When is an aircraft overweight?

[DogTailRed2]

Please move if in the wrong forum.

Reading about the Enola Gay during it's bombing of Japan and was intrigued to read that at take off the B29 Enola Gay was 7 tons over weight.
I had always thought that the maximum weight of an aircraft was set in stone and an over weight aircraft was not going to fly very well if at all.
So when is an aircraft actually overweight and how overweight can an aircraft safely be?

[charliegolf]

A better explanation will be along soon, but broadly... In war, the book becomes advisory. An overweight aircraft will take a longer run to lift off and climb, but the crew will be fairly certain that it will. Safely overweight? I believe mil aircraft have published normal operating weights, and military operating weights for when the going gets tough. I seem to recall the c-130 could go 20,000 lbs overweight in extremis- 155,000 up to 175,000.

CG

[Lonewolf_50]

Please move if in the wrong forum.

Reading about the Enola Gay during it's bombing of Japan and was intrigued to read that at take off the B29 Enola Gay was 7 tons over weight.
I had always thought that the maximum weight of an aircraft was set in stone and an over weight aircraft was not going to fly very well if at all.
So when is an aircraft actually overweight and how overweight can an aircraft safely be? It depends.
Here's a second hand story from my CO (many years ago) who flew Huey Gunships (HAL-3 in Viet Nam).
By the book, you need to be able to hover at 4' without a 2% droop in your Nr in order to take off without offloading some fuel ...
The Huey's tended to be overloaded, so they used an "in ground effect running takeoff" to get around that limitation.
As the helicopter got a bit more airspeed, the power required reduced so there was enough power to fly away and once through translational lift, you'd soon burn off enough fuel to be back in limits.
But during non war training ops, you'd not have done that.

Also, charliegolf's point on having extra runway to get airborne would play into a go/no go decision.

[GeeRam]

Reading about the Enola Gay during it's bombing of Japan and was intrigued to read that at take off the B29 Enola Gay was 7 tons over weight.


I'd be astonished if it was 7 tons overloaded.
B-29 max combat overload of take-off weight was listed as only about 500kg at 61,000kg max.

And why would it need to be overloaded?
Tinian Field to Hiroshima and back was well within the combat radius of a B-29 with a full 20,000lb bomb load, and the Little Boy atomic bomb only weighed half of that at just under 10,000lb...?

[DogTailRed2]

Thanks for the replies. Very interesting subject.

The book I'm reading is "Shockwave: Countdown to Hiroshima" by Stephen Walker.
The book also states
"The day prior to Hiroshima four overloaded B29's that failed to get airborne and burned with their crews had been bulldozed to the side of the runway"
So I guess the problem of overloading the B29 was not just limited to Enola Gay.
I can't comment on the accuracy of the amount they were overloaded except from memory from reading the book Enola Gay was 7 tons over weight.

[Herod]

charliegolf; your figure is correct, at least for the 130K back in the seventies. It's nearly 50 years since I last flew one, but it shows the long-term memory is still working!!

[BEagle]

Normally, an aircraft will have a certified maximum take-off mass (MTOM). For specific operations, military aircraft can be authorised to operate at greater mass within certain carefully defined criteria.

However, in peacetime, MTOM may not be the maximum mass which meets scheduled perfomance limits, either because of a short runway, high OAT, partial unserviceabilities etc. When all criteria are taken into account, the result is the 'Regulated' Take-Off Mass.

To take an example, although the French Concorde which crashed was probably below its MTOM, it was well above its RTOM - hence it was operating illegally from the moment of brake release. Not that you'll read that in the DGAC accident report....

[isaneng]

The pedants will love this...
Define 'overweight'....
Will fly on all engines operating? Will fly with one engine failed, possibly with a propeller that is not feathered within 'X' amount of time? Will make a screen height, with or without such an engine failure, gear up/down as per the relevant requirements of the day? Even down to is it beyond the published certification criteria? Military or Civvie criteria?
Sorry...Not trying to be a muppet, but the Q is too vague to answer?
However, like all good Pprune questions, I'm curious as to an intelligent answer??

[Heidhurtin]

IIRC Enola Gay had to undergo significant modification in order to accomodate the bomb; related to dimsions and size of the bomb bay, not weight. I think these mods included changes to the wing structure where the spar(s) passed through the original bomb bay. Could this have resulted in a reduced theoretical MTOW for this aircraft only?

