Letters January 2000

Tachs, PC Sims
Some notes on articles in past issues: With reference to your piece on electronic tachs in the September, 1999 issue, most owners depend on the tachometer hour meter to determine when ADs, TBO inspections and so on are due.

The Horizon tach records hours at a constant rate beginning when the engine reaches 800 RPM, thus it essentially accumulates time like a Hobbs meter. As we all know from our airplane rental experiences, the Hobbs meter accumulates time more rapidly than the tachometer hour meter, often significantly so. Thats not good for the pocketbook because the faster running clock makes all of those pesky maintenance items come due earlier and more frequently.

The discrepancy in the EI tach is reduced because it starts recording at 1300 RPM. In fact, averaged over a long period, the EI tach may actually record a bit slow compared to the mechanical tachometer.

No replacement electronic tachometer correctly emulates the mechanical tachometers proportional recording rate, which is centered at a calibrated cruise RPM. And thats what is needed in a product.

On the subject of the September article about PC Simulators:You mentioned Elites control interface box like it was an advantage. But you neglected to mention that the box requires a serial port be available on the PC, which may or may not be a problem depending on what devices you already have attached. The box also precludes you from taking advantage of the new USB control yokes, which seem to be the industry direction.

Bob Newman
Warrenton, Virginia

Audio Shoot Out
You made many excellent points in the article on audio panels, the most critical being the importance of high-quality audio in the cockpit.

Thats why we take such great pains in audio design and execution and why I have to comment on a few of the statements made in this article.

No other units, in our opinion, comparetoe-to-toe with the PMA7000MS. The units mentioned are closer in feature set to our last generation unit, the PMA6000. The features in the 6000, including dual transmit split mode, were unheard of in general aviation. One popular innovation credited to the competition, is our swap function. This was so unique when we introduced it that we own a patent on this feature.

We do take issue with your cost comparison, as well as the caveats about plug and play compatibility. The KMA24 simply didnt have the pins available for the features in the PS systems. However, the radio interconnections, more than half of the installation, is the same as the standard KMA24. To the extent possible we are plug-compatible. This compatibility reduces the installation costs, compared to other modern audio panels. What you failed to mention is any other audio panel requires a complete re-wire of the radio stack, compared to the PS Engineering units. It can cut about eight hours off of the installation time.

Aviation Consumer readers should get the full story about costs from their avionics shop. The dollar difference can disappear when all of the considerations are accounted for, including warranty. The total cost of ownership of PS Engineering products is typically less than the competition.

The digital warning and recorder system is just that, an option. If fully implemented, it adds seven wires to the installation, about $7 in parts and three hours in labor. In return, this gadget provides another set of eyes on the gauges and another set of ears on the radios.

The PMA6000 is still a great option. The price differential is probably greater than that described in your article, because we offer versions without the marker beacon and stereo music.

There is no need to pay for a marker beacon receiver, or stereo capability if you dont need it. You mentioned the button tactile feel. We have tripled the spring tension since the unit Larry Anglisano installed, another example of PS customer responsiveness. We believe that the positive switch action and the visual indication provided by the physical position are critical to giving multiple operational feedback to the pilot. These switches are far from fragile. They are rated for 100,000 cycles and environmentally qualified far in excess of the aviation standards. They may seem fragile compared to a membrane style switch, but these are independent switches that can be configured for many different purposes, both reducing the cost of ownership as well as increasing the flexibility.

Todays audio panel, from any manufacturer, is a quantum improvement. When all the facts are known, were confident that our systems represent the best value. Still, all pilots will be well served by any audio system that incorporates effective sound quality and logical audio control.

Gary Picou
PS Engineering

Garmins View
I am the principal designer and engineer for the Garmin GMA 340 Audio Panel. I was pleased to read Larry Anglisanos article about audio panels in your November issue.

I appreciate your magazines impartial treatment of the Garmin GMA 340, given that one of your contributing editors is an employee of PS Engineering. Although I expected impartiality from a publication such as yours, one never knows these days.

I didnt understand the comment about the all-or-none operation of the marker beacon and the comment about it not being possible to be armed with audio off. Summarizing marker beacon operation: Like all other major players, the GMA 340 allows the user to select or deselect the marker beacon as an audio source. However, in no way whatsoever can the marker beacon receiver be turned off. While it is possible to de-select the audio, the receiver itself cannot be turned off.

This is done on purpose. If the audio is not selected, the lamps continue to operate normally. So the GMA 340 is always armed. Based on the comments in the article, Im not sure this operation was understood completely.

Additionally, a particular area where the GMA 340 excels is in its SmartMute marker beacon muting function, which monitors the marker beacon signal after muting and then automatically un-mutes when the signal is no longer detected. This feature was not described in the article but has been an overwhelming success in the field.

Here are some other items regarding the GMA 340 that may be informative:

No electromechanical relays are used except for the fail-safe function. Slew-limited FET switching is employed through out. Continuous bias of the headset microphones by the GMA 340 during comm transmit to prevent large pops in the headset. Other audio panels use relays to switch the headset mic directly to the comm, thus creating a disconnect/reconnect transient at the onset and release of the PTT.

GMA 340 development included rigorous HALT (Highly Accelerated Life Test) testing with temperature extremes to 100 degrees C and jackhammer-type vibration profiles with RMS and peak G-loads far exceeding any TSO requirement. While another manufacturer claims to be KMA24 compatible, they currently do not have a chassis design with the same sloping shape as the KMA 24. We do, however.

Because the GMA 340 is a high fidelity product, it utilizes audio-quality shielded connectors rather than the old-fashioned card-edge connectors, that are frankly more suited for consumer grade PCs than aviation. When have you seen a high fidelity stereo that didnt use shielded connectors? With surface mount technology in widespread use, PC board flexure can cause latent cracks and other failures in SMD components.

A card-edge connector forces the PC board to become a mechanical component in the connector design and is not optimal in terms of PC board stress and flexure. The D-SUB connectors on the GMA 340 transfer mechanical stress to the chassis of the unit, not to the PC board, a superior mechanical design. Last, user configurable MASCM processing greatly reduces ambient noise from the aviation radios when no activity is occurring.

Brian Poindexter
Garmin Senior Design EngineerOlathe, Kansas

Early versions of the GMA 340 did not have always on marker beacon receivers. We should have pointed that out. As for our objectivity, we seem equally adept at pissing off all sides of any argument.