Leaning at Take-Off

[SkyHawk-N]

Two questions to all of you who have high-altitude airfield experience.

My understanding is that you should lean your mixture when departing a runway that is at an altitude, or has a density altitude, greater than 5,000ft (see edit below). Is this correct? I have read the Cessna 172 POH but cannot see anything on the subject.
EDIT: I just found it in the POH, section 4-14, although it states 3,000ft and not 5,000ft.

Secondly, I was taught to lean before or at the power checks, but have been told that for cooling reasons it is best to lean during the take-off roll. Does anyone have a preference or an idea of which method is preferable?

[Islander2]

was taught to lean before or at the power checks, but have been told that for cooling reasons it is best to lean during the take-off roll.Best practice will have you taxiing and undertaking power checks with the mixture substantially leaned in any event, high density altitude or not. There are NO cooling reasons that go against this SOP.

Obviously take-off at sea level should be done full rich. Fine tuning of lean mixture adjustment for high density altitude take-off can either be done during the power check or on the take-off roll. Some (e.g. Deakin) argue the latter gives the best feedback of optimum setting.

[Adrian N]

Fiddling with the mixture control during takeoff is likely to be a bit approximate in a 172. How will you tell when you've leaned to maximum power when you have a fixed-pitch prop and rpm is increasing as you accelerate?

My experience taking off at high altitude in simple aeroplanes, based on trial and error in an O-360 engined Jodel, is that full rich works fine for altitudes up to about 3000 ft. By 5000ft pulling the mixture lever back about 1cm seems to work, and by 10,000ft 2-3cm gets you off the ground (or rather snow) nicely. My very rough and ready method of finding the right amount to lean is during the power checks, at 1800rpm; weaken the mixture until rpm stops rising then enrich it slightly, throttle back, finish my checks then line up and go.

[Cusco]

For my High Altitude check out prior to rental at Big Bear (6748amsl) in an Arrer was to conduct standard power checks at 2000 full rich, then just prior to take off at the hold to apply full power on the brakes, then slowly lean till engine slows then richen up again, just to peak power keeping an eye on EGT.
Still took most of the 5800ft to get airborne but I did have a pie eater in RHS.
Trying to lean while preoccupied with take off IMHO not a good idea: if you over lean and the donk stops there's a 'kin great (cold) lake at the end of 28
Cusco:rolleyes:

[Keef]

I never eat pies ;)

That Arrow, 2-up, ought to have been able to get out of Big Bear with no bovver.

[Codger]

I use Adrian's approach.
At 4000 feet ASL with an ambient of 40C getting all the power available is something that I'm looking for. It used to be a surprise seeing just how far out that mixture knob is in those conditions.

[SkyHawk-N]

Best practice will have you taxiing and undertaking power checks with the mixture substantially leaned in any event, high density altitude or not. There are NO cooling reasons that go against this SOP.

Islander2, I have been told by several people who regularly fly from high altitude airfields that they do not like sitting at the end of the runway with power on while doing their leaning as there is inadequate air to cool their engines sufficiently. Some of the places I flew at a few years back it was sometimes difficult to breath after a short walk and overheating engines were a regular occurrence.

Leaning while doing the take-off roll seems to be a common practice. Full power, check temperatures and pressures and immediately start to lean until the engine begins to feel rough, then adjust back from there until it is smooth again. All this while continuing the take-off roll! It surely must lengthen the take-off distance.

Adrian N, that's pretty much what I was taught on Cessnas (without fancy gizmos like EDMs), you lean until the revs start to dip then you turn the mixture control forward two complete turns, this was done at power checks.

Maybe this leaning-at-take-off method also has something to do with operating from non-tarmac surfaces and the risks from shot blasting your prop on gravel while you sit stationary leaning at full power (?).

[172driver]

What I was taught and use if operating from high (and mostly hot) fields is to lean as the last step in the power checks. Basically, you bring the mixture control out until the engine starts running rough and then make two full turns towards rich (this obviously on a Cessna with the plunger-type controls). Doing this the first time it is quite amazing to see how far out the mixture knob is.

On most of the fields I had to do this the luxury of playing with the mixture during t/o roll most definitely does not exist - if you are trying to coax the thing into the air off a 1000m sand strip at a DA of something like 7 or 8000 ft you need all the power you can get from the word 'go'.....

OTOH, on this type of strip you don't have to wait for any traffic or clearance, so you go immediately after the power checks.

[tmmorris]

Surely better to hold it on the brakes right at the end of the runway for a couple of seconds to get max static RPM before releasing the brakes? Any engine overheating quickly reduced by airflow once the brakes are released. If ATC ask for an immediate takeoff say no!

Tim

[RatherBeFlying]

It's been a long time, but I recall the drill at Calgary Springbank in a Cheetah was to lean to max revs, then add a bit more.

Make note of that setting for touch and goes -- that way the gophers at the far end of the runway don't take fright:}

[fernytickles]

On a ferry flight in hubby's old Citabria, which ran rich at the best of times, I stopped in Flagstaff, AZ. According to Airnav.com KFLG is 7014 ft. / 2137.9 m (surveyed) high, and the runway is 6999' long.

Having no prior experience with high altitude take offs/landings, apart from landing there an hour earlier, I just listened to the engine and leaned it until it sounded happier. Left that setting for take off and everything went fine. I seem to remember the mixture lever was almost 2/3 leaner than its normal setting...

If it had been something more complex, I'd have read up more in the books, but the Citabria didn't have much in the way of detailed operating books. It was more a case of try to recall what is written about high altitude operations, then "sook it and see".

[Codger]

At the risk of making an ASSumption... Everyone here does actually calculate Density Altitude before even walking out to the plane... Yes?


RatherBeFlying
Ever use the Banff strip back in the day? That one could increase the pucker factor on a hot day.

[172driver]

At the risk of making an ASSumption... Everyone here does actually calculate Density Altitude before even walking out to the plane... Yes?

Yes - IF flying in hot n'high conditions from marginal strips. At SL with 10.000ft tarmac ahead, I admit, I don't. That said, I once took off in a standard 172 from Seville (which is more or less at SL) in 49C temp, no wind - interesting t/o run and climb performance :eek:

[RatherBeFlying]

Codger, sadly Parks Canada decided to improve the ecology by closing the Banff strip to non-emergency traffic before I got the chance, but maybe someday I may have to make an outlanding there;)

[Adrian N]

At the risk of making an ASSumption... Everyone here does actually calculate Density Altitude before even walking out to the plane... Yes?

RatherBeFlying
Ever use the Banff strip back in the day? That one could increase the pucker factor on a hot day.

Well to be honest, no, I almost never work out the density altitude. Most of the flying I do is from high altitude strips, between 3000 and 8000 ft altitude in summer, and up to 12000 ft in winter. They are mostly sloping and the combination of slope and wind makes it impractical to calculate take off distances. Thanks to the terrain, you can almost always descend after take off, so if you can stagger into ground effect before the end of the strip you'll be ok. If it's very hot there are a few strips that I wouldn't go to unless the aircraft was very light.

It is, of course, a different story if you are using a flat runway. But again, I confess that I wouldn't normally calculate the density altitude. For one thing, the take off and climb figures in the POH for my 45 year old aeroplane don't bear much resemblance to reality, and I find that the best approach for me is to be familiar with its actual performance in different situations, to know how it climbs at different weights at altitude, and to take a very cautious approach to take off distance and climb angle in "hot and high" conditions.