High Density Altitude Operations

[flyer338]

My previous Mooney was a much modified 1965 C model and over more than a thousand hours in the plane, I came up with a rule of thumb and take-off technique for high density altitude departures. The rule of thumb was that at gross weight I needed a runway 2/3 of the density altitude in length. I think it was conservative, but it worked.

The take-off technique was this: at 70 mph raise the nose wheel one inch, then accelerate in ground effect to 80 mph; at 80 mph raise the gear (this plane was converted to electric gear by Dugosh at 25 hours TT); at 90 mph raise the flaps, and climb out at 100mph. This worked well, but one must have a feel for the plane.

Now, many years later, I have a J model, and we live in the western desert  where the summers are hot. I am based at North Las Vegas. The temperature this time of year is 90 degrees at sunrise. The J model requires right at a 1,000’ to lift off at 2500 lbs under these conditions. I calculate the density altitude to be 4,650’.

Other airports in the area are higher and still hot. I took my wife (her 2nd flight - she is doing well at becoming acclimated to Mooney flying) to look at a piece of property at Panguitch Lake, Utah. We landed about 0900 at Panguitch airport (6700’ field elevation and 5700’ runway with no obstructions). My plan was to be airborne again by noon or 1:00. We were light on fuel, and we are light as a couple, so our take-off weight was about 2350 lbs. The temperature when we departed was 88 degrees. That is beyond where the POH charts provide guidance for gross weight take offs.

I calculated the density altitude as just over 9600’. I remembered the rule about 70% of take-off speed within 50% of the runway length.

My plan was to raise the nose at 60 knots, remain in ground effect until 70 knots and raise the gear, raise the flaps at 80 knots, and climb out at 90 knots (Vy+). It worked perfectly. We lifted off in 1500’ and were at 90 knots within 5000’.
On climb out I needed 100 kias to keep the CHT at 380 degrees or below. But we still were able to climb at 500 fpm. 
When planning this departure I considered the rate of climb I had so far experienced in this plane at heavier weights and at density altitudes as high as 18000’.

I am interested in others’ experiences with high density altitude departures in normally aspirated Mooneys.

Edited August 22, 2020 by flyer338
because

[carusoam]

Great details, Brian!
 

Got any pics to go with that?  

Best regards,

-a-

[flyer338]

Here is a picture of the love of my life and my Mooney at Panguitch Airport.

[msh9]

I have a M20E based in Salt Lake, by mid-morning in the summer the DA is typically 6700'-7200'. I don't necessarily make drastic changes to my technique. I always brief what I'm about to do before doing it, typically some flavor of,

• Apply gentle back pressure at 70 MPH to start lifting off
• Initial climb out at 85 MPH
• Gear up when I feel there is a comfortable amount of air beneath me, usually ~100' AGL
• Start accelerating, wait a beat, and then bring the flaps up
• At this point I'm normally past 90MPH and accelerating to 100 for a Vy climb to 1000' AGL.

Some notes, I have an early M20E with the lower flap speed and a manual gear. Even at high DA the flap speed limit and the speed limit for my arm to bring the gear up come up pretty fast. My normal technique is to tidy up and set for Vy quickly because of those low speed limits. I suspect you can be a little more leisurely about things in a later model M20.

75% of rotation speed at 50% of runway used is a rule of thumb I have used before at new to me airports. One other thing I do, I use a gopro to film my take-offs and landings for review later. I use the take-off footage to roughly estimate the real amount of runway used. After the flight I note how much runway was used in the video and correlate it to the DA around the time of take-off. It's not super accurate, but I am slowly making a scatter plot chart those shows a nice correlation between DA and runway use. The other nice thing about this is these numbers are accurate based on my (sometimes lackluster) technique instead of a factory pilot.

Edited August 22, 2020 by msh9

[kortopates]

2 hours ago, flyer338 said:

I am interested in others’ experiences with high density altitude departures in normally aspirated Mooneys

Yep, that's exactly the way to do it. Perhaps an even better way to learn the technique is in an Arrow - only because the service ceiling with the gear down is significantly lower and well below 9K, I recall about 7700'DA from memory. Thus till you learn to pull the aircraft up into ground effect and accelerate to a safe airspeed to retract the gear, the pilot is not going to be successful climbing out with the gear hanging out! Then add some mountain field turbulence and some wind and things go down hill real quick. Mooney doesn't publish a service ceiling with the gear down or provide a performance table of climb rate with the gear down like some others do, so every pilot has to aware of that. 

The only other consideration I would add is that if the winds are gusty as they often are in the mountains, I raise the gear retract speed up to 1/2 the gust factor over Vx  at a min or even Vy with lots of runway for added margin in settling. Solo pilots taking off for short hops have the advantage here, but that has never been me with two pilots and enough fuel for at least 2 -3hr flight back. 