Edit: I see they produced a series of these modified B29's, called "Silverplate". I really should read before posting.....link below

https://www.nationalww2museum.org/war/articles/delivering-atomic-bombs-silverplate-b-29

[tdracer]

Max takeoff weight is often defined as the max weight that you can takeoff and meet:
TO runway length (including the ability to safely stop from V1);
The ability to maintain a minimum defined climb rate and obstacle clearance if you loose an engine right at V1 while operating the engines within their certified limits;
Comply with structural limitations of the airframe.

The third of those is usually the least critical - in wartime (especially WWII), those first two are regularly abused. Heck, in peacetime, some less than scrupulous cargo operators will often abuse them.*

*Story told to me years ago when Boeing was discussing building a 1,000,000 pound MTOW 747F. Allegedly, when talking to a certain Asian freight operator, the operator responded to the effect 'So What, we've already done that.' At the time, the maximum available 747F MTOW was around 800,000 lbs. :uhoh:

[Commander Taco]

IIRC Enola Gay had to undergo significant modification in order to accomodate the bomb; related to dimsions and size of the bomb bay, not weight. I think these mods included changes to the wing structure where the spar(s) passed through the original bomb bay.
A little known part of that story relates to some level of uncertainty as to whether the modified B29 would be ready in time. To that end, the RAF was asked to and formed an Avro Lancaster “black squadron” which was tasked with preparing to drop the first atomic bomb should that be required. The training included inflight refueling techniques as well as the bomb blast escape manoeuvre - the Lancaster, flying considerably lower and slower than the B29, had a 12 second bomb blast escape window.

[megan]

at take off the B29 Enola Gay was 7 tons over weightLittle Boy weighed in at 9,700 lbs and Fat Man 10,300 so I venture the B-29 was not overweight structurally, details for the Enola Gay give the max permissible gross as 140,000, same as the regular B-29. Due to failures of the US system, British Type G single-point attachments and Type F releases as used on the Lancaster B.I Special (https://en.wikipedia.org/wiki/Avro_Lancaster#Variants) to carry the 12,000-pound (5,400 kg) Tallboy bomb were used for the atomic bomb.

Ditching on take off was a regular event for the Tinian B-29's, engine over heating being a major issue, maintenance learnt that engine baffles had to be in good order to avoid overheating. At the start they were only getting 100 hours out of an engine. On Tinian take off began with gills full open with the engineer keeping an eye on CHT's and progressively closing the gills as the aircraft accelerated, important factor as open gills caused so much drag and impeded acceleration.For specific operations, military aircraft can be authorised to operate at greater mass within certain carefully defined criteriaWWII limits were rather loose. Pilot flying the C-46 across the Hump says many flights were above even the emergency overload weight of 50,000lb, normal being 48,000lb.

For ferry flights the authorities will grant an overweight operation permit, if flying in Alaska the FAA grant a 15% increase in gross weight under FAR §91.323 for aircraft under 12,500lb.

[ancientaviator62]

The RAF Hercules tanker was often operated at 20000 lbs over the normal TOW OF 155000 lbs.

[Doctor Cruces]

charliegolf; your figure is correct, at least for the 130K back in the seventies. It's nearly 50 years since I last flew one, but it shows the long-term memory is still working!!
IIRC had also to carry wing relieving fuel in the outer tanks that couldn't be used once over 120,000lb ZFW.

[Saintsman]

Perhaps more important, is where that additional weight is located.

[ORAC]

Perhaps more important, is where that additional weight is located.


​​​​​​​https://youtu.be/ggCb7NUueuI?si=chsv0PiKXnWjjBuY

[Milarity]

I often wondered what Mrs Enola Gay Tibbets thought of her son naming his aircraft after her.

[NutLoose]

It’s like anything structural from a cable, to a bridge, to an aircraft, the maximum loading etc permitted is less than actual figure at which point it could fail, to allow an extra safety margin, the aircraft’s MAUW limitations will be similar and set the allow optimum performance for runway lengths, reliability, fatigue , longevity etc, similar will be with engine outputs.

i can give one example of it working the opposite way.

