Flying Overweight

Unregistered said:
Hello, had a question/ concern. I took a couple flight lessons on a cessna 152, because it was a cheaper option, and was told after the second flight that I can no longer fly that plane because the airplane is overweight. I was told I need to fly on the Cessna 172 with them which is much more expensive. I no longer want to stay with that school because they have proved that they are not safe because they only did the W & B after two flights, on which we did maneuvers. Now I need to look for a new school, and spend money getting used to a new plane. I feel that the overweight flights were illegal and negligent, and wondering whether I should ask for money back. So far I only have around 5 hours logged.
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Lesson 1) Now you know about weight and balance calculations. Next time you have a lesson, be sure to do a weight and balance up front. Know how to get the numbers for any aircraft and don't depend upon anyone else to do the calculation for you. Get some books to find out what else you don't know that you should be watching out for.

Lesson 2) There are other options besides flying a 172. You could reduce the weight you are carrying or be able to carry more weight. For example, you can fly with less fuel or you can fly with a smaller instructor. Or, even better, you could lose some weight yourself. Now don't get mad at me. We are starting to learn that overweight pilots are going to have a tougher time in the future passing their medicals. You might be able to fly a 152 with a more powerful engine--or some other aircraft.

Lesson 3) It is a good idea to question your school or your instructor if you believe they have done or taught something unsafe or questionable. Be ready to "fire" either one when things continue to go the wrong way. On the other hand, be open minded when that makes sense. Changing instructors or schools can be good. Doing it too often can be bad. As you have probably noted, the overweight situation is one that is skirted often. If you are not comfortable with it, then that is OK. You will be PIC one day and it is your call.

Lesson 4) Don't ask for your money back. You learned a lot in those five hours. Your school did look out for your safety when they did the W&B and they came up with a reasonable solution. There will be times when you cannot fly the plane you chose because it is in maintenance or because someone else has rented it or because ... whatever.

Lesson 5) Welcome to POA. You don't really need to hide as an unregistered guest. Log in with a real id, tell us a little about yourself and join in.
 
I think we might be missing the OP's point. He said that he did two flights with the school in the 152 and then the instructor told him that they were overweight and can no longer fly that plane.

It's one thing when you knowing fly a plane that's overweight, not saying it's safe, but you know what your doing and you accept the risks. It's another thing when you fly it a few times, then do the W&B, realize that your overweight, panic and call the student saying that it is no longer safe to fly that plane.

The way I see it the instructor proved that he is incompetent by not even knowing that they were overweight.
 
MachFly said:
I think we might be missing the OP's point. He said that he did two flights with the school in the 152 and then the instructor told him that they were overweight and can no longer fly that plane.

It's one thing when you knowing fly a plane that's overweight, not saying it's safe, but you know what your doing and you accept the risks. It's another thing when you fly it a few times, then do the W&B, realize that your overweight, panic and call the student saying that it is no longer safe to fly that plane.

The way I see it the instructor proved that he is incompetent by not even knowing that they were overweight.
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Yep, that's the main problem with this story.
 
AuntPeggy said:
Lesson 2) There are other options besides flying a 172. You could reduce the weight you are carrying or be able to carry more weight. For example, you can fly with less fuel or you can fly with a smaller instructor. Or, even better, you could lose some weight yourself. Now don't get mad at me. We are starting to learn that overweight pilots are going to have a tougher time in the future passing their medicals. You might be able to fly a 152 with a more powerful engine--or some other aircraft.
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That was my reason #1 reason for weight loss, dropped from 240+ to 190lbs, I had a goal to weight and balance the Cessna 152. (Plus get off the BP meds)....it worked.
 
Henning said:
Yes, W&B is an important safety factor, and that the instructor departed with you and he knowingly overweight is not a good sign. If you all are so large that you can't take full fuel in a 152, a 172 will be considerably more comfortable, and a 182 even more so and give you better take off and climb performance. That said, the 152 with 13 gallons of fuel will get you through your lessons, even the long cross country if you pick up fuel at the stops.
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In the 150 I burned 11-12 gallons total for my long XC.