[Comet]

I have a M20F based at an airport with an elevation of 7171’.  DA in the summer is usually well over 9000’; afternoons are often over 10.  Once DA is over 10k, I run the numbers and decide if I really need to go.  Obviously, lean the mixture during run up for best power (I still find it weird to go full rich at low elevations).  The runway is 6000’, but I still usually use Short field technique; hold breaks until MP is over 20”, then release.  I pass 60 mph at ~1000’, if I don’t rotate by 2000’, I would reject the TO (hasn’t happened yet).  Gear and flaps up with sustained positive rate (~85 mph); the terrain drops off after the end of the RW, so after I get to ~100’ AGL, I’ll  accelerate to 110 or so to help with engine cooling.  When I go to nearby airports (which are lower, but don’t have the terrain advantages), I’ll usually climb at 90 mph up to pattern altitude.

[jaylw314]

Nice!  Thanks for the description of your procedure!

The highest DA I've taken off in was about 7000', from a 4000' runway, with 4 people and near max gross weight.  It took almost no runway to get off the ground with a 32 knot headwind (luckily not much of a crosswind, but a pretty exciting parking job!), but it seemed like it took forever to climb.  Luckily it was in a pretty flat area (Tulelake CA)

[0TreeLemur]

I used to live in Laramie, Wyoming, which has a field elevation of 7200 ft.  Often windy with deep gusts.  Std. temperature at this altitude is 0C, so any time the air temperature is warmer than freezing, the DA is above 7200'.  Winters are really cold and windy.  Terrible place to fly a NA aircraft.   When I lived there I didn't even consider owning an airplane.  We bought our Mooney after moving away to greener, warmer, lower, pastures.

The last time I took off from there in August, 2018, after visiting our son in our C-model the the air temperature was 26C, and the DA was over 10,000'.   The POH t/o performance chart only goes to 5000' but with winds of 270 at 20 gusting to 30, I figured the 6300 ft long rwy 30 would get us off the ground.  If not off by taxiway B3 I would have over 1200' remaining to abort.  That morning my son wanted to go for a ride, but I thought best that my co-pilot and I had better just get out of there before it got any warmer/gustier.

Despite having a decent headwind component, it took over half the runway to get off the ground.   At about 20 or 30 ft I raised the gear and she really didn't want to climb so I lowered the nose and flew level to build up some airspeed then we climbed out. 

Often when flying on large aircraft I'll time the t/o run.  A 30-40 second t/o run is normal in a fully loaded B747 or A380.  It felt like our t/o run in the Mooney that day took over 30 seconds, and that was pretty weird.  Having an abort point in mind was comforting.

[flyer338]

Our Mooney POH are deficient in data for take-off at high density altitudes. The lack of data does not mean the aircraft is not capable given the right technique. I am firmly convinced that using the runway available after lift off to gain speed while in ground effect is a game changer that works for Mooneys and other NA low wing airplanes.

[EricJ]

2 hours ago, kortopates said:

Yep, that's exactly the way to do it. Perhaps an even better way to learn the technique is in an Arrow - only because the service ceiling with the gear down is significantly lower and well below 9K, I recall about 7700'DA from memory. Thus till you learn to pull the aircraft up into ground effect and accelerate to a safe airspeed to retract the gear, the pilot is not going to be successful climbing out with the gear hanging out! Then add some mountain field turbulence and some wind and things go down hill real quick. Mooney doesn't publish a service ceiling with the gear down or provide a performance table of climb rate with the gear down like some others do, so every pilot has to aware of that. 

I flew home from Sedona once with the gear down due to a mechanical issue, and my airplane (M20J) doesn't climb worth crap with the gear down.   The difference was remarkable.

[MooneyMitch]

My most memorable high density departure [aside from several summer time departures out of South Lake Tahoe] was Big Bear, CA, August 2008, '65 M20E.

We watched an M20C depart earlier at 105 degrees with full tanks, 3 souls aboard.................we all thought it was the end for them.............somehow they made it, barely!! Lots of gasps from our gang as we watched the Mooney nearly stall a couple of times upon liftoff................then settle into ground effect long enough to gain enough speed to pull up and barely clear the trees at the end of the runway.  Once over the obstacles, we saw the Mooney sink, thinking that was the end............amazingly they made it out .....................We decided to wait!!! 

Several hours later, half tanks, two on board, we calculated and decided to go.  Per digital sign at beginning of runway, 95 degrees and 10K density altitude!  50/70 theory was used......... gently and carefully away we went....................note to self.................NEVER AGAIN!! 

[Ragsf15e]

4 hours ago, EricJ said:

I flew home from Sedona once with the gear down due to a mechanical issue, and my airplane (M20J) doesn't climb worth crap with the gear down.   The difference was remarkable.

Any idea how high you were able to climb?  This would be nice info to have. 

[EricJ]

1 hour ago, Ragsf15e said:

Any idea how high you were able to climb?  This would be nice info to have. 

7500 ft.    That was all I needed for that particular trip.    It would have gone higher, it just wasn't climbing very fast.