The Beechcraft Baron B-58P was designed with a max all up weight of something like 2800lbs , but in the U.K. the cheaper maintenance programme at the time LAMS had a limit of 2730Ibs.
Therefore Beechcraft introduced the P-58PA ( with an A on the end ) for the U.K. market that had a reduced MAUW of something like 2700lbs, thus being under the requirement for the cheaper service scheme and all that was changed was the MAUW and the badge on the side, plus some paperwork,

[trim it out]

It depends.
Here's a second hand story from my CO (many years ago) who flew Huey Gunships (HAL-3 in Viet Nam).
By the book, you need to be able to hover at X' without a 2% droop in your Nr in order to take off without offloading some fuel ...
The Huey's tended to be overloaded, so they used an "in ground effect running takeoff" to get around that limitation.
As the helicopter got a bit more airspeed, the power required reduced so there was enough power to fly away and once through translational lift, you'd soon burn off enough fuel to be back in limits.
But during non war training ops, you'd not have done that.
That was our rule of thumb in the 212 in the Jungle. If you had 80-82% in the 4ft hover then you'd be able to get out of the LP vertically.

[Geriaviator]

All's fair in love and war, they say. By 1944-45 the RAF Bomber Command offensive was at its peak and Lancasters regularly flew overweight with bombs and fuel to the extent that crews were cleared to use war emergency power for takeoff. Source: my father's friend Flt Lt Bob Nash, RCAF, who could not speak highly enough of the Lancaster and its Merlin engines.

[Dr Jekyll]

If you refuel in the air, are you still restricted to the MAUW or can you take on fuel up to a higher figure?

[tdracer]

If you refuel in the air, are you still restricted to the MAUW or can you take on fuel up to a higher figure?
Short answer is: It depends... Thrust, etc. to get off the ground isn't the concern, but structural limits can come into play.

No first hand knowledge, but I've been told that when a SR-71 was going on a longer range mission, they took off with less than full fuel, then refueled up to the max fuel carrying capacity before climbing to cruise altitude and the Mach 3 speed.

[ORAC]

Quite usual for a military aircraft, whether transport or bomber, to get airborne at maximum weight with a large freight or bomb load and a reduced fuel load and then fill to full from a tanker at the top of climb.

The weight restriction is usually due to runway length and associated engine/climb limitations rather than structural issues.

[421dog]

A very good friend of mine was a crew chief on B-52s based in Wurdsmith AFB (Oscoda, MI) in the early ‘70s. They carried a big load of real nukes, and, as a SAC asset, were required to be on ready alert (15 min in the air from initial notification)
Apparently, in order to get off the ground, the BUFFs took off with less than 2 hrs fuel (from a 11000ft+ runway) and immediately refueled from the attendant KC-135s that were based across the field and took off intercurrently .

There was no way those eight engines were going to get them off the ground otherwise, and he said that the maximum takeoff weight (which he was responsible for calculating) was “fairly fluid”…

[vascodegama]

Quite usual for a military aircraft, whether transport or bomber, to get airborne at maximum weight with a large freight or bomb load and a reduced fuel load and then fill to full from a tanker at the top of climb.

The weight restriction is usually due to runway length and associated engine/climb limitations rather than structural issues.

If memory serves VC10 K3 MTOM 151.9 , max mass post AAR 151.9

[ORAC]

Depends on the runway you are getting airborne from…
At takeoff its maximum weight is 840,000 pounds, including 270,000 pounds of cargo and 332,500 pounds of fuel; after being refueled in flight, C-5s have weighed 920,836 pounds.

[stevef]

I remember reading about a WW2 C47 apparently loaded with PAP (Pierced Aluminium Planking) for airstrip construction that took a very long time to get into the air and climb to its cruising altitude. It turned out at its destination that the freight was actually Pierced Steel Planking! Some error! I forget what the actual weight was but it greatly exceeded the already 'generous' Douglas wartime limit.

[Fitter2]

I worked in the oil industry for a while. One of my colleagues recounted that in Libya in the 1960s a DC3 was loaded up, mostly freight, for a supply run to his oilfield. It barely got airborne by the end of Tripoli runway, and flew very gently and at very low level round a circuit and landed. It transpired that the load had been calculated in pounds, but the same number measured in kilograms. Half the cargo was unloaded, and they then set off for (and in due course arrived at) their destination.

