Density altitude and runway length quick guide?

jspilot

jspilot

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jspilot
So I’m a flatland pilot guy who has almost no experience with density altitude so forgive the ignorance. When I was training my CFI’s admittedly did not spend a lot of time on density altitude but on a recent flight I was confronted with a situation I was not sure of.

I went to Pittsfield Mass(PSF) which is up about 1,000 feet above sea level. Winds favored the shorter runway( 3,500 feet). Landed with no issue but was just making a quick stop. Listened to the weather and it was reporting density altitude of 2,800. That’s higher than I’m used to( I know many are probably laughing if you fly in the mountains) but it occurred to me that the runway was kind of short and the density altitude was high(ish) and it was warm so I decided to do a short field takeoff. It worked fine and I probably took up about half the runway to get airborne.

It occurred to me though, that I really had no way to quickly judge runway length vs density altitude and was wondering if anyone had a quick guide or “rule of thumb” they use in a similar situation. I’m glad I did the short field takeoff but really was not sure how I should know for sure( without taking the time to break out the handbook.). Does density altitude within 1,000 feet of runway length work as a quick reference for when a short field takeoff should happen? Does anyone have a rule for this?

Clearly I should have looked at the handbook but my knowledge in this area is admittedly not as strong as it should be.

Thanks for the help!
 
Not sure why this duplicated the post. Mods, sorry about this- feel free to delete one of them!
 
You should pick an abort point and always consult the POH for performance numbers. We don’t think about it too much in Long Island but when summer rolls along, it’s definitely something to review.
 
This video might be a bit dated, it the lesson is still valid.

 
Dana said:
Your POH should have the information.

2800 DA isn't that much, but 3500 being the "short" runway made me chuckle.
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Yeah I totally get this. I knew I had plenty of space but I wanted to know if anyone has a quick way to determine when to use a short field takeoff va regular. I know runway length needed it’s in the POH( If I remember correctly, the conditions on this day meant I’d take up about 1800 feet-I did check before the flight as I always do) but we all know those numbers are often not at all real- as they are determined based on zero wind which never happens and conditions do change.

Thanks to everyone who has helped so far. I really appreciate it!
 
Unless you're flying experimental, your Pilot Operating Handbook should have this info. If you don't have experience at high DA takeoffs and landings add some safety factor to the numbers.

Always have an abort point decided upon before you start the take-off roll. The 70% - 80% of your takeoff speed at the halfway point is a good starting point for a decision. I would add a bit of a safety factor on to that as well.

Lean the mixture for max. takeoff power.

If you're not certain where the halfway point is walk the side of the runway and pace it out. Pick some reference point after pacing your abort point. Then don't take off if your not off the ground by that point.

High DA is a sneaky killer. You're sitting at the end of the runway and watching a bunch of other airplanes take off ahead of you, and it's easy to say, "Certainly I can do this!".

So you start your roll and you are just past the abort point. Then the plane lifts off the runway and you feel vindicated that your decision was ok.

Then you pull the nose up to start climbing and the stall horn goes off. You realize your airspeed is disturbingly low, as you watch the end of the runway disappear below you. Then you look at the trees straight ahead and your mind swirls as it cannot reconcile the idea that you are going to slam into the trees. Then the realization hits that this one little error in judgement is going to kill you and the people in the plane with you.

It's a helluva a penalty for one small mistake.

And yes, I've almost killed myself twice due to high DA. One time with my son sitting next to me. I got stupid lucky both times...

Check your POH, lean the mixture, add some safety factor, pick an abort point - and follow your plan. If you do this you will likely be ok.

Incidentally, the airport where almost killed my son and I was only 2500 feet elevation. It was a very hot morning, we were heavy, and I did not lean the mixture. Also, it was out of the way and I could not get an accurate temperature at this remote airport. My stupid thought process was that "we were only at 2500 feet elevation, how bad can that be?" Bad enough to almost kill my son and I.
 
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AggieMike88 said:
This video might be a bit dated, it the lesson is still valid.

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Does that video seem prophetic in regards to internet forums on just about any subject? Guy does it once, runs over to the other guy to now give his expertise in, this case, density altitude.
 
benyflyguy said:
Have to admit, for its day they put a lot of production value into these vids. Was enjoyable to watch.
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I agree about the production value. It does help tell the story which also helps set the lesson into your synapses.

