High Density Altitude Operations

[jlunseth]

With all respect, I think that kind of mathematical calculation may be helpful in the purely "high" situations, as in up in the mountains in normal temps, but I would not rely on them in the "high, hot" situations. It seems to me aimed primarily at whether the engine is putting out sufficient HP to make a speed, and forgets the part where the wing may not be willing to fly. You may be flying an aircraft with a normal liftoff speed of 100, and it may still not be flying at 120 in a high heat high altitude situation. I am probably misremembering the exact speed numbers, because it was decades ago, but I was in an aircraft in pretty much exactly that situation. We made it by the way, good pilot who know the limits of his aircraft. Its not so much a matter of whether you have reached a certain speed by a certain point, but whether the airplane wants to fly or not. On a strip the length of KDRO, if the airplane is not at least light on its feet by the half way point I don't think I would want to be in the air regardless of what speed the plane is traveling at. That's 4,600 feet, nearly a mile.

[Hondo]

Not sure I understand that part. I've never seen an airport with a sign saying "1/2". OTOH, one can always point out a location on the runway that is 1/2 the runway length.

Exactly!  

 

Having an abort point in mind is a good idea whether the runway has a 1/2 sign, runway remaining signs or no signs.

 

I am assuming no one would attempt to fly unless they thought they had sufficient margin to safely takeoff and clear surrounding obstacles.

[midlifeflyer]

With all respect, I think that kind of mathematical calculation may be helpful in the purely "high" situations, as in up in the mountains in normal temps, but I would not rely on them in the "high, hot" situations. It seems to me aimed primarily at whether the engine is putting out sufficient HP to make a speed, and forgets the part where the wing may not be willing to fly. You may be flying an aircraft with a normal liftoff speed of 100, and it may still not be flying at 120 in a high heat high altitude situation. I am probably misremembering the exact speed numbers, because it was decades ago, but I was in an aircraft in pretty much exactly that situation. We made it by the way, good pilot who know the limits of his aircraft. Its not so much a matter of whether you have reached a certain speed by a certain point, but whether the airplane wants to fly or not. On a strip the length of KDRO, if the airplane is not at least light on its feet by the half way point I don't think I would want to be in the air regardless of what speed the plane is traveling at. That's 4,600 feet, nearly a mile.

 

Well, "high" and "high and hot" mean exactly the same thing to pilots who regularly deal with high density altitude. When we talk about altitude and aircraft performance, there is only one kind that counts - density altitude. Airport msl or pressure altitude means zip without temperature added in.

 

But that aside, what you say (bolder)  is true, but due to different calculations.  If you got the airplane up to 300 in your scenario (assuming a long enough runway) you might still not be able to be airborne.

 

Keep in mind that the takeoff performance charts stop at a certain altitude and you are test pilot if you extrapolate above that (we in fact do fly beyond the tables in the Rockies, but its based on decades of experience passed down). 

 

So, density altitude may be above the absolute ceiling limit of the airplane. But it's the altitude at which the airplane will simply not climb and above which the airplane will descend even at full power. It's not listed in a lot of manuals, but if the service ceiling is the density altitude at which the airplane will still climb at 100 FPM (without assistance) the absolute ceiling is coming up pretty fast. So, yeah, if the density altitude was that high, I wouldn't even get in the airplane to begin with.

 

So one still needs to think in terms of maximum aircraft performance capabilities in whatever situation we are in.

 

But the rule of thumb is still pretty useful in flyable situations, even on the edge. Recall that density altitude saps both airfoil efficiency and power. High enough and your nice big but weakend engine and that prop trying to pull you through whipped cream against the formidable drag of the tires on the runway* might not even get you up to your 70/50  in the typical situation.

 

[That drag is the reason for a common high density altitude technique of doing a modified soft field takeoff]

[midlifeflyer]

Exactly!  

 

Having an abort point in mind is a good idea whether the runway has a 1/2 sign, runway remaining signs or no signs.

 

I am assuming no one would attempt to fly unless they thought they had sufficient margin to safely takeoff and clear surrounding obstacles.

 

The problem that occurs in many high density altitude related crashes is that one word. In flight situations that are unforgiving, thought without knowledge can be fatal. That's why mountain training courses, better yet if accompanied with a dual flight, are always a good idea.

[jlunseth]

I don't disagree with you about what high density altitude is.  What I am trying to get across is that whether you have enough horsepower to achieve the Vref speed listed in the POH is not the only concern.  The "lift nose" speed in my POH is 67 (that is the wording of my POH).  I have seen conditions, not in my aircraft but in others, where the nose won't lift at the speed in the POH or even within several knots of that because the wing is not ready to fly.  The problem is compounded if the aircraft is heavily loaded or worse, overloaded.  To me, it is not very useful in high, hot conditions to give yourself a Vref marker from the POH, because the plane may still not fly for you when you get there.