13 gallons is half tanks
 
Lesson #6:

Fly defensively. The airplane will try to kill you. Controllers will try to kill you. So will other pilots, deer/geese, your own sense of balance, the wind, and, yes, even your flight instructor. It's a bit early (but only slightly) for this really important lesson, but the person responsible for safe flight is YOU.

That may seem a bit harsh, but it will likely determine when you solo just as much as stick skills, if not more.

I'd want a talk with the instructor as well, but it's a mistake to think 3-4 letters after a name means infalliability.

In my case, my instructor never figured out we were overweight. I read the AFH and did the calculation. We were about 15 lb over, so we could easily have left 3 gallons in the truck, but for logistical reasons (auto-fuel) I decided to try a 172 anyway.
 
Max gross weight is at standard conditions, sea level at 57f. So a plane at max gross weight on a 85f day taking off on a runway at 2000 ft above sea level would be the equivalent of over gross on take off, right? And I have done that a few times.
 
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tinerj said:
Max gross weight is at standard conditions, sea level at 57f. So a plane at max gross weight on a 85f day takinging off on a runway at 2000 ft above sea level would be the equivalent of over gross on take off, right?
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Many of these discussions lead me to conclude that some think that maximum weight limitations are primarily performance limitations.

That may be part of it, but hardly all:

The aircraft gross weight is limited by several weight restrictions in order to avoid overloading the structure or to avoid unacceptable performance or handling qualities during operation.
Aircraft gross weight limits are established during aircraft design and certification and are laid down in the aircraft type certificate and manufacturer specification documents.The aircraft gross weight is limited by several weight restrictions in order to avoid overloading the structure or to avoid unacceptable performance or handling qualities during operation.
Aircraft gross weight limits are established during aircraft design and certification and are laid down in the aircraft type certificate and manufacturer specification documents.
 
Unregistered said:
I feel that the overweight flights were illegal and negligent, and wondering whether I should ask for money back. So far I only have around 5 hours logged.
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I'd say you got a valuable lesson in what an airplane flies like overloaded.

RT
 
FastEddieB said:
Many of these discussions lead me to conclude that some think that maximum weight limitations are primarily performance limitations.
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That's because they are.

The structural limitations will kick in close to the 3.8G structural design limit (plus the required 50% margin) for a normal category aircraft. At 1G, you would have to overload that aircraft by multiple tons to break it.

Most student pilots never feel 1.5G, let alone 3.8.

By far the largest risk is failure to leave ground effect, with associated risks of departure stalls.

The next risk is insufficient control surface authority, but that's really a balance issue. High weight makes that more difficult.
 
Only bad things can happen when you load a plane over its gross weight limit.

You could get caught between the Moon and New York City....or find your self in Los Angeles

From June 2-4, 1959 (Max) Conrad flew Comanche 250 N110LF non-stop from Casablanca, Morocco to Los Angeles, a distance of 7,668 mi (12,340 km). This distance record (for aircraft in the 1750-3000 kilogram weight class) stood until 1987. With interior seats replaced by fuel tanks, the aircraft was loaded 2,000 lb (910 kg) over its production gross weight limit when Conrad took off from Casablanca.
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MAKG1 said:
That's because they are.

The structural limitations will kick in close to the 3.8G structural design limit (plus the required 50% margin) for a normal category aircraft. At 1G, you would have to overload that aircraft by multiple tons to break it.

Most student pilots never feel 1.5G, let alone 3.8.

By far the largest risk is failure to leave ground effect, with associated risks of departure stalls.

The next risk is insufficient control surface authority, but that's really a balance issue. High weight makes that more difficult.
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Well, the structural part kinda depends on the plane, gear strength is not calculated to 3.8gs.
 
MAKG1 said:
That's because they are.

The structural limitations will kick in close to the 3.8G structural design limit (plus the required 50% margin) for a normal category aircraft.
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Big maybe on the 50% margin - designs carry margins for very good reasons. One cannot count on using any of that margin. Many folks have learned that lesson the hard way.
 
