Weighing smaller planes

[Broomstick]

True story:

One time I was going to take a passenger up in a Cessna 150 (that’s a 2 seat airplane where total weight and the balance are very important. With full fuel my maximum legal weight for a passenger is 138 lbs). My passenger was about 6 inches taller than me and wider than I am. I weigh 145 lbs. I asked her what she weighed. She claimed 120 lbs.

I told her this wasn’t idle curiosity but a matter of safety. She was either going to step on a scale, or stay on the ground.

She elected to stay on the ground. Which was fine by me.

As a general rule, if I take men up in that plane I only fill the fuel tank half full (that’s still two hours of flying time)

My personal take on this “weigh the plane” issue is that it’s partly the NTSB and FAA being overly thorough since the media is watching them, and the media selling cornflakes because they understand the concept of “weight” better than “jammed elevator”. OOOOO! Dangerous small commuter planes!!!. Good lord, these people would freak if they saw what I fly.

Anyhoo…

originally posted by Johnny L.A.
Air conditioning? (Okay, I haven’t seen a 172 with radar or air conditioning, but it could happen.)

The Skyhawk SP offers air conditioning as an option. Me, I just as soon open the windows and fly around with my elbow sticking out in the breeze like I do with my pickup (No, I’m not kidding)

But wait! There’s more! You also have to take in the centre of gravity (CG). Not only must the aircraft not be overloaded, but the load must be distributed. If the CG is too far forward you may not have enough elevator authority to pull out of a dive.

Or you may not have enough elevator authority to even get it off the runway…

Here’s the rule of thumb: The smaller the airplane, the less margin for error with total weight and center of gravity. The two-seat Cessnas and ultralights are so small and light that a pilot can turn left or right merely by leaning left or right, point the nose up or down by leaning far enough forward or back. It’s not as precise as using the controls, but useful enough that this is taught as an emergency techinque. You can’t do that with a 747! Going 10 lbs over gross weight in an ultralight can be a serious problem. Going 10 lbs over in a 747 won’t even be noticed. 19 seat commuter jets are between these two extremes.

In some ways more important is the center of gravity and the load balance. Although not approved, aicraft CAN take off, fly, and safely land when overweight IF the balance is satisfactory. Not a good practice, but it is done for things like emergency evacuations and occassional military operations where the slight additional risk is deemed acceptable. The balance of the load, expressed in terms of a center of gravity (CG) “envelope”, can significantly affect the controllability of the aircraft. If the load is too far forward or backward you may have an airplane that you literally can’t control. And for this, the margin of error is very small. That’s why when I carry baggage in a Cessna 150 I place it directly behind the seats rather than in the rear of the baggage area - it’s closer to CG and less likely to put me into a dangerous situation. If I’m in a 4 seat airplane and the back two seats are empty I’ll put baggage in the back seat rather than in baggage - again, it’s a matter of balancing the load in the safest possible manner. In a Piper Warrior (4-seater) with just me and less than a full tank of fuel, it’s quite possible and to put enough in the baggage area to render the airplane extremely unsafe. The amount required to render the airplane unsafe in such circumstances is much smaller than you would guess - which is why I never load it that way.

A 19 seat commuter’s CG is not quite so touchy - the Warrior’s acceptable limits are measured in inches. A commuter jet might have a CG with forward and aft limits that might be measured in feet (maybe), but not yards. A 747’s will be more flexible still - but every bit as crucial. Thou Shalt Not Screw With the CG!!!

With the recent Denver crash - I too, lean towards the “something wrong with the elevator” theory. Where the weight becomes a factor is that the more weight in the plane the more deflection required by the elevator, and the more stress put on the part. If the part was questionable already, the fact that that particular flight was at or near full allowable weight might have been the final straw. Earlier flights were loaded lighter, putting less stress on the part. Maybe it would have broken anyway. Maybe if the loads had stayed light it would have held together long enough to be caught on inspection. THAT we will likely never know. I don’t think weight was the primary cause of this accident, but it may have been a contributing factor.

[pilot141]

Calculating an aircraft’s CG is a very precise process, if done correctly. Loading of the aircraft is done in a set manner, and if done correctly will keep the aircraft within gross weight and CG limits.

As other people have pointed out (thanks Johhny L.A.), the weight limits for small airplanes can be VERY narrow. As the weight of the airplane goes up, the allowable variance goes up as well. While knowing the exact weight of all four passengers is vital in a Cessna 172, an estimate within 20 pounds or so is good enough for a Beech 1900. When you get up to commercial jet sizes, the larger number of passengers means that the mean weight will fluctuate less and settle closer to the “actual average passenger weight”.

