Sunday, July 21, 2013

Density Altitude to the Max

I'm headed to Truckee, CA tomorrow and the forecast high temperature is 102F/39C.  Since Truckee is at 5901MSL, the density altitude is effectively infinite.

So how will I handle it?

First, I had originally planned a fuel stop, but the DUATS flight planner using the route I actually  intend to fly (rather than some impossible shortcut: see here) shows me arriving with a little over one hour's fuel remaining.  Less weight is a big help in this kind of situation, reducing stall speed and landing distance, so I don't want to have much more fuel than that.

Second, I will keep an accurate navigation log, and if things are not going as expected I will make the fuel stop anyway, but I won't fill the tanks.  It should be a little less than 2 hours from truck to Truckee, so I'll depart with 3 hours of fuel.

Playing with a Koch chart shows that my expected landing and takeoff distances are about 260% of normal.  Truckee's longest runway is 7,000' long, more than 4 times my normal landing distance.  With the high density altitude, though, touchdown true airspeed will be pretty high, so I'll need to brake carefully.  Again, less weight means less energy means less braking.

Checking the TAFs shows 8 - 15 knots of headwind on arrival; that's a big help, too.

Truckee is very noise sensitive and there is lots of glider activity; the airport publishes arrival and departure routes to help reduce the noise and keep the powered airplanes (in this case, me) separated from the gliders (someday, me).  I have downloaded the guide and put it into my tablet device using Dropbox.

I would like to depart for home with full fuel, but with density altitude this extreme I might decide to depart with less and stop someplace lower for more.  "Lower" is a loaded term, because fuel is relatively inexpensive at Truckee, so lower might be higher when it comes to money.  But safety trumps money.

The only remaining question I have, then, is what will the heat do to the fishing?


At July 22, 2013 at 7:29 PM , Blogger Tim Brennan said...

29 (the default and best runway, all else being equal) is plenty long enough for landing anything at any DA, and the terrain off the departure end is pretty friendly as well on the way out. I fly out of there every other week or so in a 172XP and even when the DA sign says '9000' I don't find it to be difficult even at gross. In my opinion the bigger worry is the heavy mix of traffic: gliders on 20, big jets on 29, etc.

Have a great trip!

At July 23, 2013 at 2:18 PM , Anonymous Shamim said...

Hey Jim and Tim! I just got back from KTRK on Sunday, and we saw the density alt sign go to 9900. Bets were being taken on when it would hit 0000....

I also found that at den. alt. of 8600' and 7kt right down the runway my (150 HP) Citabria with half fuel needs almost 900' of runway to lift off. I was only getting 500 fpm but found a nice thermal at Frog Pond that got me to 10.5 in a couple of minutes. And no headwind on the flight back to KSQL so I didn't have to make the planned fuel stop -- unusual weather all round.

At July 26, 2013 at 1:16 PM , Blogger Dr.ATP said...

Wind calm and 24C, the Archer used less than 1500 feet of runway. I was light: me and full fuel. Climbed to 9500 before VERDI, called Norcal Approach, and cruised home non-stop with zero headwind/tailwind.

Remember, there is no longer any such thing as "usual" weather.

At August 7, 2013 at 10:04 AM , Anonymous Curious said...

What's your tablet device and software? Cheers.


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