A similar FAA production is out there about flying a light twin. This reviews the details of shutting down an engine, flying with a prop feathered, and then restarting in air. Fits in well with the video about “The Drill”.
 
There's many 'rules of thumb' to help with this, but probably one of the easiest that I've used is 70/50. 70% percent of rotation speed by the 50% of the usable runway.

That said, it is no replacement for thoroughly studying and understanding your performance charts (and how close they are to your planes actual performance) as well as learning about all the other 'rules of thumbs' about DA.

Remember, DA is the altitude the aircraft thinks it's at. So how is your aircraft going to respond at that altitude, with that runway surface, in those wind conditions, with your current load and the current weather conditions?
 
jspilot said:
Yeah I totally get this. I knew I had plenty of space but I wanted to know if anyone has a quick way to determine when to use a short field takeoff va regular. I know runway length needed it’s in the POH( If I remember correctly, the conditions on this day meant I’d take up about 1800 feet-I did check before the flight as I always do) but we all know those numbers are often not at all real- as they are determined based on zero wind which never happens and conditions do change.

Thanks to everyone who has helped so far. I really appreciate it!
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Best I can come with is, if your wondering whether you should or not, then do it
 
POH. Do some reading or get an instructor to help you understand it. There are apps from some airplanes that will calculate it for you, but you should know and understand how to do this.
 
Always have an abort point. It’s not a set 50% or 75%, it all depends on the DA and runway you are on and obstacles.

Use your POH and do the math BEFORE you fly there. Where I live people fly to Big Bear every day and every year there are multiple people who don’t make it out.
 
Jamie Kirk said:
Always have an abort point. It’s not a set 50% or 75%, it all depends on the DA and runway you are on and obstacles.

Use your POH and do the math BEFORE you fly there. Where I live people fly to Big Bear every day and every year there are multiple people who don’t make it out.
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Just to make sure there's no confusion, the abort point is 50% of runway, it's never 75%.

It is 70% of take-off speed or better (or 75% if you prefer-that's fine) AT 50% of the runway.

The rest is good advice.
 
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Can we factor in weight on this takeoff? Being on the lighter side is a way to mitigate degradation of performance.
 
My POH has a table and correction factors to apply for runway surface, temperature, loading, and headwind component. Too many variables for a simple rule of thumb. The POH is (mostly) based on real test data. However it also assumes good technique and a new airplane. It may be wise to add some margin. The POH data is well worth taking the time to inspect when flying in unfamiliar conditions.
 
It's not that complicated. I took the pseudo parabolic relationship in my POH chart and linear normalized it with a right bias (conservative aka high read approximation), and boiled it down to simple linear calculation. For my Arrow II, ground run for flaps 0* (worst case) is basically 1000 feet plus 17% of the reported density altitude. For the 50-foot clearance, no kidding just double the resultant ground run number. Done. Stupid easy and close enough for govt work.
 
1FEF334A-34AF-42C6-B55A-6CDB559DABF8.jpeg Every pilot should have the T.O.P. Slide Computer

An amazingly simple takeoff performance computer. Set temperature window, altitude, runway surface, runway slope, wind, and gross weight on... easy and much safer than trying to plod through the poh with all its extrapolation estimates. IMO.

You can buy it at Sportys. The best $30 I ever spent.
 
jspilot said:
So I’m a flatland pilot guy who has almost no experience with density altitude ....

It occurred to me though, that I really had no way to quickly judge runway length vs density altitude and was wondering if anyone had a quick guide or “rule of thumb” they use in a similar situation.
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The rule everyone is saying regarding 70% of your speed by the halfway point of the runway works well ... except when it's windy, gusty with downdrafts (aka Ruidoso NM KSRR). Pre-plan how to turn to lower terrain if you get high DA **AND** a downdraft (downdrafts won't stuff you into the ground like a microburst, but in my area won't release you unless near 500AGL) and are headed towards higher terrain at less than 400 AGL, you're in trouble.
 
I’m headed to Idaho with a group at the end of July going to stay at Johnson Creek. We are a bit concerned being I fly a Cessna 140- they are not powerhouses unless there’s a stc engine in em. We plan on going to a field of high elevation that’s more open than JC and then decide. If me and my experienced cfi flying buddies aren’t convinced I’m good to go in, leaving by bird in McCall and hitchhiking in with one of them.

I will be solo and nothing onboard but me n gas as my extras will be in either their c180 or supercub. But I want my first real world test of my skills and her 73 year old self to not be at a field with limited to no options if they aren’t up to snuff...
 
