Understanding the center of gravity
Sometime back in the dark ages, I was getting ready to take my instrument instructor check ride, and the examiner, who was an actual FAA type from the FAA headquarters, asked me if I had done a weight-and-balance for the flight. Two thoughts flashed through my mind, the first being the obvious question: What has a weight-and-balance calculation got to do with an instrument check ride? The second was a little panicky thinking while I tried to remember how to do the calculations.
I whipped out the POH and fumbled around, finally coming up with a number. Then I ran the pencil point across the center of gravity (CG) envelope in the back of the POH and realized I had made a mistake. The point was outside the front edge of the envelope for this brand-new Piper Cherokee 140. I went back and rechecked my calculations and frowned. It still came out saying the CG was out of the front edge of the envelope. While all of this was going on, the examiner was sitting behind his desk, doing paperwork with a knowing look on his face.
Finally, I admitted, “I don’t know what I’m doing wrong, but it says that with just you, me and fuel, we’re out of the front edge.”
He half-grinned and said, “So, can we go flying?”
I thought back to my CFI training and said, “Not legally. What do we do?”
With a triumphant look on his face, he led me out to the Piper Cherokee, flung the door open, dropped his big, airline-style map case in the back and, while strapping it down, said, “This will do it. Let’s go.”
He had obviously done this to check-ride candidates before. After I had passed the check ride (thankfully!), we sat down, instructor to instructor, and he told me that he always had his candidates jump through the weight-and-balance hoops to teach them a lesson: You can never take something as important as weight and balance for granted because it’s often not what you think it is.
The good news is that weight-and-balance problems are run and rerun in groundschool, and it’s pretty hard to graduate without, if not mastering CG calculations, at least being familiar with the concepts. Unfortunately, shortly after passing your private-pilot license check ride, both the details and the concepts begin to get a little fuzzy in our minds because precious few people actually do weight-and-balance calculations as a matter of course.
There’s a valid reason why most people pretty much ignore CG calculations: 90% of the time, they’re flying the same airplane with the same load (two people, fuel and a little baggage), so although they don’t know where it is in the CG envelope, they know it’s safe because they’ve flown it so often in that condition. And it usually is safe. But is not checking it safe?
As an airplane starts bulging because so much stuff is being crammed into it, even the dimmest bulb on the tree knows it ought to run a CG check. Intuitively, we know we don’t want the CG to go too far aft. We’re not exactly sure why, but we know it’s not a good thing, so we try to avoid it.