the POH without knowing why we do the things it tells us?

Most of the aircraft I fly have no POH because they are PFA type home builds.

It is only through understanding why we do the things that we do that we can make informed decisions over how we do them. Isn't that your job as instructors to do? If not then we just as well simply go along assuming anything written down in manuals, text books or (dare I say?) POH's are always and infallibly correct?

SS

[Say again s l o w l y]

Yes, that is exactly what you are supposed to be taught.

Monkey see, Monkey do is not acceptable whether it's blindly following what an instructor tells you, or blindly following the POH and not understanding why you are doing something.

Mind you if I had to pick one of the two, I'd rather people blindly followed the POH. Not that I want anyone to do either of course.

PFA types usually have some kind of manual, certainly the engine you are using will have one.

[Lomcovack]

PA 28 Carburetor heat

So there we are on the downwind leg at a medium power setting and our check for carb icing shows nothing significant.

We put the carb heat to cold, Turn base and reduce the power to a lower power setting.

On finals we notice a slow drop in rpm, indicative of carburettor icing, Is carb heat going to clear any ice, not at these power settings.

Applying power may make the situation worse and if a go around is required an over rich mixture is possible leading to a rich cut at the worse possible moment.

If on the other hand we flew down the final approach with carb heat hot (would the engine produce enough heat to stop ice build up !) and if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1.

Reading and understanding the POH is a must but also thinking for yourself in the way the POH is used is just as important.

As Genghis has pointed out with ref reading the POH for the shortfield landing technique.

But a pilot must be aware of the limitations of the POH for example TODR are made with a new aeroplane, new engine and experianced on type pilot, I would not want to see a student pilot with a 20 year old aircraft and 1800 hr engine try to match these figures.

[Sans Anoraque]

Don't agree at all Lomcevak.

Downwind, I don't just check for carb ice, I leave CH on to prevent it. Why would you put CH to cold on downwind?

Is carb heat going to clear any ice, not at these power settings.

I don't see how the exhaust manifold can get so cold, so quickly that it wouldn't still warm the air.

an over rich mixture is possible leading to a rich cut at the worse possible moment.

Is it?

[Say again s l o w l y]

Applying full power quickly with the Carb heat set to cold, can lead to a rich cut. I've certainly experienced it with a student and it is not funny at all, especially at low level over trees........

Carb ice is an isidious thing and can creep up on you at any time, For many people getting it on final approach would often be masked by the constant power changes.

How quickly will an engine ice up? It all depends on type. Some like the C150 with the Continental engine can be pretty bad, whereas other (most PA28's) don't suffer any where near as much.

[bookworm]

if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1.

What is the proposed mechanism for "detonation"? Other than "it says so in the POH"? It seems counter-intuitive to me. The richer mixture would seem to move further away from the detonation regime.

Genghis? You raised this...

[hugh flung_dung]

An interesting question. I've always assumed that detonation is more likely because the intake air is hotter, but does that argument make sense when the engine is approved to operate in parts of the world that are pretty damned hot? I guess the difference is that the mixture would be leaned to cope with the less dense air but this means less cooling fuel.
Mmm. I await the answer with interest.

HFD

[shortstripper]

Oh dear!

My point seems to be proved! All these instructors and yet not all seem to really understand the principals of carb heat and its affect on mixture and engine performance themselves, let alone teach it. I guess the POH should be followed then as most of us are unlikely to be taught the why's anyway!

I've not yet come across a POH for a VW aero conversion btw, but I suppose they may exist?

I'm no expert, and stand happy to be corrected, but here is what I "think" are the correct answers to the last few posts.

Applying full power quickly with the Carb heat set to cold, can lead to a rich cut. I've certainly experienced it with a student and it is not funny at all, especially at low level over trees........

Unlikely, but theoretically correct. Have you ever tried the same thing with hot air set? ... You'd probably get the same thing and I'd hazard a guess that it wasn't to do with the carb heat setting!

Carb ice is an insidious thing and can creep up on you at any time, For many people getting it on final approach would often be masked by the constant power changes.

Very very true! and yes, in a prolonged descent the exhaust can cool down enough to allow ice to form as the air sucked over it isn't heated enough.

if we carried out a go around forgetting to put the carb heat to cold, detonation may occur causing problems in our engine especially over time and a reduced power output, with less immediate concequences as senario 1.

This is quite incorrect, a leaner mixture (carb heat cold) is less resistant to detonation than a richer mixture (carb heat hot). Bookworm has picked up on this and is right. You may be confusing detonation with rich cut or stumble?

So?

What's wrong with leaving carb heat on hot all the way down? It's unlikely to do the engine any harm and as for unfiltered air? Well that only really applies on run up (and even then, if you do your mag check first, any dust kicked up will have normally blown away before carb heat check). If you are in the habit of pushing carb heat to cold as you apply power it should never become an issue as it's a non-event and full power is available. Keep all power adjustments smooth and things like rich cut or stumble will also be very unlikely to occur.

SS

[Say again s l o w l y]

Actually, I meant to say "with the carb heat set to HOT", that'll teach me not to pay attention!

I tried the same scenario in the same a/c (robin HR200) and it did seem to like rich cutting and a momentary loss of power. It was absolutely fine when the carb air was cold and it required quite a violent push forward of the throttle. If increased at a more sensible rate, then there was nothing untoward noticed whatever the carb heat setting.

With regard to detonation, it could be that Piper were worried about the fact that lots of hotter air than normal going into the cylinders could in theory cause pre-ignition by increasing the temps too much and this then leading to detonation.

As has been said, Detonation is usually caused by too lean a mixture, but other things can cause it. This could be exactly why reading the POH is useful, the manufacturers may know something we don't.

All the PFA a/c I've flown have had Rotax engines, so there are definately manuals for them, as for VW engines........ No idea, they are hateful things in acar or van, let alone in an a/c!

(I've spent far too long trying to get an old camper van back to life after my sister knackered it, what a bag of bolts!)