Aftermarket modifications have been the lifeblood of the general aviation industry for years, ever since the production of new aircraft dropped precipitously in the early 1980s. Thanks to the increasing sophistication and affordability of microprocessor technology, much of the attention has focused on avionics, displays, and instruments in the cockpit. And although they may be less sexy, there are great and useful modifications for engine and airframe as well.
My partner and I have propelled our airplane into the twenty-first century with an extensive array of mods inside and out. We now have two GPS systems - one for IFR navigation and the other for identifying special-use airspace (it has a small moving-map display). We have an electronic engine-monitoring system, sophisticated autopilot, multi-feature intercom, an HSI, and an annoying but highly desirable gear-up alert. We have a new interior, a new upper door latch, sleek new engine cowls, firewall-mounted oil filters, "spats" to smooth the airflow over the main-gear wheels when they are retracted, and cuffs on the leading and trailing edges of the wing roots. The list goes on.
Each of the mods has added something to the airplane in the way of increased utility, safety, performance, comfort, or aesthetics. That's why we've spent the money. But there is an additional price to pay beyond dollars and cents. Modifications add to pilot workload. Even something as passive as our new nose cowls increases our workload. The cowls feature miniscule cooling-air inlets, which result in lower drag. However, engine cooling now is more of a concern.
When we installed the new cowls, we also added an oil temperature probe to our electronic engine monitoring system. Now we get a numerical readout of oil temperature to the degree. Before, we had to rely on the position of the faded needle on the airplane's standard but suspect analog oil temperature gauge. The result is that we now know the oil temp on the right engine runs hotter than the left engine. Because of this new knowledge, I routinely open the right cowl flap slightly while in cruise to allow a bit more cooling air to pass across the oil cooler.
In this case, the combination of two modifications - the new engine cowls and the oil temperature indication on the engine monitoring system - has caused us to pay closer attention to oil temperature in all phases of flight, and to continuously manage cowl-flap position.
There's more. The wing-root cuffs are intended to smooth the airflow over the horizontal stabilator at low speeds, primarily on final approach and landing. They work as advertised, but the welcome change in handling in that critical phase of flight meant that I had to learn and practice a slightly different, but different nonetheless, piloting technique.
Even the new interior forced a small change. The upholsterer replaced the old seat cushions with new foam. Now I sit higher, which means my view out of the windshield is slightly different. The change in perspective has affected my perception of the airplane's height above the runway, which affects my round-out technique, which affects the quality of the touchdown. I've had to adjust.
Some modifications, like the new interior, require a subtle change in piloting technique. Others have more significant consequences, including specific changes in the checklist and procedures and a supplement added to the pilot's operating handbook (POH). One such modification is the addition of an auxiliary fuel source, like tip tanks. Our airplane doesn't have aftermarket tip tanks, but I occasionally fly one that does. The additional fuel is great for making nonstop cross-country flights of more than seven hours, but managing the fuel is a work-intensive task.
One reason is that the pumps drawing from the tip tanks take too much fuel. The excess fuel beyond what is needed by the engine is fed back into the left main tank. So, as the fuel quantity in the tip tanks is diminishing, the amount in the left main tank is increasing. You can imagine the detailed notes and close attention to time that are needed to properly manage the fuel as a result. And it's all explained (although not very clearly) in a supplement to the POH.
Because my partner and I have been involved in purchasing and installing all of the modifications to our airplane, we are familiar with the resulting changes in performance and operation. We also have seen how each change has required changes in our piloting technique.
You may not have been so involved with changes made to a rental or club airplane. Unless you've received a thorough briefing by a flight instructor familiar with and current in the airplane, you may not be aware of modifications that call for a change in procedures or technique. It's best not to assume that a rental or club airplane is completely stock, even if you have had a good checkout. Ask the instructor if there are modifications such as auxiliary fuel, a standby vacuum pump, or a complex audio panel - each of which calls for specific instruction. Also, study the flight manual or pilot's operating handbook supplements that describe modifications. Any procedures and limitations will be spelled out in the supplement.
Modifications are how our older airplanes keep pace with newer ones. But the benefit that a modification bestows also carries with it a responsibility for the pilot to know about the nature of the change, its operation, and its limitations.
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