[PT20J]

A couple of points:

1. Remember that Vy decreases with density altitude and Vx increases. At DAs above 8000’ I like to use a climb speed halfway between SL Vx and Vy. 

2. The oft used 70% takeoff speed at 50% runway length mathematically works out to liftoff speed being reached at the end of the runway (I posted the derivation a while back). So, better be more conservative unless there are no obstructions.

Skip

[Ross Taylor]

What @Comet said ^^^.  Our field is just about 7k actual, with afternoon temps in the high 80s or even low 90s...so our DAs are pretty impressive.

[midlifeflyer]

The quasi soft field technique of getting into ground effect to remove ground friction and accelerate to climb speed at very high density altitudes is fairly standard across makes and models. Even works in a light sport at 10,000' D-Alt.

Captain Obvious: you gotta know what you are doing.

[Ibra]

3 hours ago, midlifeflyer said:

The quasi soft field technique of getting into ground effect

The soft air technique?!

[brndiar]

To calculate for my C I use "Koch Chart"

Lg,

M

[Ibra]

Yes one can use Koch Chart but it assumes 16kft ceiling under ISA and normally aspirated aircraft (which is sensible IMO but can be adjusted to 20kft or 10kft ISA ceiling by setting 0% ROC on those lines), but it has a temperature factor, as the adjustment is applied as multiplier to seal level ground roll and sea level ROC it is far accurate than using abosolute POH numbers...

Same as 50/70 rule it is agnostic to POH absolute data but assumes constant acceleration along the runway but it may not work with slope or tailwind but it will work if you are flying on left mag only unlike POH data 

One can derive relative factors using some rule of thumbs: +10% ground roll for each +1kft DA or ground roll = 2/3*DA under ISA temps but they rarely account for temperature corrections (about +10% ground roll for +10C (or 18F) toward infinite ground roll at 60C/140F) or aircraft ceiling, this can be takeoff elevation in underpowered types...

In nutshell, you only need to know YOUR Sea Level climb rate and YOUR Sea Level ground roll and use aircraft ceiling to interpolate a good guess, for climb perf, if you have aircraft POH ceiling you can interpolate YOUR Sea Level climb rate at various altitudes

For ground roll, roughly % distance change = 3 * %TAS change - % available power change, this comes from simple energy/distance/power/speed formulas 

Ignoring temperature adjustment, TAS change is 2% for each 1000ft DA and power loss depends on aircraft POH ceiling and % available power vs altitude curve but for NA engines it goes down linearly to zero at the ceiling 

So for each 1000ft DA one is looking to add 11% or up to 16% depending if his aircrafts POH ceiling is 10kft or 20kft and this is assuming it is not a hot day 

Turbo guys have better power vs altitude profile flat untill 12kft before it decays but they still need to add 3*TAS % change to get % increase in ground roll 

Edited August 23, 2020 by Ibra

[gsxrpilot]

If anyone would like to practice or work this out in the airplane, come visit Denver in the summertime. We've got DA's that are sometimes over 10,000 ft. and long unobstructed runways. Take off from Runway 08 at KCFO (formerly Front Range), and as long as you can clear the airport fence and the odd prong horn antelope, the ground drops away for the next 650 nm all the way to the Mississippi River. 

[midlifeflyer]

 

3 hours ago, gsxrpilot said:

If anyone would like to practice or work this out in the airplane, come visit Denver in the summertime. We've got DA's that are sometimes over 10,000 ft. and long unobstructed runways. Take off from Runway 08 at KCFO (formerly Front Range), and as long as you can clear the airport fence and the odd prong horn antelope, the ground drops away for the next 650 nm all the way to the Mississippi River. 

BTW, Paul, the 10,000' D-Alt takeoff in an LSA was after breakfast from Kelly Airpark. The have (or used to have?) an annual fly-in breakfast. 90°F. If they are still doing it (2020 excepted), it's worth the short flight.

[gsxrpilot]

Just now, midlifeflyer said:

BTW, Paul, the 10,000' D-Alt takeoff in an LSA was after breakfast from Kelly Airpark. The have (or used to have?) an annual fly-in breakfast. 90°F. If they are still doing it (2020 excepted), it's worth the short flight.

Thanks I'll have to look it up and attend.

[PT20J]

13 hours ago, gsxrpilot said:

If anyone would like to practice or work this out in the airplane, come visit Denver in the summertime. We've got DA's that are sometimes over 10,000 ft. and long unobstructed runways. Take off from Runway 08 at KCFO (formerly Front Range), and as long as you can clear the airport fence and the odd prong horn antelope, the ground drops away for the next 650 nm all the way to the Mississippi River. 

Fondly remember my morning trip up to Leadville in my ‘78 J to get a T-shirt years ago. An army CH-47 dropped in to get T-shirts also.

Skip

[carusoam]

I was wondering... helicopter and high DA...

https://www.army.mil/article/137584/ch_47_chinook_helicopter

Go Army!

-a-

[J0nathan225]

Blackhawks visit up there too.  I didn't get a shirt, but got some good pizza!