Or why I had to turn down a potentially lucrative charter flight.
One of the things I’ve said again and again is that it’s nearly impossible to load a Robinson R44 helicopter out of CG. Nearly, but not completely.
What is CG?
For those of you unfamiliar with the term CG, it stands for center of gravity. All aircraft have a specific center of gravity or point at which they could (theoretically) be lifted and hung level. While an aircraft doesn’t need to be in exact balance to fly, there are limitations to which it can be loaded out of balance. These limitations form an envelope of acceptable loading and if you’re loaded within this envelope, you’re said to be within CG or simply in balance. The aircraft controls are rigged with this in mind.
If you load an aircraft out of CG, you’re asking for trouble. For example, if I load my helicopter too heavy on one side, I could run into trouble in a turn by not being able to move the cyclic enough in the opposite direction to come out of the turn. After all, all controls have limits, normally defined by a physical stop. Running out of right cyclic while trying to come out of a left turn would be very scary indeed. Of course, I probably wouldn’t get to that point because I’d feel the problem as soon as I pulled up into a hover — I simply wouldn’t be able to keep the aircraft from drifting left.
[Note to all you flight instructors out there; if I completely mangled this description — since I’m not a CFI — feel free to step in to clarify in the Comments. This is my understanding after 10 years and 2,000+ flying hours, but I never had to teach it to anyone.]
Pilots are required to have an aircraft Weight and Balance (W&B) calculation on board for every flight. This is part of the Federal Aviation Regulations (FARs) in the U.S. In non-commercial flight, it’s usually enough to have the W&B for the empty aircraft. But in commercial flight, there are usually requirements for an individual W&B to be calculated for each flight with the given load.
So yes, when you fly on a commercial airliner, there’s a computer program somewhere that’s spitting out a W&B calculation for your flight. Your pilot has it in his possession in the cockpit.
Now you might say, “Hey, wait a minute. How do they know what I weigh?” They don’t. They’re allowed to use estimates. It all depends on the airline’s Operating Specifications (Ops Specs), which are established with the FAA.
I have Ops Specs, too, but I’m not allowed to estimate for my Part 135 Charter work. That’s why I ask for the name and weight of each passenger when I book a flight.
Four Fatties is Too Many
When I asked for names and weights yesterday while booking what was supposed to be a 2-hour aerial survey charter, I got three weights that I knew would be trouble:
A: 240 lbs
B: 220 lbs
C: 195 lbs
That’s 655 pounds of passengers alone.
Add the pilot (who is trying hard not to reveal her weight; don’t do the math, guys!) and you could only put on about an hour and 20 minutes worth of fuel to stay below the 2500 lbs max gross weight — the absolute maximum weight of the aircraft at takeoff time — limitation of my Robinson R44 Raven II.
Of course, the situation gets worse when you factor in the simple fact that all passengers lie about their weight. Every single one of them. If I put a scale out and made them stand on it, I guarantee anyone over 200 lbs. has shaved at least 10 pounds off their weight when reporting it. They either don’t figure the weight of their clothes or they’re in denial about their weight or they’re afraid that I’ll say they weigh too much. Even folks under 200 lbs are guilty of this. So I routinely add 10 pounds for each passenger. That 30 pounds corresponds to 5 gallons of 100LL fuel or 15-20 minutes of cruise flight.
Since I’m really supposed to have 20 minutes more fuel on board than I expect to need — per FAA reserve requirements — I was really sunk. Apparently, I’d be able to load up my passengers and just enough fuel to take us on a brief flight around the departure airport.
This is just the weight portion of the equation, which is easy enough to do. Add empty aircraft weight to passenger, baggage, and pilot weight. Then add the weight of required fuel. If the number exceeds 2500 lbs, something’s got to come off the aircraft. It can’t be the pilot and it can’t be the fuel required to complete the mission. Simple as that.
How the CG Stacks Up
While I could have done the math in my head, I did it as part of a complete CG calculation. It’s a pain in the butt do to one of those manually, but I have a spreadsheet solution that I worked up to do it for me. I punch in the weights and amounts of fuel and it draws the CG envelope with points for takeoff weight and empty fuel weight. So while manually doing this task would likely take 15-20 minutes of calculator punching, I can do it in about 30 seconds. I can also easily play “what if” by changing fuel quantities and moving the passengers into different seats.
Here’s what I got for the proposed flight and 2 hours of fuel on board:
Note that both points (square and triangle) are outside the boundaries of the CG envelope. The red line indicates the rotor mast. The points clearly indicate that the CG is way forward. In other words, I’m front-heavy. If I pick up to a hover, I’m likely to start drifting forward immediately. I may hit the back stop of the cyclic when I try to stop that forward motion. In other words, I won’t be able to stop.
Of course, the aircraft is also 100 lbs over weight.
Just for grins, I moved the passengers around in a what-if scenario. I’d put the biggest guy up front, since that’s where the leg room is. After all, maybe he’s not fat. Maybe he’s a former professional basketball player. It doesn’t matter for my calculation how tall a person is — all I care about is weight. But if he’s got long legs, he’s likely to be miserable in the back seat.
So I put the light guy up front and got something like this:
A little better, but not safe or legal. But I kept playing. I really wanted to do this flight. The only thing left to fiddle with was the fuel, so I started off-loading fuel on my worksheet until I got within weight limitations. I needed to drop 99 pounds to get down to 2500 takeoff weight. That’s 16.5 gallons or about an hour’s worth of fuel. This what-if scenario would produce be for a short flight, with only 56 minutes of fuel on board:
And this is where the sad truth of the matter emerged. It didn’t matter how little fuel I had on board — we would always be out of CG for this flight. Too many fatties on board. Both points remain outside the envelope.
I called the client back and told him the problem. I said that together, we weighed too much. I gave him two options: leave one of the passengers behind or fly with a company that had larger aircraft. I suggested a company based in Scottsdale. He wasn’t happy, but he understood.
I’ll be interesting to see if the big fatty (A in my list above) gets left behind. If he does, we’ll be good to go — with full tanks, as you can see here:
Parked in the desert north of Phoenix. Photo by Jon Davison.
