Letters: 08/09

Diamond Fuel Burn?

On your review of the Diamond L360 in the July 2009 issue, a note on the fuel burn: It cannot be 10.8 GPH per side, as you stated. I had a Twin Comanche-same engines-and burned 11 GPH total for years. Your correspondent mustve meant 10.8 GPH total.

My Aero Commander straight 500 with 250-HP engines burned 22 GPH both sides,

Lycoming Engine


which corresponds with what your correspondent stated the Diamond would burn. Just so you know.

Nico van Niekerk,
Thousand Oaks, California

No, we actually meant 10.8 GPH per side, not total. But some clarification is in order. First, your Comanche had IO-320s, not IO-360s. The larger engines have 20 additional horsepower. Second, the fuel burn figures we gave correspond to about 70 percent power and the comparison was made to the Thielert diesels, which burn about 5.5 GPH each to deliver cruise in the low 150-knot range.

If the L360 were used in the training role-which is probably its primary market aim-the engines would likely be operated at lower power settings, so a fuel burn of 8.5 GPH per side is more realistic.

Gear of the Year

The Garmin GPSmap 696 is a great product, Id buy one tomorrow-except for the horrible subscription prices. If one gets an annual subscription for all of the data needed for the basic functions, it would be close to $1000, not including AOPA info or weather.

The real kicker is plates at $395! And thats the reason a lot of folks are not buying. Garmin needs to get their data prices in line.

Larry Olson,
Via e-mail

Tape Displays

Perhaps I can answer your question relating to tape instruments versus dials, as reported in the July 2009 issue. As an Air Force pilot, I flew both types of instrumentation and feel qualified to comment.

I started out in the F-100 with round gauges. After a little over 500 hours in that airplane, I transitioned to the F-105D. The earlier F-105B had round gauges, but the D-model had the vertical tapes.

In case you are not familiar with the F-105, it was a single-seat supersonic nuclear attack aircraft capable of over Mach 1 on the deck. Republic did quite extensive studies of human engineering for the F-105 cockpit because the single pilot would be quite busy operating the radar and weapons systems, as well as flying and navigating the airplane. As far as I know, it was the first production aircraft to use vertical tapes for airspeed and altitude.

I was an instructor pilot and also a ground school instructor teaching “airplane general,” meaning the basic airplane systems. This was during the Southeast Asia conflict and we were getting pilots from all different types of aircraft being transitioned to the F-105 prior to going to Vietnam. One of the biggest questions the students had was: What was it like to fly the tapes?

I always told them not to worry; by their second flight, they would feel right at home. The reason was that in the 105, we could preset the airspeed and/or altitude desired with a marker-a narrow strip of metal with two white lines separated by a black line. We called it “the captains bars.”

This marker would stay at the bottom or top of the instrument tape until that airspeed or altitude came into view and then it would lock onto the tape at that airspeed or altitude. You would then fly so as to put the selected numbers on the lubber line on that instrument.

The lubber line also went straight across the attitude indicator between the airspeed and the altitude and-this is the important part-when everything was centered, all you had to do was stare at the attitude indicator. You could see the captains bars in your peripheral vision, greatly reducing or eliminating the need to cross check and making the instrument flying part of a mission much easier, leaving more time to accomplish other tasks .

I later flew the F-101 and F-4D and greatly missed those tapes. When I went to the airlines, the first tapes I saw were the radio altimeters in the B-727 and they worked just like the tapes in the Thud.

Why the round gauges for the RPM and MAP? My guess is they dont change much after they are set, so you really dont need to watch them as closely as the airspeed and altimeter. If you ever fly an airplane set up the way I described for the F-105, youll throw stones at steam gauges. I love my Comanche, but I sure wish it had tapes.

Marty Case,
Via e-mail

Experimental Avionics

Your article on used experimentals (June 2009) is extremely timely, but glosses over one very important cost factor: the cost of upgrading avionics. For instance, an electronic flight bag such as the Flight Cheetah 21 from Aviation Safety has the functionality of a Garmin GNS530, except the VHF component. True, its not certified for WAAS, but it has the functionality and your comments on the safety of non-certified icing equipment would apply here.

The TruTrak EFIS is another example. Its a brilliantly easy piece of equipment to use. There isnt a directly comparable piece of equipment available for certified GA. As avionics are about a third of used aircraft acquisition costs if current equipment is contemplated, the omission skews the comparison.

You also glossed over the inevitable costs of maintaining an older airframe, especially aging rubber, window seals, wiring and hydraulics, let alone the cost and reliability of vacuum-driven DGs and attitude indicators.

Perhaps one of the reasons that the Delta with older certified aircraft and experimentals is converging is that more potential purchasers are reaching the same conclusions as I have set out.

David Wilder,
Via e-mail