Summer flying and density altitude

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NoBShere

Pre-takeoff checklist
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NoBShere
Summer is here and I am brushing up on density altitude. I'm guessing this is an easy question that I'll feel stupid for asking for all of 15 seconds. When planning for a flight, where do you get the altimeter setting for the destination airport other than AWOS/ATIS? Or do you just call the phone number attached to the AWOS (or closest AWOS) and then I may not need to calculate because the AWOS may just tell me the density altitude?
 
AWOS/ATIS will tell you the altimeter and AWOS often times includes the density altitude as well.
 
You want to get the alt setting as you go being it will change... so AWOS or FF if you have ADSB in
 
DA = Pressure ALT + 120(OAT-ISA)

rough but gets pretty close
 
NoBShere said:
Summer is here and I am brushing up on density altitude. I'm guessing this is an easy question that I'll feel stupid for asking for all of 15 seconds. When planning for a flight, where do you get the altimeter setting for the destination airport other than AWOS/ATIS?
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Don't sweat too much over the altimeter setting to forecast density altitude at your time of arrival or departure. Change of air pressure is a factor, but a minor one compared with elevation and temperature. If you know the field elevation and the expected temperature at your time of arrival or departure, just use the current altimeter setting and you'll have 95% of what matters for density altitude and performance.

- Martin
 
Thanks for the responses. I wanted to do some review before I took off so that I could confirm I could get home.
 
Martin Pauly said:
Don't sweat too much over the altimeter setting to forecast density altitude at your time of arrival or departure. Change of air pressure is a factor, but a minor one compared with elevation and temperature. If you know the field elevation and the expected temperature at your time of arrival or departure, just use the current altimeter setting and you'll have 95% of what matters for density altitude and performance.

- Martin
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Explain what you mean by "95% of what matters...."

Afternoons at KAPA...ground is 5800 MSL, DA can be 9000 ft, sometimes higher depending on temp & barometer.
 
NoBShere said:
Thanks for the responses. I wanted to do some review before I took off so that I could confirm I could get home.
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Well, unless you're in mountainous areas, and flying something 90 HP or less, you shouldn't have a problem, other than being miserable in the heat.
 
If the destination AWOS is part of the NWS or FAA data networks, the current weather, including temp and altimeter setting, will show up right on your EFB. You can also call the AWOS station if available, or get info directly from the web via several resources. If your destination airport is not in one of those data streams you can use weather at a nearby airport as a guide, using the altitude at the destination airport in your calculations. You can use weather data from the NWS or other weather source to get more detailed temp and barometer data at future times at your destination location. Windy is a nice app/service for aviation format data.
 
murphey said:
Explain what you mean by "95% of what matters...."
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What I mean is DA is mostly a function of field elevation and temperature.
Altimeter setting affects DA, but less so than field elevation and temperature. Therefore, if you don't have a forecast for the altimeter setting (which is what I thought the OP's scenario was), just use the current setting - it'll be close enough.

Let me put it yet another way:
  • I have heard of accidents where pilots underestimated the field elevation.
  • I have heard of accidents where pilots underestimated how the warm temperature affected DA.
  • I have yet to hear of a DA-related accident which occurred only because of low barometric pressure.
- Martin
 
I always treat density altitude as an aircraft's performance altitude. Your POH has these numbers.
 
If you are curious about the destination and your are sure you are going, just enter the AWOS/ATIS phone number in your cell and call it anytime as needed. Lets say you overnight there. Sure, in the morning you can look up a METAR but it will surely be 52.999999 minutes old LOL! However just dial in and its 1 minute old - probably most useful for winds and ceilings. But most stations give DA once it exceeds a threshold. The other day I called one of the stations about 100 miles north of me in central Minnesota. The DA = 4100 - holy crap for around here in our flat low lands! I think their field elevation is only 1400.

Similar question as others: What plane? What route? If you are up in the Mountains lots of people (NOT ME) to help you out.
 
Airport altitudes are published on vfr charts and in the chart supplement. If you can't get an altimeter setting, set the altitude to the field altitude, this should be a good enough approximation. From there you can get the altimeter setting from your Kollsman, you should have a thermometer in your airplane, so you can figure it out yourself. Be careful, some airports have different elevations at different locations. These are usually spelled out on the airport diagram. Probably not an issue, but be aware of it. Much easier to listen to the AWOS though.
 