Those suggesting the design margins for structural strength allow for overweight operations ignore the long-term effect of repeated stresses beyond intended values. Eventually, those repeated excessive stresses result in strain, the safety margins are compromised, and the structure can then fail even with a stress well below the book limits. That's the really scary part about these things.
 
Re: Flying Over Gross

By the way, I think a better term for this is flying over gross.

I fly "overweight" all the time, but I'm working on it! :D
 
Here's that promised personal anecdote...

In the mid-1990's a friend and I bought a (1966?) Citabria 7GCBC, both for fun and to rent out.

11803594746_73c18a5723.jpg


We incorporated and I put together both a Tailwheel Transition Course and a Basic Aerobatics Course. This was out of Hollywood Aviation at N. Perry airport in Hollywood, FL.

Once I checked people out, I would give them a key, with the express agreement they were never to take the plane without first confirming availability. They were also not to do aerobatics unless approved by me for certain maneuvers we had covered, and, of course, had to arrange for me to leave the parachutes.

On the Christmas day, 1996, a renter called to ask if the plane was available the following day, so he could take his fiancé's son up as a Christmas present. It was, but I advised him that contrary to normal procedures the plane had been topped off - another renter had filled it for a proposed cross-country that he had canceled. He said it would not be a problem. Incidentally, I had not given him aerobatic instruction nor did he ask for the parachutes.

On the 26th, I heard on the radio about a small plane crash. A very short time later one of my employees called me and said the FAA had called.

The sad story is told here (Summary, but you can click on "Full Narrative" for the particulars):

http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20001208X07205

Anyway, I think its a decent example of "links in a chain".

Had he just been overweight and flown normally, he might have been fine.

If he had just done aerobatics, but not been overweight and done them at altitude and with parachutes, he might have been fine.

And if there had not been something suspect about the strut attachment point*, he might have been fine notwithstanding all of that.

Add them all together though, and you have a double fatality.

It did get to me, and I took a couple years off flying, allowing my medical and CFI and BFR to expire and my Tiger to pretty much just sit in the hangar. Was not sure I ever wanted to fly again, but I eventually got back on that horse.

Anyway, I thought this story might fit in here somehow. We should all try to learn from the mistakes of others - life's not long enough to make them all ourselves.


*This is a mystery the answer to which I will probably never know in this lifetime. The NTSB said it looked like a bolt was missing from a strut attachment point. We had never had the wings off the plane, and had just inspected the wing root as part of the annual. We removed the wing root fairings and looked at all the wing attachment points as did the A&P and nothing was amiss a few hours prior to the accident. I was in the highly competitive gym business, and have always wondered if someone sabotaged the plane with me as the target. Either that or the NTSB got it wrong. It was referred to Broward County Homicide but I never heard another word. Weird.
 
Interesting, perhaps the bolt failed both precipitating and getting lost in the crash.
 
Henning said:
Interesting, perhaps the bolt failed both precipitating and getting lost in the crash.
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That should be relatively easy to determine if by inspecting the bolt holes. You can tell if it's recently been in contact with something.

Well, assuming it wasn't consumed in a fire.
 
Henning said:
Interesting, perhaps the bolt failed both precipitating and getting lost in the crash.
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When I suggested that, they said the bolt holes would have looked different had the bolt been there and failed.

It's now 18 years hence, but I recall they said it looked like the bolt had simply not been there.

No fire.

Interestingly, they did ask why there was no W & B documentation in the plane. I described the black loose-leafed notebook that contained it and I recall they went back to the crash scene and recovered it. They really did check every legality and I was glad we were conscientious with our paperwork.

The scene was essentially "Everglades" - the swamp that exists west of Miami/Hollywood/Ft. Lauderdale. As in the ValueJet crash, that kind of swamp can consume a lot of debris.


I have a huge file in my hangar with a lot more on the accident, it may be in there somewhere.
 
