When is an Aircraft Too Heavy to Fly?

[jimdavis]

Something to think about when considering whether or not your aircraft is in "good condition":

When we do the performance tests (take-off, landing, climb, etc) the aircraft is set up to be representative of one that is within specifications. Often specifications will come with tolerances, and in general we will set the aircraft to "most adverse" in order to be conservative. A classic example is tire pressures. Your tire pressure in the AFM / POH typically has a range (lower limit to upper limit). Assuming that underinflated tires degrade performance, we will set the tires to be slightly under the lower limit. And that is "slightly". The assumption is that your tires will always be inflated to the specified pressure range. Think about that next time you look at your tires on preflight and think "it looks a bit soft, but it'll be OK".

So what are the things that make your aircraft "in good condition" over which you have direct control? If you can mentally identify those factors, and make sure of them in your preflight, you're probably improving your odds on the day that you really need the performance.

Thanks so much Stressors - these people have been causing me to have sleepless nights.

[jimdavis]

Come Brand - what's your answer to the question? We have all has a go at it.

New year's greetings to you and yours.

jim

PS I have to say I am amazed how few people are prepared to say "Yay - I learned something new today".

[Brand Wessels]

Come Brand - what's your answer to the question? We have all has a go at it.

New year's greetings to you and yours.

jim

PS I have to say I am amazed how few people are prepared to say "Yay - I learned something new today".

Come on Jim,you know me - I know nothing.

What I can say is this - based on the answers to my question, I have no doubt that many PPL's do not ever open a flight planning performance manual before any flight...

[Joe S]

In terms of the aircraft performance information in the aircraft POH, Jim is correct as the POH information is used to make decisions that involves peoples lives and thus needs to reflect what can be expected from an aircraft that has been declared airworthy by the relevant authority. The aircraft manufactures are very aware of product liability claims, particularly in the USA, and will ensure the performance data published can be met by any aircraft declared airworthy.

My own experience is with the two aircraft I own, one built in 1968 and one in 1985 is that the takeoff and landing performance information is easily met and both aircraft general perform better than the POH states. I tend to focus on take and landing distance due to the various airstrips I use.

Flying at MAUW and minimum can be very different and should be part of training as there are some unexpected changes. Taking off it is expected that there will be a longer take off run and slower climb. The landing however introduces some interesting changes, you would expect the longer ground run, however the unexpected change would be in the final approach. The change comes in the air distance (measured from above the 50ft obstacle to touch down) for landing over a 50ft obstacle, the aircraft at MAUW will have an air distance approximately 20% -30% longer than the same aircraft at minimum weight. This is much the same as gliders adding water ballast for better penetration on the glide.

[jimdavis]

Thanks Joe. I'd be interested to hear what two aircraft you are talking about.

All the best for 2025

jim

[Lood]

For what it's worth, a very competent instructor and I once tested the POH graphs for my V35B Bonanza.
We took the actual weight, fuel, temperature, etc, etc very accurately and plotted it on the take-off distance graph to determine the distance given by the POH.
I can't recall if we tested the climb performance also, but the take-off distance turned out almost exactly as predicted by the POH.

I can't say the same for my Twin Comanche though. It had just about every imaginable mod, which was very nice to have, but it also resulted in a very, very heavy Twinco with a very poor useful load.
At MAUW, it flew well, but it took it's time to get airborne and VY was often a 150 - 200 ft/min affair. I never tested it against the POH, but I'm pretty sure that it didn't come close to the POH take-off and climb performance figures.

[Joe S]

Jim, the two aircraft I am referring to are a M20G and a G109b, . In the past I flew a Piper Arrow that performed very close to the performance stated in the POH. Personally I think the POH is the place to start when considering if an aircraft is too heavy to fly and then to consider the weather, physical conditions of the airstrip and surrounding terrain and only then decide.

[propstrap]

Jim, the two aircraft I am referring to are a M20G and a G109b, . In the past I flew a Piper Arrow that performed very close to the performance stated in the POH. Personally I think the POH is the place to start when considering if an aircraft is too heavy to fly and then to consider the weather, physical conditions of the airstrip and surrounding terrain and only then decide.

Joe, you are correct, the POH is where you should start. I am unfamiliar with the aircraft and how their graphs look, but the graphs should provide performance calculations based on weather, conditions of the runway and density altitude. Pardon if I repeat what you know, but the WAT referred to here sometimes refer to weight, altitude and temperature, which should be variables that the graphs provide for. The runway slope and wind component should also be included in the graphs.