[chrisk]

It would seem impossible not to have greater than 70% of airspeed by the 1/2 way point at KDRO. That would be 47kts at 4600' of used runway, I don't think I would want to be in the air at that point.

 

If I was not at least in ground effect by 50% of the runway at KDRO, I would abort.

 

And fly by airspeed.  If your used to judging by ground speed, you will be shocked.

[Rhumbline]

Your approach from TX will be relatively easy though you should carefully plan your crossing of the Sangre de Cristo's.  Right out of high school I learned how to fly at Animas Air Park about five miles to the west of KDRO.  Over the decades, I've flown everything from a Traumahawk to a four-holer in & out of Durango.  The worst experience I've ever had in an airplane was departing what was then RWY 20 (now 21) during the summer with a CFI student in a C182RG.  We suffered a power loss over the departure end of the runway and the airplane began to sink toward the Florida River drainage with nothing level but ground covered with juniper and pinion.  Obviously, at least one of us survived.  In fact, the bird was nursed back to 20 at about 200 AGL and it's the only time I've ever kissed the ground!

 

Regardless of direction, the runway is more than adequate for a loaded C but don't expect to tackle higher terrain to the north.  I regularly fly in & out of KDRO and always choose RWY 03 if the winds and/or traffic favor it for the options are somewhat better should circumstances force the exercise of contingencies.  You will be above 5,000' so aggressively lean the mixture on the ground to avoid fouling (I have the mixture pulled about half way out after priming for a quick smooth cold start and don't need to prime at all if the motor's warm) and follow the leaning tips available here or elsewhere that you find reasonable for other phases of flight.  Apply good, conservative judgment and you should have no trouble.

 

Apart from the above, I would not venture into the hills without mountain training.  You will note that the terrain gets very rugged very quickly to the north.  Youthful invincibility left me along time ago and I feel I'm pressing my luck to venture further up the valleys than Vallecito or Electra Lakes when I want to do some sight seeing.  I would now never go any further without another motor.  

[Rhumbline]

P.S.  Hope the weather's okay.  We were panning on heading to KPGA, KINW, or P14 for lunch tomorrow but it looks like the weather's going to suck.  Safe travels and enjoy your stay in Durango!

[gsxrpilot]

We had a nice flight from Austin to Durango today.  It was easy VFR the whole way at 10,500. The last 75 miles or so were in light rain and moderate turbulence. Of course it was 1pm which is the wrong time of day for flying in the mountains.  We had stopped half way at Andrews County for cheap fuel and restrooms. I was using flight following and the controllers were certainly concerned about my navigation first through the MOA's in west Texas and then the high terrain in New Mexico and Colorado. 

 

The route I chose was 84R LLO SJT E11 (fuel stop) HOB CME CNX WARRE CABUS BEANN ABQ PEDRA KDRO. 10500 was a safe altitude and ATC was happy with it as well.

 

In fact the only really interesting event was dodging a Coyote on the runway in Durango. But with a 150ft. wide runway, there was plenty of room for both of us.

 

We intend to lift of at dawn on Tuesday. Thanks for all the advice. I'll let everyone know how it goes.

[Ragsf15e]

Good luck.  I fly out of a lot of high DA airports and usually try to leave my M20F10-15 gallons shy of full, but that still leaves 50 gallons in the F model and 4 hours plus 1 hour reserve is more than my bladder can handle anyway.  

 

I'd highly recommend making sure it's as cold as possible and that you're not near or right at gross... or leave off some fuel to give yourself a couple hundred lbs of safety margin.  She's not going to climb well.

[gsxrpilot]

And we're back... The strategy for leaving KDRO

1) Leave as early as possible -- we were wheels up about 6:15am this morning.  The temperature was only about 45 F and the DA was reported as 6400ft.

2) Stay as light as possible -- I figured with full fuel we would be about 150 lbs under gross. I also asked the line guys to leave me 3" of air in the tanks. I calculated this to be about 38 out of the max 52 gal of fuel saving an additional 84 lbs.

3) Lean for best power -- thanks for the advice on this. I leaned for max rpm and then richened it just a bit. This worked for take off, but when safely established in the climb, I had to richen it a bit further to keep the CHT's under control.

4) Airspeed is everything -- I took off no flaps, to have as much airspeed as possible before lifting off, I had 9K ft of runway to work with but only needed about 4000. 