Cargo is already weighed before its put on board an airplane. Sometimes it’s at the check-in counter (convenient, because if your bag is too heavy they can hit you up for the extra money right there!), other times it’s behind the scenes. Of course, this does NOT prevent human error - ie someone not weighing the bags and just “guesstimating” what they weigh. Assuming the cargo numbers are entered correctly, the only true variable becomes passenger weight. The larger the airplane, the less of a factor this becomes.

For example, an MD-82 can carry around 130 people.

130 x 180 lbs = 23,400 lbs of passengers.

The maximum fuel load for an MD-82 is approx 27,000 lbs. So in this case, the fuel and passenger variables are around the same (although you will never see less than 5,000 lbs of fuel in the tanks, so the fuel variance ranges from 5K-27K)

In another aircraft I am familiar with (nods to Robot Arm ), the C-141, the cargo capacity was about 60,000 lbs. We could carry up to 151,000 lbs of fuel also. On a long flight you will burn off more fuel than the weight of the cargo (or passengers) that you are carrying.

Bottom line: the larger the aircraft, the less significant passenger weight fluctuations are to airplane performance. On an airplane like a Beech 1900, I doubt if passengers alone could account for an out-of-CG condition. With a full airplane (as is the case with the one that crashed), this is even less likely - it would take (WAG here) eight NFL linebackers in the back rows, the rest of the airplane empty and no gas on board to get the airplane out of CG limits. Any gas (in the wings, near the CG) and any passengers up front would counteract the people in back.

To answer Robot Arm about weighing the passengers vs going over a scale: this would only be feasible with small aircraft, and even then I don’t see it being worthwhile. The other variables (fuel, cargo weight) are so much greater on large airplanes that it is much easier to WAG passenger weights than to make them all get weighed (imagine the line to get weighed while boarding a 777 to London!) The smaller the airplane, the fewer the passengers, the less hassle…and at some point you reach the actual cost/benefit tradeoff value. What this number is, I don’t know. But I figure even at the 20± number (of passengers), it’s easier to use the standard number and have the gate agent enter in any numbers for some obviously large people (and assign them a seat under the wing!)

/Hijack reply/ Robot Arm, yes all the C-141s are gone from active duty. A few Reserve units are still flying them but the Starlifter is in its sunset days.

[Berkut]

*Originally posted by Robot Arm *
Joey, do you think weighing all the passengers (and knowing their seat numbers, assuming they don’t move around) and knowing the weight and position of all the baggage is more of a pain in the ass than just stopping twice on the taxiway?

It’s not the fact that you would have to stop twice on the taxiway, it’s the scale installation, calibration, certification, etc. This is the FAA we’re dealing with, after all. And think of the setup you would have to use for something like a 747.

Do you think if the airlines were forced to do am itemized W&B for every flight, that they would do it with huge scales on the taxiway? Seems unlikely to me. I bet they would go with assigned seating and weighing the passengers when they check their luggage or something.

Or, they would just keep doing it like they do know, except with 190lbs per passenger rather than 170 or 180. (seems this is the way they’re headed)

[Robot_Arm]

If you had to know exact weight and exact CG, you could do it in theory with scales on the taxiway. But practically, it would be more trouble than it’s worth.

Just to continue the hijack (not the best word to use, considering the subject) with pilot141; I hear they’ve already selected a C-141 to go into the Air Force Musuem at Wright Patterson. It’s the one that flew to Hanoi to pick up the American POWs, and it’s been repainted to the old white-over-gray.

[Berkut]

*Originally posted by Robot Arm *
If you had to know exact weight and exact CG, you could do it in theory with scales on the taxiway. But practically, it would be more trouble than it’s worth.

I wonder if you could get an accurate weight measurement from the pressure in the landing gear struts?

Maybe I’ll try this next time we gas up the 727, and see how accurate I can get it.

[aeropl]

[QUOTE]
*Originally posted by Broomstick *
**
With full fuel my maximum legal weight for a passenger is 138 lbs). QUOTE]
Broomstick, How do you come up with this 138 lbs figure? I fly a Cessna 150, and the Maximum useful load (1977 Commuter II model) is 471 lbs according the POH. I’m guessing your figure includes a lot of luggage or you weigh 333 (471-138) lbs. Please tell me if I’m missing something.

[Broomstick]

The C150 in question is an aerobat, which has a heavier empty weight than a normal C150 but the same gross weight. At some point in the past, a prior owner also installed enough avionics to make it a fully IFR capable airplane. Which is a little strange, but eccentrics are everywhere. Between that, fuel, and lil’ ol’ me, there just isn’t much left for a passenger.

I also made a 10 lb typo - it’s 148 lbs for the passenger.

