Densitity Altitude, its real

B

bahama flier

Pre-takeoff checklist
Joined
Jan 15, 2014
Messages
143
Location
Deland, Florida
Display Name
Display name:
bahama flier
When I was in training, not much time was spent on issues like Density Altitude, it was covered but briefly, and its importance was not stressed enough for me to take note of it when I departed on hot days.

I left the Bahamas on a hot day later that I usually do, I was borderline overloaded if not for real. Normally half a runway is all I need for departure, so I back taxied half of the runway, and started departure.

I pulled back on my yoke when I reached my lift off airspeed, but nothing happened,, I went faster still pulling back, the plane would not lift off,, I was at the point of no return and was looking at all my departure setting, all correct, the trees and the end of the runway were coming up fast, my pucker factor was maxed out.

I just got my wheels off and still pulling like crazy to miss the trees, and just did.

I had my first experience with density altitude, now by habit I always use all the runway even if its cold and I am light loaded, lesson learned.

If you are a new pilot, density Altitude can be dangerous if over looked on hot days or high altitudes. Pilots that live and fly in high elevations live with this with each flight, in the South, just on hot days. Just my 2 cents....
 
Wow scary stuff. But I'm baffled how lift off speed increased.
 
Something sounds a little off to me.

At SL and 90°F, that would only be a DA of around 2,000'.

There would be performance effects, but nothing that drastic.

Also, while it would take longer to get to liftoff speed, once there performance should not be markedly different either.

I suspect something else was amiss.
 
Last edited:
FastEddieB said:
Something sounds a little off to me.

At SL and 90°F, that would only be a DA of around 2,000'.

There would be performance effects, but nothing that drastic.
Click to expand...

I agree. Sounds like either overloaded or underpowered, or both.
(Single mag or fouled plug(s) can easily do that to you.)
 
You were on the back side of the power curve. Lucky most of it was in ground effect, or you could have mushed right into the trees. DA made it worse, but you did most of it yourself by pulling it off green.
 
Had that issue leaving Osh one year, hotter than the blazes and we wound up departing during the hottest part of the afternoon. I thought I was going to have to abort. Jeeze that was scary.

But I know what the OP means. I admit to learning it's importance the hard way. Some of you were there when I did.
 
this thing is no matter WHAT the density altitude is- once you reach the take off IAS the plane is going to fly - it might not climb worth a damn, but its gonna be flying.

I'd never ever accept less than 5000' for departure if offered an intersection - and if the runway is shorter than that . . . .then you do a full length. Whats with half way? How was taxiing all the down to the end not the smart idea?

whats the saying ? Runway behind you, fuel on the ground. . . . etc etc./
 
I could understand that at 10,000 DA, but that really doesn't sound right even on a stupid-hot day at sea level.

I think you were overloaded (a lot), perhaps nose-heavy (again, a lot), had an engine problem, low tire, or misconfiguration, or perhaps a tailwind.

The real lesson for high DA is to have an abort point. If you know you should be off given conditions at 2000 feet, if you're still on at 3,000, abort. Figure it out ahead of time so you don't get past the point of no return.
 
Nothing more useless than the runway behind you,the sky above you,and the fuel you didn't take. Learned a great deal about my aircraft performance and density altitude ,on a trip to the Midwest and Alaska last summer.
 
Weight was more likely the culprit.
Airspeed is airspeed regardless of density.

Groundspeed on the other hand will increase.
If I were you I would go back and retroactively do a W&B and make a perm. mental note

Whatever weight you were at would get you killed if DA was higher.
I think you got a pass now learn from it.

I got a similar pass over the summer and changed some behaviors as a result.
 
Too much nose down trim or wrong flap setting can give me the sense that she doesn't want to fly without and until I use a large amount of back pressure.

What was the DA?
 
Runway behind you is useless. A pet peeve of mine is following that stupid yellow line onto the runway if in a small GA aircraft.
 