 

It all seemed to work well. We used about 4000 of the 9000 ft available, stayed in ground effect a little longer and then slowly climbed out. We maintained a shallow climb all the way to 11.5K and set the course for home. A fuel stop at E11 (Andrews County) for cheap fuel, and this time filled the tanks. Again up to 11.5K for a smooth and easy flight home to 84R.

 

Thanks for all the advice... Next time maybe I'll stick my toe a little deeper into the mountains.

[bonal]

Checked it out on Flight aware looks and sounds like you had a great adventure.  I'm thinking why not take off flaps I know when I have had similar DA I get much better climb out with flaps set for take off. I understand the need for air speed but you might try that on your next opportunity

Glad you had an excellent trip

[gsxrpilot]

We really had a great time.  I'll explain my reasoning for the no-flap takeoff. And I'd love to hear where my reasoning might be flawed. 

 

KDRO sits on a very slight mesa. So instead of rising terrain just off the ends of the runway, the terrain actually falls away. Not much, but just a bit. Also, my route of flight to the south was also fairly low. And I've got 9K ft. of runway to work with.  I also know that I'll be fighting CHT in the summertime climb especially with a leaned mixture.

 

So my thinking was that 1) I didn't need a good rate of climb, just positive rate, and 2) a shallower, higher speed climb would help keep the temps in check.

 

It did work out fine, but I welcome the comments. I would have certainly used takeoff flaps if I needed the climb rate to out climb rising terrain.

[Andy95W]

I definitely won't second guess your decision because it was well thought out and worked well.

I think I would have used take off flaps to give an initial positive rate of climb to get out of ground effect and get above the radiant heat that is close to the surface, and accepted the 30 sec. to a minute of slightly high CHTs.

But now that I've read your analysis of the environment, I will keep that in mind if I'm in a similar situation.

[Super Dave]

It's a common misconception that flaps improve climb, but it isn't true. Setting takeoff flaps will result in a shorter takeoff roll, but climb rate and climb angle both suffer with flaps.

I think it was a wise choice to use no flaps when runway length was no factor.

[donkaye, MCFI]

It's a common misconception that flaps improve climb, but it isn't true. Setting takeoff flaps will result in a shorter takeoff roll, but climb rate and climb angle both suffer with flaps.

I think it was a wise choice to use no flaps when runway length was no factor.

 

Some airplanes say not flaps, but that is NOT the case for a Mooney.  You get ⅔ more lift than drag with approach flaps.  They should always be used in the Mooney on takeoff per every Mooney POH I have seen.

 

In fact with the M20A the POH says for short field no flaps until lift off then FULL FLAPS, retract gear, then for Vx climb with one notch of flaps until clear of an obstacle; for high density altitude, then lower the nose increase speed to 90 and retract the flaps.

[bonal]

Yoshimura, I'm not criticizing you flew a well thought out flight plan I know that when I opporate on plus 100 degree days my initial break with take off flaps is much sooner than without and initial climb is much better. The best part is you have executed the flight and documented the results perhaps next time when you have lots of runway try with flaps and note the difference. Either way nice job

[Super Dave]

Some airplanes say not flaps, but that is NOT the case for a Mooney.  You get ⅔ more lift than drag with approach flaps.  They should always be used in the Mooney on takeoff per every Mooney POH I have seen.

You've never seen a POH for an F model? My '76F says "FLAPS - Set for TAKEOFF or as desired". I think several other models say something similar. Regardless, unless it says otherwise in the limitations section, flap settings are only a recommendation, not a rule.

As far as the notion that flaps generate 2/3 more lift than drag at approach setting; It is simply false. I know that the vast majority of flight instructors make some kind of claim that flaps at a takeoff or approach setting create "more lift than drag", but the reality is that flaps at ANY setting will ALWAYS increase the coefficient of drag more than the coefficient of lift. if it were true that approach flaps created "more lift than drag", we would use them in the climb all the way up to cruise altitude. We would also us them to increase glide ratio in the event of an engine failure.

[carusoam]

The 'More lift than drag' statement is something I have heard during training decades ago...

Sounds like an easier way for an instructor to explain the reality of the situation to a new student that is already loaded up with a book full of physics knowledge with only a little background in physics...

The T/O flap setting significantly lowers stall speed, which is helpful getting off the ground. The penalty is some added drag...

The full flap setting lowers the stall speed more, but adds too high of a drag penalty. There is also a huge trim adjustment required if the engine quits on T/O.

Unit analysis doesn't really allow a direct comparison of drag to lift.

- Thrust and drag are balanced forces...

- Lift and weight are balanced forces...

Comparing Lift and Drag are a nice way of an instructor saying 'follow the POH' because it is a proven way to get the result you are looking for...

Thoughts of a PP,

-a-