Here’s how it breaks down:

Empty weight: 1146 (actually 1145.84 lbs rounded up)
Maximum gross weight: 1600
Full fuel: 156 lbs
Pilot (me): 150 lbs <= that’s me + my usual gear

That leaves only 148 lbs for a passenger + anything else.

With half fuel I can legally carry up 226 lbs of passenger in that airplane.

I think most C150’s have an empty weight in the high 900’s or just over 1000, which allows for heavier people in it. But since the installed equipment can vary considerably you have to go by the actual weight of the particular airplane (which I sure you know, that’s for the benefit of the non-flyers)

[Broomstick]

*Originally posted by Joey G *
It’s not the fact that you would have to stop twice on the taxiway, it’s the scale installation, calibration, certification, etc. This is the FAA we’re dealing with, after all. And think of the setup you would have to use for something like a 747.

I’ll second the above - the idea of giant scales is appealing, but problems come up with installation, maintenance, callibration… not to mention airplanes are considerably more variable size-wise than trucks. That means a scale that can handle a huge range of weights, or multiple scales.

The weight of the airplane itself should seldom vary (after any work on it that could affect the weight and balance - installation or removal of equipment usually - these figures are re-calculated.) Fuel is measured in pounds (or kilograms, presumably, outside of the USA) going into the airplane. The big variables are the passengers and their baggage, and scales for people and portable luggage are much smaller and cheaper than the large industrial scales that would accomodate airplanes.

And, as already mentioned, total weight alone is not enough. You have to know how everything balances You can be at a safe weight but a dangerous out-of-balance condition. I’m not sure how you would go about building a scale that can also tell you the way the load balances, and without that, you’re back to calculating the balance for each item (including people) on board.

I also agree that you’re less likely to be out-of-balance with a full load (provided you don’t blow the weight limit). Passengers in the back would tend to balance passengers in the front, and with a full load any random passenger getting up and moving is a much lower percentage of the total and thus has less effect. If you have only 5 passengers in a plane that holds 19 and they all decide to clump far forward or far back that could pose a problem.

I don’t have a problem with the FAA checking their “averages” - I have no problems stepping on a scale myself - but imagine some passengers will wail. I’m sorry, weight CAN be a matter of life or death for a smaller airplane. You would think that folks would be eager to weigh in as a means of personally contribuing the the overall safety of the flight.

Also have a :smack: - I said “crash in Denver” The Beech accident was in one of the Carolinas, wasn’t it? Have to make sure the caffiene level is adequate prior to posting.

[heresiarch]

*Originally posted by Broomstick *
**Here’s the rule of thumb: The smaller the airplane, the less margin for error with total weight and center of gravity. The two-seat Cessnas and ultralights are so small and light that a pilot can turn left or right merely by leaning left or right, point the nose up or down by leaning far enough forward or back. It’s not as precise as using the controls, but useful enough that this is taught as an emergency techinque. You can’t do that with a 747! Going 10 lbs over gross weight in an ultralight can be a serious problem. Going 10 lbs over in a 747 won’t even be noticed. 19 seat commuter jets are between these two extremes.
**

That’d make a good short story: A commuter plane has a mechanical failure that prevents the pilot from operating the control surfaces. She can still operate the throttle, but it’s a single engine plane, so she can’t use differential thrust to change course.

She ends up giving instructions to passengers over the intercom (“Everybody move to the left side - now!”) On final approach, all the passengers have to move aft little by little, until everybody is crammed into the very back of the plane when it finally touches down.

Is it feasible? What kind of plane would that be?

I’m guessing that the smaller planes, small enough to be maneuvered by the “Simon Says method”, are extremely unlikely to fail in that manner. Don’t they have direct mechanical links to elevators, rudder, and other flight controls?

[Johnny_L.A]

I’m guessing that the smaller planes, small enough to be maneuvered by the “Simon Says method”, are extremely unlikely to fail in that manner. Don’t they have direct mechanical links to elevators, rudder, and other flight controls?

I heard about a guy who built a Thorp T-18 (I think – Could’ve been a Mini Mustang) who could dive or climb by moving his feet.

Airplanes in general are unlikely to have a mechanical failure. Still, failures do happen occasionally. Some pilots are especially cautious after a 100-hour or Annual inspection, as they want to be absolutely sure the aircraft is properly rigged. I read in one of the flying magazines about a year ago, of a student pilot who took off in an airplane just after its 100-hour check. The A&P forgot to bolt on the landing gear, and it came off when she lifted off. (She landed safely.) A forgotten cotter pin can be a disaster. Controls can break or become disconnected over time. So can oil lines, fuel lines, etc. When I was working at Edwards AFB an aircrew was on a routine flight in an O-2 (the military version of the Cessna 337 Skymaster). The landing gear cable became disconnected in-flight. The pilot happened to be unusually short and wiry, and he was able to get under the instrument panel and hold the cable after attaching a keyring to it while the other pilot brought the aircraft in for a safe landing.