So, this came to mind during a quality review meeting (snooze fest). I was just thinking about this weekend and it’s going to be hot. Might as well think through what kind of density altitude I would be dealing with. Totally blanked on the metar, darn quality metrics.

Anyway, I fly in California so mountains are a factor and yes I have done two mountain flying lessons. One during my dual xc, and then one to firm things up after ppl.
 
The theory of density altitude was just a concept to me. I was flying at sea level mostly and fairly level terrain. Then one day I was flying an unfamiliar aircraft out of a 2500 foot elevation airport, fully loaded, on a 96 day. And I didn't lean the mixture. And there was a significant hill off the end of the runway. And my precious son was in the plane with me. I almost killed us both. I couldn't out climb the hill and flew directly over a winding river, below the tree tops for three miles. We finally came out over a lake where we were able to gain altitude. That day, the terrifying reality of density altitude performance took on a very real meaning. It's a very easy way to kill yourself and your loved ones. Stupid easy really. Just push the book numbers a bit because you've gotten away with it before... and then one day the trees are filling your windscreen and your mind is scrambling trying to figure out how you got here.
 
Lndwarrior said:
The theory of density altitude was just a concept to me. I was flying at sea level mostly and fairly level terrain. Then one day I was flying an unfamiliar aircraft out of a 2500 foot elevation airport, fully loaded, on a 96 day. And I didn't lean the mixture. And there was a significant hill off the end of the runway. And my precious son was in the plane with me. I almost killed us both. I couldn't out climb the hill and flew directly over a winding river, below the tree tops for three miles. We finally came out over a lake where we were able to gain altitude. That day, the terrifying reality of density altitude performance took on a very real meaning. It's a very easy way to kill yourself and your loved ones. Stupid easy really. Just push the book numbers a bit because you've gotten away with it before... and then one day the trees are filling your windscreen and your mind is scrambling trying to figure out how you got here.
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And the lowering of air density with temperature is a double hit: not only does it rob lift, it also robs engine power. And if it's humid, the water molecules in the air (which don't contribute to combustion) steal a little bit more power from the engine as well. The percentage changes of density and water content of the air are pretty small with changes in temperature (10% change in density between 0-30 deg C), but climb is due to EXCESS power (and lift), which is but a fraction of the total power output of the engine and total lift developed. So even small changes in power output or lift have have a large impact on the ability to climb by diminishing the available excess. (That's the rationale behind the high compression STC for the AA-5 and AA-5A models, which essentially provide an additional "10 hp" over the stock engine. This small increase in power results in a large increase in climb rate.)
 
chemgeek said:
And the lowering of air density with temperature is a double hit: not only does it rob lift, it also robs engine power.
[...]
climb is due to EXCESS power, which is but a fraction of the total power output of the engine and total lift developed. So even small changes in power output or lift have have a large impact on the ability to climb by diminishing the available excess.
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That's exactly right. And I am amazed how many pilots do not know how to properly lean the mixture for high density altitude take-offs to squeeze out all available power. This is not the time to fly full rich.

- Martin
 
Martin Pauly said:
That's exactly right. And I am amazed how many pilots do not know how to properly lean the mixture for high density altitude take-offs to squeeze out all available power. This is not the time to fly full rich.

- Martin
Click to expand...

And you don't want to be full rich for landing like you would at lower elevations in the event of a go around.
 
First there's been a lot of good replies to this thread and I have enjoyed reading them. But my question to NoBShere is why are you trying to calculate Density Altitude at all? What is really important is the takeoff and landing distances required at the airports you will be using or may have to use. Let me explain my reasoning.

Once you are airborne and clear above terrain and obstacles, density altitude doesn't matter that much. The airplane will do what it's going to do and you don't have to calculate it. That is a problem for the engineers, not the pilots. When you get close to the ground and are considering landing at some airport the important information to know is whether or not you can land at that airport safely, and take off again when you're ready to leave. Most poh manuals give you take off and landing distances based on pressure altitude and air temperature correction, at least the ones I have read.

When I was learning how to plan a cross-country my flight instructor told me to plan a flight to land at my destination with FAA minimum fuel left on board. From Fredericksburg VA KEZF his suggestion for my Cherokee-140 was Knoxville TN. He wanted the takeoff and landing distances required with prevailing weather at each airport, at an alternate airport nearby, and at 3 airports along the way.

His point was driven home to me in a big way. Depending on the prevailing weather nearly any airport can be too short or just long enough, and you as a pilot need to know that information before you get in the cockpit. Density altitude by itself is an arbitrary number, but takeoff and landing distances are hard numbers that you need to know, and add a reasonable margin of safety for.