For you guys who never fly close to the edges, you won't understand. If your obstacle clearance is semi-close on a cool day it can be scary close on a warm day. Factor in a larger than normal load???? Been there, done that.
 
Young Eagles. 2008. Windham CT. July. Temp 86. Dew 65.

Runway 18. Wind, 190/10. downward sloping - 70' trees end of the runway.

3 skinny kids plus me in a 160hp Warrior. 100# under gross.

Took a long time to accelerate. Took a long time to climb. Those kids thought it was neat having the sensation of speed going over the trees . . . . yep - was fun for them.

The book said it was going to be fine. But a tired old Warrior trainer was barely up to the task. If I hadn't popped two notches of flaps and dragged it to Vx I'm not sure I would have cleared the trees. . . . .
 
I'm baffled at the use of half of the runway.
Hopefully you'll never do that again .
 
Cpt_Kirk said:
Runway behind you is useless. A pet peeve of mine is following that stupid yellow line onto the runway if in a small GA aircraft.
Click to expand...

Interesting, it took 12 posts before someone picked up the "1/2 the runway" comment.

3 things a pilot does not need:
Runway behind you
Air in the tanks
And altitude above you

And yes, forcing (pulling) an aircraft off the runway before it is ready to fly puts you instantly on the backside of the power curve.
 
This is why I like our skywagon and the Sportsman cuff.

I've never took off mid-field, but then again the wagon has never taken more than half a 2000' strip to get off. Loaded or unloaded. High DA or not.

Our country strip is 2000' with 40' obstacles both ends. Half way if I'm not off, that's my abort point. But it's never happened ... not yet. Sitting in on a backcountry forum at OSH, the host said you should abort shorter than half the length if it's high DA or downhill, or huge obstacles etc....
 
Come on up this summer to KAPA, load the family truckster to max gross and enjoy the 3000' takeoff roll. We've got 10,000' of pavement so it won't seem quite as scary. ;)
 
SmashTime said:
I'm baffled at the use of half of the runway.
Hopefully you'll never do that again .
Click to expand...

same here...don't see any logic in that at all. Heck, if your engine sputters out on you at the worst time while your still on the ground you're gonna be in trouble. No sense in not taking an extra minute or two to taxi to the end of the runway and use it all.
 
kenjr said:
same here...don't see any logic in that at all. Heck, if your engine sputters out on you at the worst time while your still on the ground you're gonna be in trouble. No sense in not taking an extra minute or two to taxi to the end of the runway and use it all.
Click to expand...

Depends on the runway.

If it's 12000 feet, using only 6000 may save a ton of taxi time and still give you a huge warning if something isn't right. In anything smaller than a King Air.

If it's 3000 feet, well, that's not a very good idea.
 
MAKG1 said:
Depends on the runway.

If it's 12000 feet, using only 6000 may save a ton of taxi time and still give you a huge warning if something isn't right. In anything smaller than a King Air.

If it's 3000 feet, well, that's not a very good idea.
Click to expand...


This. When I'm at GRR, I'm not taxiing an additional MILE to the end of the runway. When half of it is still over 30% more than what I have where I'm based.
 
Some days taking an intersection is no big deal. Sometimes it isn't a great idea. I believe the OP made an error and was sharing his recognition of it. As he said, lesson learned. Chalk it up to things we do along the way.
 
Learned the same lesson. 152, max gross, high DA, climbing out of a mini-valley. Used every inch of that runway, wheels within 5 inches of runway edge and then lined up. I also did short field procedure. Used more runway than I thought I would, glad I was cautious. Still had a huge pucker factor climbing just above the pine trees up out of the valley. LIke the original poster, passenger thought it was cool flying just above the trees on departure. After landing, explained and he said "oh..." DA was about 6k - 7k.

My lessons learned:
1) Mixture for best power - not full rich. CFI may have told me this, but we never practiced. I've since read up.

