A Little Hangar Flying: July the Month of Fly Ins

Boy, I tell ya, the summer is just here and the Fly ins are all over the Northwest. Not only here but all over the USA. Too many to go to and get those summer chores done. So you have to pick and chose the ones you want to attend.

I have a Cessna 180 that looks really bad. That’s why I’m going to a few fly ins. One is the 180/185 fly-in in the Back Country of Idaho. It’s called the Johnson Creek Fly Inn. Mostly 180/185’s from all over the US.

Now when I was up in Alaska flying, every place I went to was a back country airport or off airport landing strip. Why do I go to this place? Maybe it’s the comradery of the guys with like airplanes! Johnson Creek has a runway in between the hills. The hard part is it’s HOT/ HIGH and the WINDS are squirrely. Then there’s mean ol’ Mr. Density Altitude coming into play. That’s the idea of this article this month.

Here’s a quick way in the cockpit to check the Density Altitude. Also, some other things to think about and things I’ve learned over the years.

Take your altimeter and SET the Field Elevation, then set 29.92. See the difference? The 29.92 will give you Pressure Altitude. Now there is temperature. That’s where Density Altitude comes into play. Density Altitude is Pressure Altitude corrected for Temperature. You can get the Temperature off your cockpit thermometer. There’s a formula that will give you Standard Temperature at any altitude. That’s addressed below. Basically the colder the temperature is from standard day the lower the Density Altitude. Vice versa, the hotter it is, the higher the Density Altitude. Sometimes in the winter in Anchorage, Alaska while figuring the Density Altitude we would be way below Sea Level. Great performance.

Next: you can then go to your Aircraft Operation Manual and with this information gained below get the Takeoff and Landing distances needed to make a safe arrival and departure within the in the Lower 48 performance parameters of the aircraft. If your loads are too big, then one option is to make two trips to get all your stuff in or out. Example: Take your flying buddy/wife out last after you see how underperforming the airplane is with all the stuff you needed to camp with. Plus, go in with half tanks of fuel. In other words, fuel for the trip. There’s no need to fill the airplane up.

I did just that and took off when the air temp was low in the early morning at a fly in years ago. I also had half tanks. The place was Pagosa Springs, Colorado. Density Alt. when I landed was 11,300 at 1:00 PM. When I took off it was 7800 @ 8:00 AM. (NOTE: Most AWOS will, if listened to, tell you the density altitude to help make those go, no go decisions. )

You can calculate Density Altitude on your wiz wheel/E6B. Wait! No one uses those anymore; I mean the GPS in the airplane. Your laptop or iPad. There actually is an E6B in the bottom of my Flight Bag. For use when the electricity is out and to show people I know how to use one. Go to the performance page. Most GPS’s have them. Learn to do it before you go somewhere that is guaranteed to have a higher than normal density altitude. Just be aware of what you are getting into.

Now some simple math…It’s the only time in my life I wish I had paid attention in school. Hahaha! I was looking out the window most of the time. But I didn’t know I was going to be a Pilot. I just knew I had to find a job that allowed me to look out windows.

The simple math…Ya right. This is a pretty simple formula since two of the variables will always be the same and the other two are easy enough to find. Let’s say our current altimeter setting is 29.45 and the field elevation is 5,000 feet. That means (29.92 - 29.45) x 10 + 5,000 = 5,470 feet. That’s your pressure Altitude. You can see this by doing what I mentioned above.

Know what the field elevation is and put 29.92 in the Colesman window of the Altimeter. The indication will be the Pressure Altitude. Simple. Now let’s move on to step two, finding density altitude. Here’s the formula:

density altitude = pressure altitude + [120 x (OAT - ISA Temp)] Now, before your eyes glaze over, here’s how simple this density formula is: we already have the value for pressure altitude from our last calculation; OAT is degrees Celsius read off our thermometer (let’s say it’s a balmy 35 °C today) and ISA Temp is always 15 °C at sea level. To find ISA standard temperature for a given altitude, here’s a rule of thumb: double the altitude, in this case 5000 feet X 2 = 10, then subtract 15. Which is -5. (For example, to find ISA Temp at 5000 feet, we multiply the altitude by 2 to get 10; we then subtract 15 to get -5. So, in the example above:

density altitude = 5,470 PA + [120 x (35 - 5)] Working out the math, our density altitude is 9,070 feet.

If you have been to 9000 feet MSL in your airplane you know it is a dog. So plan accordingly.

Now before you get there if the density altitude is way up there. Guess what? So is your landing roll and ground speed due to a higher true airspeed. Your indicated speeds for all operations are the same…Flaps gear etc. What’s different is your True Airspeed, which is going to be higher than your indicated speed. Which MEANS your Landing Roll will be longer. 25 % longer for every 1000 feet elevation. So plan on using the first part of the runway not the last part. Fly your airspeeds. See the notes below.

Remember:

PA+{ 120 x (OAT-STD Temp)} = DA

Don’t forget to lean for Best Power per Aircraft Operating Manual. Aside from leaning, here are some other tips and some good ideas to consider from Crawford, who operates in Boulder density altitudes that top 5,000 feet 11 months of the year according to yearly temperature averages.

• Trust your airspeed indicator, not your eyes, when landing. Groundspeeds can be up to 20 knots faster than you are used to when using the same indicated airspeed (IAS) required by the pilot operating handbook.
• Fly in the evening or early in the morning when temperatures are lower.
• Call a local instructor at the airport where you are going.
• Before flying to a high-altitude airport, know whether your aircraft climbs more efficiently with the first increment of flaps. Many aircraft do, but your results may vary and that first notch may add more drag than lift.
• Be sure the aircraft’s weight is below 90 percent of maximum gross weight.
• Don’t fill the tanks (see previous tip).
• Fly shorter legs and make extra fuel stops (tough suggestion to accept, but it results in less exciting takeoffs).
• Be ready to ferry one passenger to a lower density altitude, then come back for the other. If you are unsure of conditions, fly around the pattern once alone without baggage to test your aircraft’s performance.
• Have 80 percent of your takeoff speed at the runway’s halfway point, or abort. For Crawford, that means having 48 knots IAS in a Cessna 172 at the halfway point. Just another story. In the B-747 old -100’s I was many times shutdown because it was too hot to get off the ground in second segment of Take-Off /Climb to safely depart. What were our options? Reduce the load…not going to happen! Takeoff on a longer runway. Haven’t made one that long!!!. Use another flap setting? Nope, second segment was limited due to engine thrust. So the only thing we could do was wait until the early morning until the temperature went down to a legal number. Then one day or many days in Anchorage the temp is down but the altimeter is so low our performance was out of limits. What options? Reduced the load because the altimeter was not going to come back into acceptable parameters until the next day. We are talking low, lows of 28.76 or so.

So Density altitude is a REAL Serious situation. Pay attention to it.

AOPA has a good online course on this subject at https://www.aopa.org/login/asiCourses/?course= mountainFlying&project_code=&_ga=2.7138043 5.1561281493.1500646370-1185227085.1500646370 Mike Radford References:

Density Altitude calculations. Flying Magazine, By Stephen Pope October 25, 2011 reworked by Mike Radford.

Mountain Flying tips AOPA Magazine, July 1, 2007 By Alton K. Marsh