How we delineate different aircraft has always been a bit murky. On one side, flight schools require detailed checkouts for every flavor of Cessna and Piper. On the other, the FAA treats an entire line of Citations built over many decades as the same type rating. Ditto the very different Boeing 757 and 767.
What we often fail to pay attention to is subtypes of the same airplane, assuming that only minor modifications from the manufacturer mean we’re left with largely the same airplane. That may be true for a Cessna 172M and 172N, but according to Twin Commander training guru Barry Lane, it’s not true of Commanders.
Take the 690 variants as an example. He calls the 690A and 690B Rockwell airplanes. They were designed and built by Rockwell, and the company’s philosophy is wholly represented. The 690C is a bit of a hybrid. Designed by Rockwell, but under Gulfstream’s ownership, changes started to creep in. By the 690D we’re still in the same type designation, but it’s fully a Gulfstream airplane with many changes—some subtle, and some significant. The power indicator changed from horsepower to percent torque, for example. It’s the same thing, but it will get the pilot’s attention.
Lane said his experience at one memorable training event cemented the idea of just how different these airplanes can be. A fleet operator was accustomed to operating 690As and 690Bs, and had recently acquired a 690D. He suggested they undergo differences training, something the pilots and technicians deemed unnecessary. During the training session they called him to the shop to show off a panel upgrade on the 690D, and something was amiss.
“I knew something was wrong, and it was just bugging me,” he said. “But it took me a minute to put my finger on it.” Finally, Lane realized what had happened. The 690As and 690Bs have interconnected fuel bladders, while the 690D is a wet wing. The wet wing airplanes have a fuel interconnect between the wings with two yellow lights on the panel with a confusing label set-up. The yellow lights indicate the status of the interconnect valve. Below those is a label for the low fuel scavenge pumps. The shop, having never worked on a 690D, had assumed the lights above were a low-fuel warning, and moved the yellow interconnect status lights to a position below their respective fuel quantity indicators.
Lane pointed out the mistake, an ironic one that clearly showed the need for differences training. Unfortunately nothing was changed, and now if he returns to do recurrent training, he must explain how the fuel system works and includes an addendum of how their fuel system works, effectively requiring differences from the differences training every year for the entire staff.
“They were making assumptions,” he said. In that case it caused a mistake that pilots and mechanics are choosing to live with, but it’s a perfect point to illustrate how knowledge of one airplane doesn’t always translate to all. Even different serial numbers in the same subtype can have significant differences.
Overcoming these differences comes down to two factors. The first is recognizing your limitations and lack of knowledge. As pilots, humility and safety often have an inverse relationship. Acknowledging you don’t know everything about every airplane opens up the possibility of learning. And second, seek out training. If you are buying an airplane that’s new to you, get a thorough briefing from the owner about he or she operates, and take the time to find some professional training.
It’s true that many airplanes fly roughly the same. Most of us can keep the shiny side up without much trouble, but with modern avionics, a diverse production run, and many decades of upgrades, it’s smart to look at every Twin Commander as unique.
