High Density Altitude Takeoff.

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rookiepilot
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High Density Altitude Takeoff.

Post by rookiepilot » Wed May 08, 2019 6:44 pm

As summer is rapidly approaching and with the short field thread, here's a complementary topic, for those hot days at a high elevation airport in a single engine AC.

This is from memory and my own observations, sure techniques will vary.

Especially appropriate to flying in the western US, which has many airports over 7000 ASL.

1. If any doubt on the length of runway, absolutely run the numbers from the POH, using the correct density altitude. As 30 + year old piston engine is unlikely to make full rated power as when new -- add a healthy safety margin. Top up your oil. Not enough oil, will make your engine run hotter.

2. As Pilot Dar stated, consider changing to an early morning departure, lighter weight, or more favourable wind if necessary.

3. Ok, numbers check out. Consider departure terrain as well as wind, both the elevation changes off departure, are turns possible towards more favourable terrain, and how friendly or dangerous is the terrain in case of engine failure. In light wind scenarios, these considerations will factor into your runway choice.

Be wary though of too much wind as well. When taking off into an area of surrounding higher terrain (or transiting). Check upper winds. Too - Strong winds can create downdrafts that you may have difficulty climbing out of.

4. RUN - up. Do your normal routine, but if it's really hot out -- I'm thinking of a takeoff in Montana when it was 35C and 5000 + ASL -- try to do run up near the end of the runway. Try not to linger on the ground once ready, as engine temps are a factor in your climb out.

Run up rich at 2000 RPM, then slowly lean out until MAX RPM -- best power -- at that power setting is achieved-- then richen no more than one - half turn. (To protect against detonation at full power) Then leave it alone, pre takeoff checks and departure with mixture in that position. (Non turbocharged) Cowl Flaps of course open and leave open for climb.

5. In the climb, watch your Oil temp and CHT's. Try to keep CHT's below 400. 380 or so is better. I stayed too long waiting for takeoff clearance in Montana, takeoff was fine -- long runway -- but my CHT's briefly hit 430 on climbout. I stopped the climb -- terrain allowed -- and richened just slightly. Be careful doing that, going full rich will reduce power noticeably. Step climbs may be required. In any case be prepared for a very slow climb in a light non turbo single -- 200 Ft / min might be a good rate!
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Re: High Density Altitude Takeoff.

Post by photofly » Wed May 08, 2019 7:09 pm

Full throttle full rich at the threshold, then lean the mixture back smoothly and rapidly until you feel the acceleration peak (variable pitch) or rpm peak (fixed pitch). If your altitude is sufficiently high, a normally aspirated engine can't generate detonation at any mixture setting because the engine can't develop enough power. You want the mixture for maximum power at full throttle, not at 2000RPM.

Once you're airborne, it's just like the last part of a regular climb to altitude. Manage the engine just as you would in that situation. If you don't have a problem with engine temperatures climbing at 7000' when the ground is at sea level, you won't have temperature problems climbing at 7000' when the ground is at 6500asl.
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Re: High Density Altitude Takeoff.

Post by rookiepilot » Wed May 08, 2019 7:44 pm

photofly wrote:
Wed May 08, 2019 7:09 pm


If you don't have a problem with engine temperatures climbing at 7000' when the ground is at sea level, you won't have temperature problems climbing at 7000' when the ground is at 6500asl.
It's not exactly the same thing, due to temperature considerations, both OAT -- it would tend to be hotter on the ground at 7000 ASL --- and in your engine when the climb commences.

From 7000 feet, to climb to 9000 i would use a cruise climb on a hot day, and that would preclude engine temp issues. Take all day if necessary. Lots of airflow. That's not always an option on takeoff.

Starting at 7000 ASL on the ground on a hot day, the engine could be already well into the mid 300's from the run up, before one advances the throttle for takeoff. Airflow has not yet become a factor to effect cooling. I've seen the temps jump really fast in those conditions.

Doing a full power run up prior is an option, but will heat the engine that much more before takeoff. I think 2000 should be acceptable too.

One thing to consider also, make sure to run up into wind, especially when it's hot.

How much of an issue engine temps would be on the ground, would vary with the engine, and how it's cowled, condition of the baffles ect.
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Last edited by rookiepilot on Wed May 08, 2019 8:05 pm, edited 1 time in total.

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Re: High Density Altitude Takeoff.

Post by Big Pistons Forever » Wed May 08, 2019 7:52 pm

Photo

How many 7000 ft + DA takeoffs have you done ?
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Re: High Density Altitude Takeoff.

