Incipient rich cut?

[QDMQDMQDM]

Wouldn't mind some advice please.

Cruising today at about 9,000 feet in the Alps at 2400 rpm (Lycoming 0-320), hot day, and thinking 'F*ckin' ell it's high up here', then initiated descent and forgot to put the mixture to rich. Got down to about 6,500 feet and realised the error, so put the mixture to full rich, whereupon coughing and rpm fluctuations from 2000 to 2400 (QDM heartrate stable however at 180). Opening the throttle made it worse and it was only cured by selecting carb heat cold.

Was this due to a sudden overrich mixture, exacerbated by the carb heat hot?

Have flown a further 1.5 hours today and had no problems witt he engine.

Thanks for any input,

David

[Tinstaafl]

High density altitude ==> leaner mixture required
Carby heat exacerbates the apparent density altitude.

6500 doesn't need full rich, especially if it's hot too. Even full throttle (normally aspirated engine) won't produce anywhere near maximum power. Odds are that full throttle will be within the permitted leaning power range.

May I suggest you don't apply full rich on descent but enrichen gradually as you descend?

[slim_slag]

The bit of information we are missing which could confirm Tinstaafl's diagnosis is what your mixture was at 9000. ie whether you leaned for max rpm at a full power run up, and whether from that point you were leaning on the way up.

I never go full rich when it is warm out except when starting, and immediately after I "lean for taxi". When leaning during full power run ups, I start to see significant extra rpm at around 3000ft DA. People are taught to go mixture RICH on the descent because it's easier, being too rich is less risky than being too lean, and as most people fly in the UK appear to fly below 2000ft it probably makes some sense.

So at 6500 ft with above standard temps I would expect you to be overrich at mixture full in. Of course this all depends on how your mixture is set by the mechanic. It's surprisingly easy to foul plugs on some engines if you taxi on full rich (of course power is low), and you also burn more fuel when overrich which if you miscalculate could leave you short.

Mixture and it's effect on power/engine damage is not well understood by most, including me I have to say, you really need instrumentation to know what is going on in your cylinders, and gamijectors are superb.

Mixture setting on an uninstrumented engine is an art, but should really be mastered if you fly in the mountains, especially for take off. Having mixture wrong when hot and high is one of the "gotchas" which can cause you distress. You learn something from every flight, and the good thing is you are less likely to do it again.

[Sensible]

You learn something from every flight, and the good thing is you are less likely to do it again.

especially if the mistake is made around mountains I Guess!!

[AC-DC]

QDM
A carburetted aircraft without EGT gauge is not easy to lean as you have to do it by the ear and the RPM gauge. The rule of thumb is that above 5000’ MSL you should lean to best RPM and then enrich a bit. Your mistake was to go full rich at 6500’ with carb heat on, actually, your carb. was at an higher alt. then the aircraft so the mixture became very rich. Always move engine controls in a slow movement.


Slim S.
One correction to your posting. The engineer can not adjust mixture for any power setting other then idle.

[bluskis]

You said you quickly advanced the mixture, a sudden advance of the mixture lever will result in the symptoms you experienced. I did it once in the UK, a nasty feeling, not forgotten I assure you.
I bet the alps looked great today.

[QDMQDMQDM]

Many thanks, everyone. It's reassuring to hear what you say!

The field I took off from is 3300feet and with an OAT of 31 degrees that gives quite a reasonable density altitude. Sounds like I should be leaning on take-off with a full power run-up.

Also, in these conditions it isn't really necessary to use carb heat at all and frankly the engine does not like it.

For info, my friend's 90HP PA18 has its C-90 wired full rich and it didn't like it by the time it got to 10,000 feet, coughing a bit etc.

Ah, the adventures of lowlanders in the mountains!

Thanks again,

David

[Flyin'Dutch']

QDM,

Nuff said about the mixture methinks.

Can you tell us a bit more about your adventure to date!

FD

[QDMQDMQDM]

Hi FD,

Yes enough about the mixture (went much better tonight actually, given the advice on this forum).