When you are flying over the terrain in hot weather, what length runway is acceptable, marginal, or unacceptable? If your engine quits, you are going to take what you can get, but there are a lot of other reasons why you might want to land sooner rather than later along your route of flight. If you pull up nearest airports on your GPS which ones can you use and which ones do you need to eliminate? Landing and takeoff distances vary dramatically with pressure altitude and air temperature, so each airport at a different elevation will have a different number. I like to add 1000 ft elevation to the highest airport along my route, and calculate the landing and takeoff distances for that airport with prevailing weather at the hottest part of the day. Those are the numbers you need to know.

Have FUN! Fly SAFE! Petehdgs.
 
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William Pete Hodges said:
why are you trying to calculate Density Altitude at all? What is really important is the takeoff and landing distances at the airports you will be using or may have to use.
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And a big factor for take-off and landing distance, along with wind, runway slope, and runway condition, is density altitude. The performance charts in the POH start with pressure altitude and temperature, which gives you density altitude based on those two variables.

Once you are airborne and clear above terrain and obstacles, density altitude doesn't matter that much. The airplane will do what it's going to do and you don't have to calculate it. That is a problem for the engineers, not the pilots.
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Agreed. However, to get to above the terrain can very much depend on density altitude. Look at your rate of climb at full power and Vy at different density altitudes - say, 2,000' and 8,000' - and it's a night-and-day difference. Terrain you can easily out climb on a cold winter day may be a real challenge on a warm summer day - all because of density altitude.

- Martin
 
It is well worth evaluating density altitude for a trip to evaluate runway length for both landing and subsequent takeoff, and to determine if you can maintain adequate terrain clearance. I don't usually fly over terrain high enough, nor to/from runways short enough, for density altitude to be a large concern, but a quick check is always prudent, especially if flying to/from higher altitudes in hot conditions, or overflying higher terrain. A temperature and a local or destination area barometer setting is plenty accurate for this purpose. The non-standard barometric correction factor is only a few hundred feet either way, typically, and could be ignored as a first approximation. You can consult a denalt chart or do a quick spin on the E6B to get the density altitude figures required. The POH will tell you the rest in terms of performance allowances required.
 
Martin,
I agree that pressure altitude and temperature are what density altitude is, but I have yet to find a POH that actually uses DA to determine takeoff and landing distances. They all start with pressure altitude and adjust for temperature. That means calculating density altitude is pointless because you can't reference it to the performance data in the POH.
Now you can assume POH climb data for Altitude varies directly with density altitude Instead of standard altitude as shown in the manual, but do you need to calculate it? No. Just listen to AWOS and add the density altitude warning to your climb altitude chart and airport elevations. Why waste all that brain power when you can determine the same information faster with less work?

Have FUN! Fly SAFE! Petehdgs
 
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Since I'm on a roll I want you to consider this:

Aug 12, 2016, KEZF 6 dead after airplane misses landing at Shannon Airport Fredericksburg VA. Look it up. Two high time professional pilots with 4 paying customers and neither pilot knew the actual landing distance required spot on short field technique to accomplish successfully. Instead they came in too hot, heavy, and unprepared.

That day at Shannon Airport the accident aircraft required 1640 feet of ground roll to stop, more than half the total length of the runway. They didn't even touch the pavement until 2/3 of the runway was already gone. They couldn't get it down and stopped, so they started a go-around very late in the ground roll, but that didn't end them, they ran out fuel on one engine during the go around and lost control at low altitude. A very sad situation at my previous home airport. This accident haunted me for a long time.

I maintain that this is a Density Altitude related accident. KEZF is just under 3000 ft (depending on which data you read) 85 ft elevation and the runway has a highway at one end and RR tracks at the other. It is deceptively unforgiving in hot weather with a light wind straight up the runway. Why? It is moderately short for larger and heavier aircraft and a large updraft is developed in the touchdown zone by a straight on light wind on a hot day. A double whammy if you are not on your game.

If either pilot had checked the landing distances required they would either have used a spot on short field technique, to ensure a positive touchdown in the first 25% of runway length, or diverted to KRMN, a 5000 ft runway, just 10 NM away. Very sad.

So Martin, do you still think I am missing the point? (You didn't actually say that, but it appears to be implied) Petehdgs

P.S. Martin and I have PMd on this and I am still laughing about his term "Violent Agreement" to describe our banter back and forth. so funny.
 
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