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MAKG1 said:
[snip]
Most 172s have flight adjustible rudder trim. That's real nice.
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Some 172's have flight adjustable rudder trim. Consulting my logbook, I've flown 8 C-172's from C model through an R model. Only one (happens to be a N) has rudder trim.

Please procede with your regularly scheduled thread...
John
 
I'm super torn when I see things like this. As a structural engineer I'm well experienced with fatigue calculations and structural design.

For fatigue loads the only two things we are concerned about are stress ranges and number of cycles. If it was designed properly then increasing the load 2% isn't really going to make a very big difference on fatigue. Number of cycles will but that's number of cycles through the high stress range. As an engineer I would consider "accidental" overload highly negligible for fatigue design.

Ron Levy said:
Those suggesting the design margins for structural strength allow for overweight operations ignore the long-term effect of repeated stresses beyond intended values. Eventually, those repeated excessive stresses result in strain, the safety margins are compromised, and the structure can then fail even with a stress well below the book limits. That's the really scary part about these things.
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You're close Ron. Unless you're loading well beyond the gross you're not going to increase the stresses by more than a few percent. It's stress cycles that matter more. Also, stresses are caused by strain, not the other way around.

The only issue that I have with overloaded flight is unexpected loads. Lets say you take off overgross by 100 lbs. Nothing should break, wear more, or be unsafe (beyond loss of performance). Now, lets say you hit MASSIVE wake turbulence. That extra 100 pounds might be all it takes to break a wing root. So, unless you can guarantee that you wont hit 3.8 g then you really can't say beyond a doubt that you'll be safe. Will you survive with no significant wear or damage on the plane, probably. Is it legal: NO!
 
How many overloaded or overgross aircraft have structural failures? Not many. I'd suggest that the real danger of over-gross flight is the adverse impact the extra weight has on the performance and flight characteristics of the aircraft.

Accident statistics show that pilots are the weakest link in the chain. Flying over-gross just means they have less stall margin to work with, need longer runways, can't clear obstacles quite as easily, etc. That's what I'd be most concerned over.

OTOH, I can show you a column from an old Flying magazine where the editor wrote "They fly much better over gross at the beginning of the trip than they do if you run out of fuel prior to reaching the destination." or WTTE.
 
tehmightypirate said:
You're close Ron. Unless you're loading well beyond the gross you're not going to increase the stresses by more than a few percent. It's stress cycles that matter more. Also, stresses are caused by strain, not the other way around.
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Maybe the engineering terminology has changed since I got my engineering degree from the University of Michigan 41 years ago, but I learned that stress is a measure of the force applied to the structure, and strain is a measure of the deformation of the structure as a result of the stress. Hence, the term "strain gauge", for a device which measures such deformation. And if you read what I said carefully, you'll see that I specifically mentioned repeated stresses as being what weakens structure over time, resulting in later failure under what should be (but for the often invisible damage done by repeated overstresses) acceptable loads.

And after further review, it appears my engineering knowledge is not so out of date as I might have thought.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/StressStrain.htm


The only issue that I have with overloaded flight is unexpected loads
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That is certainly an issue, but not the only one.
 
kyleb said:
How many overloaded or overgross aircraft have structural failures? Not many. I'd suggest that the real danger of over-gross flight is the adverse impact the extra weight has on the performance and flight characteristics of the aircraft
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In the short run, I would agree. In the long run, I would not -- look at all the structural issues with aging aircraft, especially the catastrophic failures involving ancient firefighting aircraft.
 
Ron Levy said:
Maybe the engineering terminology has changed since I got my engineering degree from the University of Michigan 41 years ago, but I learned that stress is a measure of the force applied to the structure, and strain is a measure of the deformation of the structure as a result of the stress. Hence, the term "strain gauge", for a device which measures such deformation. And if you read what I said carefully, you'll see that I specifically mentioned repeated stresses as being what weakens structure over time, resulting in later failure under what should be (but for the often invisible damage done by repeated overstresses) acceptable loads.