On Friday, I'm headed to Flagstaff, KFLG, to pick up probably seven boxes, each weighing 35#, of blood donations from a local blood drive.

When I leave, the temperature is likely to be 74degF. Anyone wanna do the DA calc on that?

:D
 
Huckster79 said:
I’m headed to Idaho with a group at the end of July going to stay at Johnson Creek. We are a bit concerned being I fly a Cessna 140- they are not powerhouses unless there’s a stc engine in em. We plan on going to a field of high elevation that’s more open than JC and then decide. If me and my experienced cfi flying buddies aren’t convinced I’m good to go in, leaving by bird in McCall and hitchhiking in with one of them.

I will be solo and nothing onboard but me n gas as my extras will be in either their c180 or supercub. But I want my first real world test of my skills and her 73 year old self to not be at a field with limited to no options if they aren’t up to snuff...
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Johnson Creek strip is quite good and plenty long. I think what intimidates most people is the fact it's in the bottom of a valley and the closeness of the terrain on downwind, base and final.

I've seen plenty of 120s and 140s in there, as well as just about everything else you can imagine. Stay light as possible and fly within limits and you should be fine.

Just remember, all you gotta do is not hit anything.
 
Challenged said:
There's a few articles I've seen that reference having 70% - 80% of your takeoff speed at the halfway point.
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All else being equal, constant acceleration would mean you'd be at 70.71% (1/sqrt(2)) at the halfway point. 70% isn't good enough. But, 70.71% also means you'll lift off as your wheels hit the end of the runway, and that's if you started with your wheels at the other end of the runway which is damn near impossible in most places.

So, better to choose 75-80% and hopefully you'll have enough room to abort if necessary.
 
SToL said:
Johnson Creek strip is quite good and plenty long. I think what intimidates most people is the fact it's in the bottom of a valley and the closeness of the terrain on downwind, base and final.

I've seen plenty of 120s and 140s in there, as well as just about everything else you can imagine. Stay light as possible and fly within limits and you should be fine.

Just remember, all you gotta do is not hit anything.
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Good to know! I’m going to take her out as soon as she’s back from the shop with new skylights and get things put back in as I’m painting the interior right now :), and take her up high and get a feel for her in thin air but still around home.

Yea I’ll be real light. I need a new w&b before going as I’ve shed about 50lbs or better off her!
 
Am I right to ascertain that In High DA one not only needs to watch take off but also be cautious on the landing as slowing a too high of sink rate or rounding out is going to take more time?
 
Huckster79 said:
Am I right to ascertain that In High DA one not only needs to watch take off but also be cautious on the landing as slowing a too high of sink rate or rounding out is going to take more time?
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Yes. And, as a landing is really a "go-around that failed", you need to think about the go-around, and how that's gonna work out.

I've left Flagstaff with a Piper Warrior, on a 75-degree day, just my wife and I on board, half tanks, minimal baggage, and there was a whole lotta "I think I can.... I think I can.... " going through my head...
 
Rgbeard said:
On Friday, I'm headed to Flagstaff, KFLG, to pick up probably seven boxes, each weighing 35#, of blood donations from a local blood drive.

When I leave, the temperature is likely to be 74degF. Anyone wanna do the DA calc on that?

:D
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Since no one bit on that - I did some guesstimates, and calculated it.

Field elev is 7,014. Guessing temp 24c dew point 03c, altimeter 29.92

Comes up to 9,726'.

Turbo's gonna earn its money on Friday.

At FLG, there's a little DA calculation sign on the taxiway - just to serve as a reminder to pilots. Thank goodness for a nearly 9,000-foot runway. The sign has five digits in the display.
 
Rgbeard said:
Since no one bit on that - I did some guesstimates, and calculated it.

Field elev is 7,014. Guessing temp 24c dew point 03c, altimeter 29.92

Comes up to 9,726'.

Turbo's gonna earn its money on Friday.

At FLG, there's a little DA calculation sign on the taxiway - just to serve as a reminder to pilots. Thank goodness for a nearly 9,000-foot runway. The sign has five digits in the display.
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If you really want to make it earn it's money, take me with you.
 
Rgbeard said:
Since no one bit on that - I did some guesstimates, and calculated it.

Field elev is 7,014. Guessing temp 24c dew point 03c, altimeter 29.92

Comes up to 9,726'.

Turbo's gonna earn its money on Friday.

