Has anyone established a firm cutoff point with regard to density altitude? I grew up flying around Logan, Utah (KLGU) and know how a loaded 152/172 performs when its 80 degrees and the field elevation is around 4,500. That equates to a DA of 7,000 and makes climbing a very slow (~300 FPM) process unless you lighten up. I suppose it depends on weight and what terrain you are trying to get around but I was wondering if anyone had a set limit with their aircraft.

I'll do DA of 11,000 ft if solo in my 85hp J3 w/ at least 4,000 ft of runway. Never with a passenger.

Too many variables to make a DA call. My general rule of thumb is consistent and applies at all weights, das, fields, surface types. If I've not reached 70% of rotation speed by 1/2 of the runway - abort the takeoff immediately. Always - no exceptions.

Better to be on the ground wishing you were in the air than (barely) in the air wishing you were on the ground.

Rule to live by.

In my old C170, I scaled the takeoff weight with the density altitude. If it was warm and the field elevation was high, I would fly with no more than two on board, 50 lbs of gear and half full tanks. It still performed pretty poorly. If it was hot and the field elevation was high, I would stay on the ground. If it was cold and high, or warm and low, I would fly near gross weight, and expect pretty marginal performance. By myself on a cool morning, the 170 was great!

In my C180, I still try not to fly in the heat of the day. I seldom fly at gross weight either. Usually just two or three on board, half tanks and 50 lbs of gear... The takeoff and climb performance is pretty good on the 180 when operating three to four hundred under gross.

No firm numbers for me, I just try to keep trends in mind to maintain nice meaty margins.

Check out the following link for some excellent info on Density Altitude and making a decision to lighten your load…
Superb review of many aspects of loading vs. DA and performance…

http://www.canyonflying.com/lightenup.html

Hers a video that can help promote serious thought on DA

And some other thought provoking tidbits:



And this thread from six years ago:

http://www.backcountrypilot.org/forum/b ... ender-2572

And the always enjoyable and informative FAA safety film from the sixties:

The fact you are inquiring about it, tells me you have the hard part figured out.
I admire you for asking the question, as it's easier to stay alive by learning from others mistakes
I'm spoiled by my current plane, but I have a few stories…
I used to fly the Grand Canyon in C model 402's.
On a typical Summer day the DA would be close to 10K.
Climbing out at 140kts the CHT's were redline.
If one quit, and you would need to slow to "Blue Line", the running engine would melt.
With both running it wasn't pretty! I sure like those big bore Continentals!

Toluca Mexico, 8466"MSL, runway length 13780'.
Challenger 601-3R.
Departing there for SVMI (Venezuela) the balanced field length for takeoff was over 11000'.
My rules were Day/VFR and I always kept near new tires on the plane.

DA is for real!
Your inquiry regarding it, tells me a lot about you

237

Since there are so many variables, as others have said there is no single number.

I have used a T.O.P. take off performance computer for a long time to come up with a go/no-go solution and or to flight plan

See: http://www.sportys.com/PilotShop/product/9305

Since I am a bit of a Luddite, I like this really simple manual tool which takes into account, Pressure Alt, Temp, runway surface, slope, % gross weight and wind to come up with a take off distance and percent Rate of Climb. It never runs out of batteries and comes pretty darn close to actual performance. Also being a bit conservative, I add on a "comfort" factor.

Since my M5 is not on the list, I came up w/ my own Sea Level at full gross numbers and marked them on my T.O.P. In addition, my plane is 37 years old; therefore, going by the factory numbers is not a good idea.

I am sure there are a number of calculation solutions out there. I am pretty happy with this one. For What it's worth.

TD

Sidewinder wrote:Hers a video that can help promote serious thought on DA

I'm not sure how much (if any) DA actually played a factor here. That airport is Mears (3W5) in northern Washington, 2600ft runway at 267' MSL. DA couldn't have been that high... You'd think a turbocharged bonanza wouldn't have had much of a problem with it.

There really aren't too many factors, but no hard and fast limit. It is easy and necessary to calculate performance for planning purposes before dropping into a place. The canyonflying.com link outlined above is an easy approach that has been around since at least WW2, including to guestimate climb gradients to avoid surprises.

Soy's 70% rule is really the thing to watch on takeoff...it has worked countless times over the years to take the worry and guesswork out of hot and heavy takeoff progress, and is great for figuring out a safe load vs giving your passengers a circus ride.

lesuther wrote:Soy's 70% rule is really the thing to watch on takeoff...it has worked countless times over the years to take the worry and guesswork out of hot and heavy takeoff progress, and is great for figuring out a safe load vs giving your passengers a circus ride.

I really like the idea of that rule. I think for high D-altitude at larger runways it's perfect.

I have been trying to apply the rule recently. It has not worked.