And after further review, it appears my engineering knowledge is not so out of date as I might have thought.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/StressStrain.htm


That is certainly an issue, but not the only one.
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You're right, I had it backwards in my head for some dumb reason. I did catch your reference to repeated stresses and assumed you were talking about fatigue. I'm half tempted to run a fatigue design and see precisely how much of a difference it would make but either way my original point stands that unexpected extreme loads are the major issue with minor performance issues being the second most important impact. That could be major or minor depending. For fatigue stresses and structural damage in normal flight I would consider them negligible (but not non-existant).
 
tehmightypirate said:
You're right, I had it backwards in my head for some dumb reason.
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And you don't have my excuse of being over 40 years out of engineering school, either. :D

I did catch your reference to repeated stresses and assumed you were talking about fatigue.
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Since elastic deformation is away of life in aircraft, I was. It's just that those strains remain within that elastic range if we keep the loads within book limits, although we do know that repeated loading within design limit stresses can eventually cause failure. Problem is when you overstress, that fatigue life goes down dramatically even if the strains remain within the elastic limit, and if strain gets to the plastic point, it's only a bit more strain to the yield point -- which can happen with an under-limit load once plastic deformation begins. That's how one person's foolishness can kill he next person even though the next person is playing it by the book.
 
There might be another fatigue-related issue here.

I've no idea if this really happened, but a real common problem in automotive shops is overtorquing bolts past the yield limit. Unless you twist the head off, it will be attached, but will fail much more easily than it should.

Having said that, I've never heard the telltale BAP BAP BAP BAP signature of that problem outside an aircraft mechanic's shop.
 
Ron Levy said:
And you don't have my excuse of being over 40 years out of engineering school, either. :D

Since elastic deformation is away of life in aircraft, I was. It's just that those strains remain within that elastic range if we keep the loads within book limits, although we do know that repeated loading within design limit stresses can eventually cause failure. Problem is when you overstress, that fatigue life goes down dramatically even if the strains remain within the elastic limit, and if strain gets to the plastic point, it's only a bit more strain to the yield point -- which can happen with an under-limit load once plastic deformation begins. That's how one person's foolishness can kill he next person even though the next person is playing it by the book.
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Heh, and I'm taking the 2nd part of my structural engineering license exam in April. :p Time to get back to studying I guess.

Ah! Now you're speaking my language. As you know, the two stresses we are concerned about for fatigue are the threshold stress range and the maximum stress range for a given service life (for x number of stress cycles).

So, if the stress is always below the threshold stress then we have an unlimited service life. I would be nice if most trainers are designed to be below the threshold stress for fatigue under normal use because, with the frequency stress cycles could happen in a plane (turbulence, landing and takeoff, stalls, etc) fatigue is a very real problem. As you pointed out we see fatigue failures a lot so it appears my hope isn't realistic.

If it was the case then as long as your overloaded stress cycles are less than 20,000 then you will never fatigue the plane to death. (20,000 cycles being the prescribed lower limit for fatigue to take place).

Now, if the plane is designed to operate above the threshold stress (which is most likely) then it will have a given service life before fatigue failure occurs. Note to the layman: above a certain stress fatigue will always happen, the only question is how many cycles does it take to get there. The allowable stress is computed based on the number of cycles. This is given by the equation

Fsr = (Cf / N)^0.3333
Fsr = Maximum allowable stress to ensure the fatigue cycles of N
Cf = Fatigue factor

Let's take Cf = 250x10^8 (though it varies wildly if you have stress concentrations)

So, Fsr = 36.8 ksi at 500,000 stress cycles.

Let's say we overload the plane by 5% and this causes a 2.5% stress increase (halved for two wings).

Thus, our allowed number of cycles to get a new stress of 1.025*36.8 = 37.72 ksi will be:

Fsr = 465,827 cycles.

That's assuming you fly overloaded every time, I don't have the data in front of me on limited stress ranges beyond the usual limits but I imagine it has a slightly reduced effect if it isn't constantly being overload.

Either way, conservatively we see that it causes a 7% drop in fatigue life at most. This means that your plane will fall apart 3 years sooner on a 40 year design life.