It is also important to consider that the graphs are only valid if the aircraft performs within limits (there should be guidelines in the POH), i.e. in good condition. The performance graphs should state what your power should be under the specific conditions, and you can use that as a check. If the aircraft underperforms the graphs are no longer valid. As long as the aircraft is maintained according to schedule, you should be good.

[jimdavis]

Jim, the two aircraft I am referring to are a M20G and a G109b, . In the past I flew a Piper Arrow that performed very close to the performance stated in the POH. Personally I think the POH is the place to start when considering if an aircraft is too heavy to fly and then to consider the weather, physical conditions of the airstrip and surrounding terrain and only then decide.

Joe, you are correct, the POH is where you should start. I am unfamiliar with the aircraft and how their graphs look, but the graphs should provide performance calculations based on weather, conditions of the runway and density altitude. Pardon if I repeat what you know, but the WAT referred to here sometimes refer to weight, altitude and temperature, which should be variables that the graphs provide for. The runway slope and wind component should also be included in the graphs.

It is also important to consider that the graphs are only valid if the aircraft performs within limits (there should be guidelines in the POH), i.e. in good condition. The performance graphs should state what your power should be under the specific conditions, and you can use that as a check. If the aircraft underperforms the graphs are no longer valid. As long as the aircraft is maintained according to schedule, you should be good.

Propstrap, I see you chose not to answer my question on the previous page about an 737 going into George on a wet runway with bald tyres that show the canvas.

Don't worry - I'll answer it for you. Yes it's perfectly legal and the aircraft will land safely - because the charts say so. As Stress Merchant says - the charts are for the "most adverse" limits of wear and performance degradation - not for a new aircraft.

I would have thought a sensible response to this chat would have been "thanks Jim - I have learned something new."

[propstrap]

Jim, the two aircraft I am referring to are a M20G and a G109b, . In the past I flew a Piper Arrow that performed very close to the performance stated in the POH. Personally I think the POH is the place to start when considering if an aircraft is too heavy to fly and then to consider the weather, physical conditions of the airstrip and surrounding terrain and only then decide.

Joe, you are correct, the POH is where you should start. I am unfamiliar with the aircraft and how their graphs look, but the graphs should provide performance calculations based on weather, conditions of the runway and density altitude. Pardon if I repeat what you know, but the WAT referred to here sometimes refer to weight, altitude and temperature, which should be variables that the graphs provide for. The runway slope and wind component should also be included in the graphs.

It is also important to consider that the graphs are only valid if the aircraft performs within limits (there should be guidelines in the POH), i.e. in good condition. The performance graphs should state what your power should be under the specific conditions, and you can use that as a check. If the aircraft underperforms the graphs are no longer valid. As long as the aircraft is maintained according to schedule, you should be good.

Propstrap, I see you chose not to answer my question on the previous page about an 737 going into George on a wet runway with bald tyres that show the canvas.

Don't worry - I'll answer it for you. Yes it's perfectly legal and the aircraft will land safely - because the charts say so. As Stress Merchant says - the charts are for the "most adverse" limits of wear and performance degradation - not for a new aircraft.

I would have thought a sensible response to this chat would have been "thanks Jim - I have learned something new."

To the contrary Jim. This is where we differ. Saying the charts assume most adverse limits of wear? If the aircraft performance is below that specified, that boat is not going to float.

From the C172 POH:

The data in the charts has been computed from actual flight tests with the airplane and engine in good condition and approximating average piloting techniques.

From the C208 POH:

The data in the charts have been computed from actual flight tests using average piloting techniques and and airplane an engine in good condition

From FAA Pilot Handbook of Aeronautical Knowledge

It is important to remember that the data from the charts will not be accurate if the aircraft is not in good working order or when operating under adverse conditions.

(already quoted)

I asked you let us discuss any possible misunderstanding but you declined, which is sad as I wanted to exchange ideas, hence this unfortunate debate.

There can be no argument about it, the graphs assume an aircraft in as new or good condition, otherwise many aircraft manuals and the FAA is wrong, which I tend to doubt. And the graphs are based on science, as the regulations which they have to comply to, are based on it as well. Graphs are there for pilots to make decisions on can I do this or not. For that, you need hard facts, i.e. the aircraft can stop from this speed within that distance, or accelerate within this distance to that speed all taking into account things like WAT, runway slope and condition, and wind for example.