The adventure so far is proving rather fun. Left the UK on a glorious day, June 12th, in formation with my travelling companion in an L21C. (I am in a PA18-150.)

1st leg, Eggesford --> Old Sarum, also in company with an Auster from Eggesford on its way to the KZ fly-in at Stauning in Denmark. Met an L4 pilot at Old Sarum about to depart for the same destination as us (the French Piper club rassemblement at a private field near Lisieux in Normandy), except he was going straight across the water to Cherbourg, two-up, and we were going the long way round in Super Cubs, one-up. Anyone familiar with the L4 / J3 will understand that this chap deserves some kind of merit award / one-way ticket to a lunatic asylum! Needless to say he beat us to Lisieux by some hours.

2nd leg Old sarum -- Headcorn via the South Coast. No problems and I can thoroughly recommend Headcorn as a departure aerodrome. The fellow in the 'tower' there is so helpful.

3rd leg Headcorn -- L2K. Three of us (the two cubs and the auster) set out in formation and we waved goodbye to the auster halfway across as he coasted off North to his night stop of Midden Zeeland. (Gasp, horror, we might have been talking to each other on an air to air frequency!) You could see the French coast from Ashford, so no big deal for the crossing. L2K itself was a doddle and an introduction to the laidback nature of things to come.

4th leg L2K -- St. Pierre des Ifs (Terrain de l'angleterre), near Lisieux. From L2K it was low level down the coast, skirting round and under the Deauville TMA to Lisieux, arriving as everyone was departing for dinner at a little bistro. There were two British Vagabonds, a Colt, the L4 and a Super Cub like mine already there, so the flag was well and truly being flown. Huge friendliness all around, and even the small contingent of short, fat, elderly gents who ran around getting a bit steamed up about where you parked were extremely pleasant and on the whole helpful. Amazingly, Richard and I were the only ones camping out of about thirty aircraft.

To be continued (when I am a bit less pissed).

David

[QDMQDMQDM]

OK, if anyone is reading, here is some more:

Friday June 13th, there was an organised coach trip to the beaches for participants, but the visibility was 50 miles +, so Richard and I took the cubs and flew along the beaches low-level. Highlights: the US cemetery at Omaha beach and the Mulberry harbour at Arromanches. Then we stopped at Lessay on the west coast, giving me my first chance to practice the inky-pinky in circuit and joining calls -- no big deal for anyone with O-level french. People there very friendly and again extremely laid back. From there to Granville for lunch. Ditto on the laid back. The taxi driver there was an ex-trucker / novelist / pilot -- good fun, and I believe most of what he told us. Then back across the peninsula to Lisieux in time for dinner at the cider factory. Mon dieu! Since Richard and I were the onlyu campers, the owner of the field was driving us back. This meant that after everyone else had been bussed back to their hotels we had to stay behind for 'further discussions', which involved 40-year old Calvados and much hilarious badinage before a swift backroads trip back to the airfield at about 01.30.

Saturday June 14th, thunderstorms, one after another. Jerome, a charming Swiss Easyjet captain with the most immaculate L4 ever seen, was terrified at the thought of hail, but although the storms were severe, hail never materialised, thankfully. Lunch was a massive affair at the airfield, with the local classic car club, hunters, chamber of commerce, dignitaries, etc etc in attendance. After various prizes were presented, each aircraft crew was presented with a massive hamper (Pommeau, calvados, apple jam etc.) by the chamber of commerce. Incroyable, hein?

The thunderstorms cleared up about 5pm and, unable to stomach the idea of more food and speeches and wishing to get on with the trip, we said our goodbyes and left to refuel at Bernay. I got caught in my companions wake in take-off which was interesting as I waited for the dropped wing to come up as we headed for the trees. (It did eventually.)

From Bernay, where a Cap 10 was doing aeros in the overhead, we headed 100 miles south east for Montargis, where we landed at abolut 20.45 to a party in full swing, after a pleasant, but soporific flight similar to a cross-country acrosss Nebraska.

TBC...