Electronics in the Summer Heat, Have a Backup!  

By Bill Cassels 


Early in the year we decided to fly our S35 Bonanza from Mogollon Airpark to Redding, CA. My youngest daughter lives in Redding, and we were going to spend the week visiting and then capture the four-year-old grandson for a month or so.

Since we’d be going somewhere near the California coast, about April I thought I’d better get my Instrument Proficiency Check completed. I hadn’t really flown on instruments in practice or actual for years, and it showed during my first flight with my CFII. The first few hours we used his Cessna 182 on basic gauges, working on turns and climbs, then simultaneous turns and climbs. After a while I had my basic skills back, and it was time to move into the Bonanza and do approaches. At first we went out to Luke AUX1 ILS and shot a bunch of ILS approaches. We flew a couple of trips out to AUX1 to do the ILS, around and around, getting beat into me “Small Corrections!” Now we’re ready to run down to Casa Grande and practice holding in the stack over the VOR and work on our foreign language skills. 

electronics summer heat 1

Even though for enroute charts and approach charts I’d planned on using ForeFlight, running on an iPad Pro connected to the Stratus2 ADS-B In, I printed out the AUX1 ILS and the Casa Grande approaches. Everything went fine, during training I can still read paper charts. I didn’t even think about practicing with ForeFlight. It wasn’t like I didn’t know how to use ForeFlight, since I used it a lot on VFR trips without ever having a problem. 

On July the Fourth we departed AZ82 at 0800 heading to Fox Field (KWJF) for our midpoint fuel stop. It was going to be severe clear all the way from AZ82 to KRDD via KWJF. I climbed to 8500 feet and went direct on the magenta line on the iPad to KWJF. Even at 8500 feet it was warm with the OAT at 75 degrees. About 20 miles East of KWJF, ForeFlight slaps on a message that the Stratus-2 is hot and will shut down soon. It wasn’t lying, and a couple of minutes later, no more magenta line on ForeFlight. No problem, I had KWJF loaded into the GNS430W, and it had its own magenta line. I took the Stratus-2 down from its place on the glare shield and put it on the floor to cool down. 

electronics summer heat 2

A few minutes later we landed at KWJF. I dumped the peeps off at the terminal and taxied over to the self-serve. At the end of fueling, the peeps showed back up, so we loaded and got ready to go onto KRDD. At this time the Stratus-2 seemed to have cooled down enough to run, so back on the glare shield mount it went. The magenta line reappeared now pointing to a 2.5 hour flight to KRDD. In the mean time I pointed all of the outside vents toward the Stratus-2.

Now about 20 miles Southeast of KRDD, about the time I’d be bring up the instrument approach plate on ForeFlight, the iPad displayed that it was overheated and was going to shut down soon. Once again electronics don’t lie, and the iPad turned itself off. Now that certainly would have been inconvenient if I were in IMC and needed the enroute charts and approach plates. I didn’t think I needed to bring along any paper charts, so back to the GNS430W to finish the trip to KRDD. 

During the week in Redding, I stopped by the store and bought a couple of different little freezer packs. I figured I would freeze those things and pick whichever one fit the best lying on top of the Stratus-2 in its place on the glare shield. That much worked, and I guess coming back at 9500 feet it was cool enough for the iPad with all the vents pointing at it this time. 

electronics summer heat 3

Since then I found out that the Appareo Stratus ADS-B Out Transponder connects directly to the Stratus-2, providing it power and an outside antenna. The Stratus-2 can be moved inside the panel in the shade. And I can point an avionics cooling hose at it. That should solve the problem with the Stratus-2 overheating. Now I don’t know what to do about keeping the iPad from overheating and shutting down. I’ve heard they make iPad holders with built in fans. But does that mount onto a Ram iPad mount? 

I certainly learned not to totally depend solely on consumer electronics, and thankfully this all happened in VFR conditions and not in IMC. I suppose in IMC it might have been cooler and the iPad would have been okay. 

I’ll probably bring paper charts with me as backup next time, or even maybe a spare iPad…. 


Now that you have reached your cruise altitude, you can switch off the fasten seatbelt sign and take a moment to relax. If you are in a technologically advanced airplane, or carrying a tablet, you are probably looking at multi-colored LCDs throwing all sorts of information at you. My Electronic Flight Information System has readouts of fuel flow, range, miles-per-gallon and indicated/true/ground speeds. Is there a way to minimize fuel flow, maximize MPG, and maximize airspeed all at once? Probably not, but we will explore the factors effecting each and some other performance metrics that we may not normally think about.


Maximum Time Aloft

The airplane's time aloft or endurance is its fuel on board (gal) divided by fuel flow (gph). To maximize time aloft, one must maximize fuel capacity and minimize fuel flow. Fuel flow is minimized by using the least amount of power to sustain flight. This is the bottom of the power required for level flight curve. On an endurance flight, as fuel is consumed and the airplane weight decreases the minimum power required will decrease and the throttle can be reduced. To fly for maximum endurance, you will be flying really slow, at or near your Vx speed, so this is rarely done in general aviation. An example of when this would be useful is on an observation mission where the airplane needs to remain on station for the longest possible time.


Best Miles per Gallon, Best Range Speed

This probably gets your attention because now we are talking about saving money. The best miles per gallon will result in the maximum range of the airplane. Another way to express it is that this cruise speed will use the least amount of fuel for a given trip distance. The fuel used on a trip depends on fuel flow and the time it takes to make the trip. So now we need to minimize fuel flow (keep power low) but make the trip fast enough to use as little fuel as possible (keep power high!). The middle ground is found at the bottom of the L/D curve of the airplane and is close to, if not the same as, the Vy speed. In the interest of saving fuel, we get to fly a little faster but still at a relatively slow airspeed.


Best Speed per Gallon

Optimum cruise speed was derived by B.H. Carson of the U.S. Naval Academy in 1980. For those of you that can remember back that far, this was after the oil embargo. Suddenly the country was focused on the fuel efficiency of cars and airplanes. Carson addressed the question of using miles-per-gallon as the optimizing metric for airplanes. He noted that for airplanes the speed for optimum fuel efficiency was quite slow and utilized only a small percentage of the airplane's available horse power. Instead he derived a new metric, the optimum cruise speed which maximizes the speed of the airplane relative to fuel flow. It is the best speed per gallon-per-hour ratio that can be achieved. The optimum cruise speed that he derived is 1.32 times the best miles-per gallon speed. It is a bit more practical than flying at best range speed.


Here are the V speeds discussed in this article derived from the performance charts I measured during the flight test period of my RV-10. The blue line is the thrust required for level flight and is read from the axis on the left in pounds. The red line is the power required for level flight and is read from the vertical axis on the right in horsepower. The maximum endurance speed is a very slow 70 KIAS and only requires 57 horsepower from the 260 horsepower engine. The maximum range speed is 90 KIAS and requires about 100 horsepower. The optimum cruise speed is 120 KIAS, still somewhat slow, but better, and requires around 120 horse power - still only 46% power.


Next month we will discuss how to determine these performance curves for an airplane through flight testing.


Please login to add a comment.

Back to Top