Mid-winter freeze, high pressure weather, we had a DA -2,000ft when I took off last The ASI was only *just* coming alive as the Bearhawk lifted off, showing about 22kts.

The other time that rule doesn't work is short airstrips. If you're on a 600ft x 8ft strip, I find there isn't any time to a) wait for the ASI to come alive b) look away from the airstrip to check ASI c) abort T/O when ASI fails to come alive in time.

It's far from ideal, but I find I just have to commit on those shorter ones. I always do a rolling run-up on back-track for take-off to make sure there are no surprises; it works really well, the prop is still in top condition.

robw56 wrote:

Sidewinder wrote:Hers a video that can help promote serious thought on DA

I'm not sure how much (if any) DA actually played a factor here. That airport is Mears (3W5) in northern Washington, 2600ft runway at 267' MSL. DA couldn't have been that high... You'd think a turbocharged bonanza wouldn't have had much of a problem with it.

I kind of agree. To me it didn't look like he held it in ground effect, just tried to climb once air born. I think this is an excellent portrayal of why building airspeed in ground effect is so important.

I've been using the Sportys Calculator (that TomD has the link to) for a long time. It was recommended to me by a mountain flier and I used it on my very first Maule trips into Idaho eight years ago and ever since. It's easy and straightforward and adds an element of real safety to my flying. The best part is that you can use your very own take off performance numbers for your actual plane, weight and skills as a reference point. All of my experience says it's really accurate. Highly recommended.

I believe in the 70/50 rule, but unfortunately it doesn't tell the whole story. Assume you do in fact have 70% of your flying speed by halfway down the runway. That means you're likely to lift off just fine, but it doesn't tell you how well you'll climb. That, to me, is the biggest bugaboo of really high density altitude, the inability to climb well.

So far, the highest DA I've taken off in was 12,100' at Leadville in October 2009. Leadville is 6400' long at 9934' elevation. My airplane pretty well peaks at 15,000' DA, but it struggles to get there. We lifted off at about the 3500' mark, stayed in ground effect until about 90 mph, and by then the end of the runway was coming up, so we climbed out, increasing airspeed to 100 mph. The climb out from Leadville was pretty anemic--maybe 100 to 150 fpm.

Here we (me and my Molly dog, plus the usual stuff in the airplane (oxygen!; survival kit, towbar, but not much else, with about 5/8 tanks--the airplane is lighter than usual.) are, as we leave downwind heading northish. You can see we haven't climbed well, as we're still a bit below pattern altitude with the runway off the right wing:



Cary

Soy

Never say never.
I have heard the 50/70 rule for over 30 years.
Heard it directly from Sparky in 1983.
I lived by it until I got to Cabin Creek.
Once you are at the 70% mark on Cabin Creek the runway falls of steeper than you can stop short of the willows in the crick.

Chris

Here is the density altitude video you are looking for:

Good point. There are no universal rules. Everything has exceptions. Of course if you are taking off into descending terrain, you can be just a little less cautious - but as Cary points out - you gotta out climb the terrain. The 70% rule will get you 50 feet off the airport - after that you need to use your noodle - and you better have thought about that before you start your takeoff roll.

wannabe wrote:Soy

Never say never.
I have heard the 50/70 rule for over 30 years.
Heard it directly from Sparky in 1983.
I lived by it until I got to Cabin Creek.
Once you are at the 70% mark on Cabin Creek the runway falls of steeper than you can stop short of the willows in the crick.

Chris

soyAnarchisto wrote:Good point. There are no universal rules. Everything has exceptions. Of course if you are taking off into descending terrain, you can be just a little less cautious - but as Cary points out - you gotta out climb the terrain.

Yup, good rule of thumb, but still gotta think. Here's another one:

I'm in the process of setting up my own grass strip (cut down the last of the obstructions yesterday, chainsaws are fun) and one of my runways has a considerable hump in the middle. Going one direction the hump is about 50% and some planes probably won't be at 70% yet but will be ok. Going the other direction they would need to abort. The difference in this case is that going one direction your can take off at about the 80% point in the runway and make a turn toward another field and plenty of room to climb. The other direction is surrounded on all sides by trees. Some closer than others, but if you aren't off by 50% then you better be using what's left of the - now downhill - runway to brake.

I just wanted to confirm Scopolax's comment on how doggy his old plane is with high density altitude. Fly's ok with nothing in it and low on fuel.

Moss farmer wrote:I just wanted to confirm Scopolax's comment on how doggy his old plane is with high density altitude. Fly's ok with nothing in it and low on fuel.

Moss Farmer,

You might reach out to Robw56, and ask him what his experience is with the 8042 prop on his 170. I have heard that it makes a huge difference, and always wanted to make that upgrade to 15C, but never had the cash when a prop was available, and never could find a reasonably priced used prop when I had the cash. They do sell them new. Call Western Propeller in Troutdale if you are interested. The price was too high for me to buy new.

Scolopax