The term "minimum airworthiness standard" can be misleading, but the graphs assume specific performance requirements, and then the aircraft is deemed in good condition. Can you quantify / define the minimum airworthiness standard? Is it the bare minimum, so when the pilot brakes once too hard on the way to the runway it is no longer valid? There is no place for open ended assumptions in performance graphs.

The manufacturer uses a new airplane on the graphs (it just off the floor for test flights), and while performance degradation is accounted for (excess power reduced over time, or flat rated in turbines) it may not mean the same to everyone reading "minimum airworthiness". Thus they may think they conform to the graphs while they do not and that is a risk. I have tested aircraft in that state, where they are used but do not produce e.g. the power they are supposed to. Such an aircraft is a flight safety risk.

End of argument, if the aircraft cannot give the same performance as the manufacturer tested and drew up the graphs, it is not valid. I can't put it more straightforward as that.

Respectfully Jim, let's just say we agree to disagree and let the thread move on. You have contributed so much over many years, but this is something which I feel strongly about. As you said, maybe we just use different semantics. Best wishes for 2025 to you.

[jimdavis]

Sorry Strappers, that was rude of me and unnecessary - I apologise..

I think it does boil down to semantics. The term good condition is an opinion - it is not definitive. As we can see the worn tyres and brakes are technically airworthy, and will therefore perform within the graphs for a wet runway, but no one would describe them as being in good condition.

Airworthy condition is definitive. It means stuff falls within the legal limits and is therefore covered by the POH..

And a great 1025 to you.

jim

[SimplyFly]

Much has been said here about the assumed state of the aircraft. The phrase that interests me equally is "average pilot". Please allow me to put a stick in the beehive with three statements:

• My impression is that more than half the private pilots that I have met are sure that they are above average.

• About half of us are below average pilots.

• Reading Avcom does not exempt us from this.

There is a tale about a trade union that went on strike because they found out that half their members were earning less than the average wage.

Simpli

[IKTAV]

To weigh in on this topic one more time. (Excuse the pun )
Airlines have Performance Factor degradations for each of their aircraft.
Because their individual aircraft are monitored so closely, a factor is applied to each of them in case they are not performing exactly to spec. Adjustments (to weight, fuel flow etc) are then made so that appropriate variables can be adjusted to meet the published performance. (This is usually related to engine performance and drag factors in case of a missing winglet or the odd dent from an ACSA truck. )

But even if the brakes are worn, the pilots can expect the same deceleration performance - provided they haven’t worn beyond the “brake wear limit indicators” on the brake pads themselves. (That’s one of the many reasons a thorough preflight inspection is so important.)
And likewise as long as the tyres aren’t showing more than one layer of canvas of a certain size, they can also be expected to perform to spec for one more landing before replacement. (Depending on the tyre’s make and the aircraft.)
And similarly the manufacturers of light aircraft make certain assumptions. Naturally always based on the conservative side regarding the average pilot and a well maintained aircraft.

But back to take off weight.
The aircraft can be considered too heavy if it cannot achieve the various performance criteria published in the Aircraft Flight Manual or POH. These criteria will include Take Off Distance, Lift Off speed, Climb Rate, WAT limit and a few others.

(This is assuming that the non-performance is caused exclusively by overloading - because it could also be caused by something else. Like strapping the door open against the strut of a C172!! As was done recently. )

[117]

Why do you lads talk about weight or MAUW as opposed to max wing loading, your CG% range and your polar curve?

Im pretty sure most of the time any normal PC will be positively nose heavy at MAUW - which will add to its issues, right? Or do I have it wrong, especially on the CG %?
I assume that 4 large saffers in a 172 plus full tanks will push the CG way too far forward which will create it own set of issues (im not talking about exceeding the published MAUW here)? Where if the CG range was adhered to in this scenario, will the 172 be able to still fly albeit it a little more slugish at the published numbers, which should make you fly that little faster on the polar curve?

For the big busses, I assume that they control the CG% by means of fuel transfers between fore and aft tanks and rely more wing loading numbers as opposed to all up weight?

[GL]

I think you are all being too smart about rules and regulations and graphs.
If you look at the question (When is an Aircraft Too Heavy to Fly?) - the answer seems to me to be simple:
When the power is insufficient to overcome the total drag at a speed faster than the stall speed.
It is a corollary of "Even a barn door will fly given enough power."