Post by photofly » Thu May 09, 2019 1:35 am

rookiepilot wrote:
Wed May 08, 2019 7:44 pm
It's not exactly the same thing, due to temperature considerations, both OAT -- it would tend to be hotter on the ground at 7000 ASL --- and in your engine when the climb commences.
If the OAT is the same, the climb is the same. Obviously if the OAT is different, then it's different. That's my point. A climb at 7000' ASL doesn't depend on where the ground is.

My engine temperature stabilizes pretty quickly once I'm moving at 130kts. The engine temperature twenty seconds ago has little relevance to the temperature now. You'll find that too - adjust the mixture and/or the cowl flaps and the CHT fixes itself pretty fast.
How many 7000 ft + DA takeoffs have you done ?
You are welcome to assume, as with everything I write, that I've never sat in an aircraft, much less piloted one. Your corrections to anything I wrote that is wrong are always welcome and appreciated.

When you takeoff at high altitude, do you fix your take-off mixture during a runup at 2000rpm or adjust it at full throttle during the roll?
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Last edited by photofly on Thu May 09, 2019 1:40 am, edited 2 times in total.
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Re: High Density Altitude Takeoff.

Post by photofly » Thu May 09, 2019 1:36 am

double post
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Re: High Density Altitude Takeoff.

Post by Aviatard » Thu May 09, 2019 3:46 am

rookiepilot wrote:
Wed May 08, 2019 6:44 pm

Run up rich at 2000 RPM, then slowly lean out until MAX RPM -- best power -- at that power setting is achieved-- then richen no more than one - half turn. (To protect against detonation at full power)
I'm curious why you would use 2000 RPM instead of full throttle. A few Cessna POH's I've read showed that leaning is allowable in climb at full throttle above 3000 feet DA because the engine can't produce more than 75% power.
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Re: High Density Altitude Takeoff.

Post by photofly » Thu May 09, 2019 4:25 am

Indeed. See note 2:
87B4BF23-5F53-4DA8-96DA-4CE6C5EE46B1.jpeg
87B4BF23-5F53-4DA8-96DA-4CE6C5EE46B1.jpeg (191.45 KiB) Viewed 546 times
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Kirk: This is a dangerous mission. Likely, one of us will die. The landing party will be me, Spock, McCoy, and Ensign Ricky.
Ensign Ricky: Aw, crap.

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Re: High Density Altitude Takeoff.

Post by rookiepilot » Thu May 09, 2019 5:31 am

Aviatard wrote:
Thu May 09, 2019 3:46 am
rookiepilot wrote:
Wed May 08, 2019 6:44 pm

Run up rich at 2000 RPM, then slowly lean out until MAX RPM -- best power -- at that power setting is achieved-- then richen no more than one - half turn. (To protect against detonation at full power)
I'm curious why you would use 2000 RPM instead of full throttle. A few Cessna POH's I've read showed that leaning is allowable in climb at full throttle above 3000 feet DA because the engine can't produce more than 75% power.
Yes, for run up. You could if engine temperature is not an issue. It's primarily not to overheat the engine on the ground. I found in a 172, temps less of an issue, climb rate was a greater issue. One can carefully adjust in the climb, too.

In the 182, climb rate obviously better, but temps were a major issue in those conditions. Temps over 400 are a bad idea for any period of time. Complicating factors can be no wind, to help cool the engine on runup, then delays from ATC to takeoff after runup is done.

I've never done a high DA takeoff off a really short runway, to be fair.

I'm also not sure, I'd want to do a full power static run up on gravel or grass.
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Re: High Density Altitude Takeoff.

Post by Big Pistons Forever » Thu May 09, 2019 7:28 am

My personal experience has been that a brief full power run up to lean the mixture has not resulted in excessive CHT’s. However the engine will definitely heat up on the ground so you want to be efficient when doing the runup checks to minimize the time the engine is running above 1000 RPM

I would not advise adjusting the mixture during the takeoff run
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Re: High Density Altitude Takeoff.

Post by rookiepilot » Thu May 09, 2019 6:11 pm

https://rodmachado.com/blogs/learning-t ... de-takeoff

Good video on the subject.

Rod recommends full throttle run up, lean max rpm, slight richen to keep mixture on rich side of peak, adjust as needed in the climb. (Fixed pitch)

For Constant speed, run up, lean to max EGT on hottest cylinder, richen 50- 75 degrees. I've used this method.
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