[Broomstick]

*Originally posted by heresiarch *
That’d make a good short story: A commuter plane has a mechanical failure that prevents the pilot from operating the control surfaces. She can still operate the throttle, but it’s a single engine plane, so she can’t use differential thrust to change course.

She ends up giving instructions to passengers over the intercom (“Everybody move to the left side - now!”) On final approach, all the passengers have to move aft little by little, until everybody is crammed into the very back of the plane when it finally touches down.

Is it feasible? What kind of plane would that be?

Well, yeah, in theory it’s feasible. The first quibble I come up with is that most commuter airlines don’t have any single-engine airplanes. It might be possible to come up with a scenario for this with a twin engine, but since I don’t fly twins I don’t feel qualified to speculate on it much.

My second quibble is trying to imagine getting panicked passengers to co-operate enough to pull this off. I’ve been on a commerical airplane with a serious problem. The second serious problem after the dead engine and flaming wing was crowd control. It’s not impossible, but you’d have to address the level of extreme fear that would be happening at the same time.

I’m guessing that the smaller planes, small enough to be maneuvered by the “Simon Says method”, are extremely unlikely to fail in that manner. Don’t they have direct mechanical links to elevators, rudder, and other flight controls?

Even the Beech that went down in the Carolinas had “direct mechanical links” - rods and cables between the controls manipulated by the pilot and the actual surfaces on the wing and tail that provide steering. I think most folks would be surprised at just how big an airplane can get and still use direct mechanical links for controls.

And direct links do fail from time to time. Cables can break, pushrods becomes disconnected, bell cranks or pulleys jam… “Control surface failure” is a scenario discussed in training and even practiced, both in simulator and actual airplanes (for instance, in the real airplane it might be landing with just trim and engine power, rather than using the elevator, in order to simulate a jammed elevator. Such practice must be done with some care, and of course you DO have the control available if the simulating needs to be terminated for safety reasons)

Load shifting affecting control does happen from time to time with parachute planes. See, with skydivers the FAA allows the jump plane to remove seats so everyone sits on the floor and you can cram more people in the plane. If the people all shift at once that can pose serious control problems - fatal crashes have resulted from folks sliding back towards the tail at takeoff. (In fact, statistically speaking, a skydiver is more likely to be killed on the ride up than on the ride down). But, even when things are proceeding normally, the pilot has to be aware of the shifting load and changing weight of the plane as folks get off at altitude, and the jumpmasters have to keep the traffic flow under control - if everyone got out at the same time it could pose control problems for the pilot. I know of an incident at a Florida air show where a large load of skydivers went up in a sizable airplane, the pilot lost control (I’m not sure why - I heard about this from the skydivers, not the pilot) and they wound up in a spin with everyone stuck to the sides of the airplane barely able to move. The pilot got out of the spin, at which point the entire complement of skydivers decided to abandon ship and did so nearly simultaneously. After which the pilot lost control again. Fortunately, he also regained it again so no one was hurt. (Some underwear had to be replaced, though…)

Another incident, not skydiving this time, involved a pilot on a checkride and an FAA examiner in a Piper Tomahawk (Also referred to as “Traumahawks”). See, the Tomahawk was given spin authorization when certified, but had always been plauged by a higher than average spin fatality rate (think it was something like six times the average). Debate as to why had raged for years, with many pilots claiming they just weren’t safe to spin and others saying they were. FAA kept saying the plans were certified spin capable and it must be the pilots screwing up. Anyhow, part of this checkride was a spin test. So the two guys put the Tomahawk in a spin - and then they couldn’t get out. Finally, one of the guys unbuckles and crawls on top of the instrument panel. That changed the balance of the airplane enough to allow them to get out of the spin. Now, there was nothing wrong with the spin-and-recovery technique of either guy involved. After landing, a thorough inspection revealed nothing wrong with the airplane. Apparently, some are safe to spin and some aren’t - but there’s no way to tell which is which before you actually do the deed. THAT’s when the FAA banned spins in Tomahawks.

And I’ve several stories of guys on military transports shuffling in a group from one part of a cargo hold to another to annoy the pilots up front, who have to keep adjusting things to compensate for the weight shifts.

So all this weight-and-balance stuff is truly important. People have both screwed up with the W&B, and used it to save their butts.

Yep, would be a potentially interesting story,