Will it hurt you down the road: yes. Will it kill you: not yet. Will it kill the next guy: probably not. Should you do it: obviously not.

As you know (and anyone reading this should learn) stresses are not linear and stress concentrations play a big roll. A high stress concentration area that is designed to be under the fatigue threshold could, under a extreme event (wake turbulence or a really hard landing) combined with overload, be pushed beyond the fatigue threshold. Repeated excursions beyond this over the course of 40 years can kill a plane. Thus, my original recommendation that you don't do it. However, you should understand why you shouldn't do it. The plane could take one overloaded flight or maybe even 1,000 overloaded flight. But, down the road, someone will pay for it either in expensive annuals or they bite the dust.

Now, that said, if you're over by 20 pounds I would almost consider that negligible error given my experience with engineering. You could PROBABLY do this with no harm what-so-ever. But, and this is the big one, IT'S STILL ILLEGAL! Anyone reading this should not fly over-gross. But, if you don't care about legal stuff, then at least understand what you're costing yourself (or someone else) down the road.
 
In my opinion it's obviously obnoxiously unprofessional and unsafe to knowingly operate an aircraft outside of its limitations.

If I can give you one piece of advice here (the same advice many others on this topic have given): Simply move on to another flight school and find an instructor that you click with. Don't cause problems with anybody in this industry because it will come back to bite you at some point.

Lastly, even if you tried to get the CFI or flight school violated it would be hard to do since it was in the past and there's not really any way for you to prove that you were over max gross weight.

Good luck.
 
MAKG1 said:
There might be another fatigue-related issue here.

I've no idea if this really happened, but a real common problem in automotive shops is overtorquing bolts past the yield limit. Unless you twist the head off, it will be attached, but will fail much more easily than it should.

Having said that, I've never heard the telltale BAP BAP BAP BAP signature of that problem outside an aircraft mechanic's shop.
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And that's why I don't understand why anyone who works on cars doesn't automatically own a torque wrench.

But, as you said, assuming the bolt was torqued properly it should be fine. Extra load will be just that, extra. Obviously improper installations could be compounded by overload but there's no real way to plan for that. In a hypothetical situation then, yes, you could kill a bolt that was under-designed but that's what the safety factor in design should be for.
 
Ron Levy said:
In the short run, I would agree. In the long run, I would not -- look at all the structural issues with aging aircraft, especially the catastrophic failures involving ancient firefighting aircraft.
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Ancient firefighting aircraft were probably not overloaded, just overflown. Do a few thousand 2 G pull-outs in a transport category airframe and it'll eventually break.

The GA fleet is overbuilt and, even overloaded, rarely sees anything close to design limits. Fatigue from modest overloading isn't an issue. Yes, they will wear out faster, but the difference is negligible.
 
kyleb said:
Ancient firefighting aircraft were probably not overloaded, just overflown. Do a few thousand 2 G pull-outs in a transport category airframe and it'll eventually break.

The GA fleet is overbuilt and, even overloaded, rarely sees anything close to design limits. Fatigue from modest overloading isn't an issue. Yes, will wear out faster, but the difference is negligible.
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I would agree with this statement and my calculations in my previous post would seem to reflect a similar agreement.
 
jsstevens said:
Some 172's have flight adjustable rudder trim. Consulting my logbook, I've flown 8 C-172's from C model through an R model. Only one (happens to be a N) has rudder trim.



Please procede with your regularly scheduled thread...

John
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I would agree with this. I have flown far more 172s (from early 60s to post 2000 models) that did not have flight adjustable rudder trim than those that did. In fact, the only ones I have flown that did have adjustable trim were 180 hp models.
 
kyleb said:
Ancient firefighting aircraft were probably not overloaded, just overflown. Do a few thousand 2 G pull-outs in a transport category airframe and it'll eventually break.

The GA fleet is overbuilt and, even overloaded, rarely sees anything close to design limits. Fatigue from modest overloading isn't an issue. Yes, they will wear out faster, but the difference is negligible.
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Exactly....and the same reason why fatigue has been such an issue with the civilian T-34 fleet.
 
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