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Mace

lean for takeoff at high altitude airport?

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Posted (edited)

Quick question as to real-world ops -- if you are in a piston aircraft, on the ground at...let's say Denver, Colorado, or Bogota, Colombia...any high altitude airport...many checklists for piston aircraft say to go full rich for takeoff, or do they mean lean for best power?    I'm assuming the latter, but wanted to see if there is some reason to go full rich I'm not thinking of here.

Edited by Mace

Rhett

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As I could recall, C172's checklist says "(lean if above 2000ft)" after that line.

Maybe some plane just assume flat land T/O and not add that on.

Haven't have my hand on 172 or so for quite while, so I might remenber it wrong, or it's just our 172, as different operator would develope their own checklist.

Some item on airliner fleet checklist also not "suit" for every conditions, like Fuel pumps...OFF for 737 shutdown, but it could be on for APU operation, so you could deviate the checklist, but these step should be in your Amplified or Supplementary Procedures.

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Ok, thanks. 

I'm going to also assume in the real world they lean during run-up/mag check, or the pilot simply knows about where he/she should lean based on experience.


Rhett

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@Mace granted I have never done it myself, but one of my CFI:s adviced me to conduct a full power run up and start leaning until you get max power (RPM) if taking off from an aerodrome with a high DA.
Be adviced that during the run up you will never achieve the RPM you will see during the actual takeoff roll, due to no relative wind.

Let's get @sd_flyer in here. 🙂


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6 minutes ago, SAS443 said:

@Mace granted I have never done it myself, but one of my CFI:s adviced me to conduct a full power run up and start leaning until you get max power (RPM) if taking off from an aerodrome with a high DA.
Be adviced that during the run up you will never achieve the RPM you will see during the actual takeoff roll, due to no relative wind.

Let's get @sd_flyer in here. 🙂

That is exactly how it done. Lean to  get max rpm prior take off (during runup) If everyone had CFI like yours we didn't have fish all those airplanes that ditched into lake departing Big Bear (L35) LOL 

Here is also another important point. Even thought one can lean to get maximum power sometimes in not sufficient if density altitude is high. Going back to Big Bear Airport example on hot summer day afternoon or evening is probably not the best time to depart. So many experience falked opt to depart early in the morning when air is coolers and density altitude ot lowest!

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On 4/19/2021 at 10:24 PM, sd_flyer said:

That is exactly how it done. Lean to  get max rpm prior take off (during runup) If everyone had CFI like yours we didn't have fish all those airplanes that ditched into lake departing Big Bear (L35) LOL 

Here is also another important point. Even thought one can lean to get maximum power sometimes in not sufficient if density altitude is high. Going back to Big Bear Airport example on hot summer day afternoon or evening is probably not the best time to depart. So many experience falked opt to depart early in the morning when air is coolers and density altitude ot lowest!

Most interesting, thanks. What about landing? The checklist generally calls for a full rich setting when starting the descent and approach. Is it the right thing always to do  ?


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Posted (edited)

What you also need to remember, is that with your fuel having less air available to mix with it when up at a higher density altitude, it follows that there will be a similar problem for the wings and the propeller, since both of those need air to function.

So not only will your engine probably not develop full power, even if it was capable of giving you full revs, your propeller wouldn't develop as much thrust as it normally would when at sea level. Likewise your wing will have to go through the air a bit quicker than it would at sea level in order to get enough lift for take off, so your take off roll is likely to be a lot longer than it would be at sea level.

So of course when landing, this also means you will be coming in faster and your higher descent rate might surprise you too. This also means you have to be ready for that potentially higher descent rate, so that you don't over-react to it, freak out and pull the stick back in a panic, which is one of the things which has caused a lot of crashes on finals when people have been caught out by that.

Have a look at this video (below) of a Stinson which had four people on board, but tried to take off from Bruce Meadows Airstrip (near Stanley, Idaho) which is at an elevation of 6370 ft. Even just reading that basic info tells you things are probably going to be problematic for such a low-powered vintage aeroplane.

It does actually make it into the air, but then it is unable to climb or maneuver and it hits the trees and crashes. Don't worry, everyone on board survived this accident, so it's not grim to watch it, but it is a nail-biter and you will be shouting 'abort that take off' as you watch the unbelievably long take off roll. There's tons of time (and room) to close the throttle and simply wait for colder weather, or tell someone to get out and wait on the ground whilst you come back for them later on.

As a result of the trees gradually slowing the thing down during impact, nobody was killed, but there were quite a few injuries such as broken bones, cuts and bruises and such. Sometimes, you have just enough airspeed to take you to the crash site...


 

 

Edited by Chock
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10 hours ago, Dominique_K said:

Most interesting, thanks. What about landing? The checklist generally calls for a full rich setting when starting the descent and approach. Is it the right thing always to do  ?

Well typically rich setting will let you land but not necessarily go around ! LOL You would lean to a pattern altitude of your destination (typically 1000ft AGL for light GA) . That would give sufficient power for in case of go around. 

 

Here what FAA recommends:

At high altitude airports (5000 feet density altitude and above), lean for taxi, takeoff, descent and landing. Use the following procedures:

  • Startup and Taxi: Lean at 1000 RPM (all propeller combinations) until RPM peaks, then enrichen slightly.
  • Before Takeoff: Go to full throttle and lean mixture. With a fixed pitch prop,lean to maximum RPM and then enrichen slightly. With a variable pitch prop on carbureted engines, lean to engine smoothness. If you have an EGT gauge, lean to +100 degrees F. on the rich side of peak. With fuel-injected engine, lean to the correct fuel flow setting according to the POH for your specific airplane.
  • Traffic Pattern Ops: Always lean at traffic pattern altitude for landing at high altitude airports, but only after you have established maximum power. This practice will ensure maximum available power in the event that you need to make a go-around.
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