The only difference between one Dodge Ram and another is the options each owner ordered. One person might want the sunroof, but someone else might not, buy the basics like fuel systems, suspensions, and engines are pretty much the same. Chevys are Chevys, and Fords are Fords. Airliners, however, are all quite different, even within their type.

Millions of passengers board commercial airliners every day clueless about what kind of plane carries their lives. Airline customers pretend to care about safety, they talk about it a lot, but in reality, they’ll ride the back of a sick duck if they can save a buck on airfare. Like the Comet and the DC-10 before it, the Boeing 737 Max will return to the air without as much as a glance from the public.

One of the most surprising things the Unknown Pilot learned from airline flight academy graduates is that not all types of airplanes are alike. For example, one might think that the only difference between one airline B-727 and their competition is the paint. Yet underneath the livery, American’s Boeings are mechanically quite different than United’s or any other airline.

Boeing, you see, has unique codes assigned to every customer. The code for United Airlines is 22, and it’s 23 for American aircraft. Each airplane ordered by United Airlines can be very different from anyone else’s. If you ever rode in an American Airlines Boeing 727-200, it was technically a Boeing 727-223, or a 727-222 if United.

The variations were substantial because the airlines themselves were individualistic. Today, we view all airlines monolithic conglomerates with a cookie-cutter product. Their foundings, however, were as extreme as the personalities of Bob Six at Contentinal, C.R. Smith at AA, and Howard Hughes at TWA. Their airplanes and their culture reflected this diversity.

American, as one of the most conservative airlines in history, bought its B-727-023 (727-100) fleet without the already proven technology of automatic pressurization. The Unknown Pilot possesses the now somewhat useless Turbo-Jet Flight Engineer certificate and knows from personal experience what an intense exercise manual pressurization is. Since every other airline bought automatic pressurization, AA had to pay extra to get their airplanes without the feature.

The fuel, hydraulic, electrical systems were all different; many other quirks existed between one companies aircraft and another. The differences didn’t pose an operational problem as long as the airplanes remained in the fleet of the original buyers. The rub came when economically obsolete planes were sold off to secondary carriers. Standardization became a challenge when the new company acquired airplanes from different mainline carriers. The cockpits, the systems, and the configurations didn’t match up with anything approaching standard.

Standardization is a critical ingredient in safe airline operation. Airlines and passengers, if they ever thought about it, certainly don’t want creative airline pilots. When pilots and co-pilots pilots from bases on opposite coasts, can climb into the cockpit and operate the airplane as if they’d been flying together all their lives, that is the benefit of standardization. There’s only one way to do a checklist or an emergency procedure, and standardization assures positive results, and it takes years to instill it. The military and all airlines seek uniform standard operation, but few consistently achieve it, especially the military.

Airplanes belonging to major airlines are like purebred horses, but after they’d expended their economic lives at the original carrier, they became the Heinze 57s of the secondary airline world—a mixture of everything. It was relatively easy for the flight engineer to preflight the cockpit of an American Airlines or United Airlines airplane because they were identical between the two companies. Most non-military and non-airline pilots understand the use of checklists—they are ubiquitous. However, work-for-a-living airplanes and their complexity require the use of flows to work in a multi-pilot environment efficiently. Flows don’t work well or at all if the fleet is composed of machines with different fathers, but they were always exciting.

So next time you’re traveling through O’Hare or any airline hub, remember that those two Boeing 777s of different airlines ahead of you for takeoff may be very different beasts underneath their skin because of each airline’s tradition and philosophy.