At FLG, there's a little DA calculation sign on the taxiway - just to serve as a reminder to pilots. Thank goodness for a nearly 9,000-foot runway. The sign has five digits in the display.
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I actually did do a quick one but since I didn't have the dew point or alt setting or field elevation (ahem, someone didn't provide them), i wasn't going to make a fool out of myself. But I did come up with something like 8538 or some such. But I couldn't remember the field alt and used 6000. Not bad for 1000ft off when I started 1000ft to low.
 
Huckster79 said:
Good to know! I’m going to take her out as soon as she’s back from the shop with new skylights and get things put back in as I’m painting the interior right now :), and take her up high and get a feel for her in thin air but still around home.

Yea I’ll be real light. I need a new w&b before going as I’ve shed about 50lbs or better off her!
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I was going to say, just fly up to some high alt airport and do some practice on their ridiculously long runways, but looking at your picture I get the impression that's probably not a close by option for you.

Oh and that new paint, that's just extra weight. :p
 
Huckster79 said:
Am I right to ascertain that In High DA one not only needs to watch take off but also be cautious on the landing as slowing a too high of sink rate or rounding out is going to take more time?
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No. It will take more *distance*. You want to keep the same indicated airspeed on landing as you normally would, but at high DA that will be a faster true airspeed and thus a faster ground speed (for the same wind) as it would be at a lower DA. However, with the higher true/ground speed (again, I emphasize, *same indicated AIRspeed*), you'll still have more momentum so the ground roll will be longer.

Rgbeard said:
At FLG, there's a little DA calculation sign on the taxiway - just to serve as a reminder to pilots. Thank goodness for a nearly 9,000-foot runway. The sign has five digits in the display.
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Worst I've taken off in was 12,200 feet, in a 182. Ground roll was about 2,000 feet, about three *times* normal.
 
flyingcheesehead said:
No. It will take more *distance*. You want to keep the same indicated airspeed on landing as you normally would, but at high DA that will be a faster true airspeed and thus a faster ground speed (for the same wind) as it would be at a lower DA. However, with the higher true/ground speed (again, I emphasize, *same indicated AIRspeed*), you'll still have more momentum so the ground roll will be longer.



Worst I've taken off in was 12,200 feet, in a 182. Ground roll was about 2,000 feet, about three *times* normal.
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Make sure I’m on same page- I understand what you say on rollout but distance is time too so I wasn’t thinking of landing roll as much as rounding out or if you are on final and ya notice your sink rate is greater than you would like it’s going to take more time and distance when you add the power in to arrest that sink rate won’t it? So you would want to make sure you don’t find yourself short as what normally could be solved with a touch of power may take a more dramatic touch of power if you misestimated initially, right or no?

The airplane is going to sink faster than normal just as it will climb slower than normal correct? So speeds should be kept the same but power setting at home (moderate temps at 800ft) May be lower than what you use in high DA to accomplish same indicates airspeed and sink rate in the pattern?
 
flyingcheesehead said:
All else being equal, constant acceleration would mean you'd be at 70.71% (1/sqrt(2)) at the halfway point. 70% isn't good enough. But, 70.71% also means you'll lift off as your wheels hit the end of the runway, and that's if you started with your wheels at the other end of the runway which is damn near impossible in most places.

So, better to choose 75-80% and hopefully you'll have enough room to abort if necessary.
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I dunno...seems to me if you’re at 70.71% of liftoff speed at the halfway point and wasted the first 100 feet, you’d have 100 feet to spare when you lift off.
 
Huckster79 said:
The airplane is going to sink faster than normal just as it will climb slower than normal correct? So speeds should be kept the same but power setting at home (moderate temps at 800ft) May be lower than what you use in high DA to accomplish same indicates airspeed and sink rate in the pattern?
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Just maintain your airspeed, and by that I mean nail your approach speed and it will work out. What will change is your ground speed, it will be faster at higher DA. That may or may not be noticeable to you. I would be more concerned in takeoff than landing, but as long as you stick to early morning operations you'll be fine.

I understand you're flying with a group, but I still recommend trying out Garden Valley to get your backcountry feet "wet"...

EDIT: Another option is Stanley (2U7). It's not confined in a canyon, but it is at 6400 MSL with a long dirt/turf runway. Smiley Creek (U87) is another option, again not in a canyon, but has a nice long grass runway and is at 7200 MSL and is an excellent eye opener to DA!
 
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