2) ALWAYS setup to use entire runway when less then 5k long. On shorter runways, I don't follow yellow line, I like the extra length - even if it is only 10-20 feet. Why not?
 
I still don't believe that even at 100lbs over gross it would have had THAT much effect with 2-3K' density altitude in the Caribbean. Longer than normal TO roll? Sure. I almost wonder if he had the throttle all the way forward, mixture leaned properly, and was departing into the the prevailing wind and not inadvertently performing a downwind takeoff. If you're going to be operating at or near the limits, at least do a calculation on TO roll and such using the DA.

The key phrase that got me was "still pulling like crazy" which sounds like an excellent way to pull it right into a stall while trying to avoid the treeline. I would only pull (not discounting basic human instinct) whatever amount was required to maintain Vx as any more is just counter-productive.
 
BillTIZ said:
Interesting, it took 12 posts before someone picked up the "1/2 the runway" comment.

3 things a pilot does not need:
Runway behind you
Air in the tanks
And altitude above you
Click to expand...

Well, unless you're on fire ... :lol:
 
BillTIZ said:
Interesting, it took 12 posts before someone picked up the "1/2 the runway" comment.QUOTE]

First thing I noticed too... Maybe the OP's instructor skipped more than the lesson on DA... There are several apps available...I have one on my cell phone that computes DA in a couple of seconds... use it everytime for sake of habit...
Click to expand...
 
TazzyTazzy said:
Learned the same lesson. 152, max gross, high DA, climbing out of a mini-valley. Used every inch of that runway, wheels within 5 inches of runway edge and then lined up. I also did short field procedure. Used more runway than I thought I would, glad I was cautious. Still had a huge pucker factor climbing just above the pine trees up out of the valley. LIke the original poster, passenger thought it was cool flying just above the trees on departure. After landing, explained and he said "oh..." DA was about 6k - 7k.

My lessons learned:
1) Mixture for best power - not full rich. CFI may have told me this, but we never practiced. I've since read up.

2) ALWAYS setup to use entire runway when less then 5k long. On shorter runways, I don't follow yellow line, I like the extra length - even if it is only 10-20 feet. Why not?
Click to expand...

Mitch, in a "mini valley," descending air can ruin your whole day. Even at sea level in a 152, let alone at high density altitude. If you're taking off directly into significant wind with terrain ahead, you are quite likely to do it in descending air.

When performance limited, it's important not to rotate early. It's a high drag configuration. That's the real error in the following video; the pilot tried to pop off early and accelerate behind the power curve while climbing. The analyst has it wrong; it's a SEA LEVEL (or nearly so) airport, under conditions where a max-gross 172 could have made it easily.

https://www.youtube.com/watch?v=ZmEVwyMRYIY
 
A lot of guessing about my problems lifting off, a lot of advice, the point of my post was to stir some thoughts for new pilots, my problem was not to use the runway available to me, I hope new pilots were reading.
 
It would be helpful to put your experience in perspective if we knew what the total length of the runway was. Which Bahamas airport was it?
 
bahama flier said:
A lot of guessing about my problems lifting off, a lot of advice, the point of my post was to stir some thoughts for new pilots, my problem was not to use the runway available to me, I hope new pilots were reading.
Click to expand...

The problem was not really DA either. So while it is prudent to use all the runway in these situations you may have needed it for reasons other than DA.
 
bahama flier said:
A lot of guessing about my problems lifting off, a lot of advice, the point of my post was to stir some thoughts for new pilots, my problem was not to use the runway available to me, I hope new pilots were reading.
Click to expand...

I hope they are reading all the way through the thread... as others have stated DA alone does not account for the issues you described, and the best thing for you--as well as the new pilots you are trying to help--is to work to determine the root cause.
 
As a newly minted private pilot back in the 90s, I departed Lake Lawn, WI in a 152.

Took the taxiway to the runway and neglected to back-taxi the rest of the way. Will never make that mistake again!
 
You must log in or register to reply here.
Back
Top