[rans6andrew]

In the UK Microlight aircraft are limited to 450kg MAUW, this is both a restriction of the licence conditions of the pilot and also a safety figure for the max loading of the airframe structure. IF the SAME aircraft is certified as a GA aircraft it is allowed to fly at a higher MAUW as the safety margins for a GA aircraft are calculated using a lower factor. Same aircraft, different max weights. Just an observation.........

Rans6..........

[DaveReidUK]

Reminds me of the very old Canadian moose hunting trip joke ...

[212man]

To take an example, although the French Concorde which crashed was probably below its MTOM, it was well above its RTOM - hence it was operating illegally from the moment of brake release. Not that you'll read that in the DGAC accident report....
Somewhat Francophobic, I'd say. The slight weight anomaly (around 800 kg vs a total of around 185,000 kg) was described in the preliminary report, and again in the final report. Rather academic, given its negligible effect on performance and likely inaccuracy, given the use of standard passenger weights. BTW, it's the BEA (Bureau d'Enquêtes et d'analyses pour la sécurité de l'Aviation civile) that conducts investigations - the DGAC (Direction Générale de l'Aviation Civile) is the regulator.

[PEI_3721]

Over weight;

There is too much mass for the certificated condition.
Any mass in the wrong position effecting control and stability.

[BEagle]

The slight weight anomaly (around 800 kg vs a total of around 185,000 kg) was described in the preliminary report, and again in the final report.

It was considerably more than that! RTOM, was well below MTOM due to the tailwind, which the Captain totally ignored...

Concorde experts told me that it was around 5 tonnes minimum!

[wiggy]

As TD has said, and certainly flying civvie heavies, this really is an "it depends".

It can be structural ( in fact very often the limiting factor out of places like London and Singapore on Long long haul routes where there's plenty of runway), other places with less generous amounts of tarmac you can end up "capped" by performance...I'm not sure there's a usual reason...

Going back to the OP - in the context of the B-29 - military, wartime, it might be worth thinking about the rules that may or may not have applied to those operations. Applying the modern rules of having to have the performance to allow for an engine failure on takeoff would almost certainly reduce "maximum take-off weight" on the day for many runways. If that wasn't a consideration, wartime and all that, I wonder how many of the B-29s that didn't make it into the air were at or below structural max weight but had an engine go at high speed but before lift off.

[Lonewolf_50]

Reminds me of the very old Canadian moose hunting trip joke ... The one that ends 'what's a panoe?' or a different one? :cool:

[Sue Vêtements]

I think it was the excellent Serenade to the Big Bird by Birt Stiles where he talks about flying B17s out of England and on one trip, he took off only to feel the aircraft was really sluggish and then started to descend! Quickly he called for wheels down just in time to be able to bounce in and out of a field, back into the climb and wheels up

Turned out it was a deep penetration raid, his ground crew had loaded not just extra but extra extra ammunition for the trip so they were well over gross - and apparenty the ammo was stored "down the back" too :uhoh:

[Ascend Charlie]

The one that ends 'what's a panoe?' or a different one? :cool:
The one that says "Yeah, this is the same place the last pilot crashed in!"

The Huey had a weight limit of 9500lb, but for a self-deployment of three aircraft to PNG we had to carry quad tanks with fuel to cross Torres Strait, pilots (2 per) to fly them, spare pilots (to be trained in PNG), crewmen (fly and train), groundies to do turn-arounds and scheduled minor maintenance, and the spare parts and tools to do so.

We needed (and got) permission from Bull5h1t Castle to operate at 10,500lb for the exercise. The hover height was just enough to do cushion-creep takeoffs. By the time we got to any destination airfield (Goroka 6000', Mt Hagen a bit higher) we were down to an acceptable weight.

As an aside to a previous post, our initial training on Hueys always included skidding takeoffs from grass, where even a hover wasn't possible. In Vietnam, PSP was slipperier than grass for such a purpose.

[stilton]

It’s like anything structural from a cable, to a bridge, to an aircraft, the maximum loading etc permitted is less than actual figure at which point it could fail, to allow an extra safety margin, the aircraft’s MAUW limitations will be similar and set the allow optimum performance for runway lengths, reliability, fatigue , longevity etc, similar will be with engine outputs.

i can give one example of it working the opposite way.

The Beechcraft Baron B-58P was designed with a max all up weight of something like 2800lbs , but in the U.K. the cheaper maintenance programme at the time LAMS had a limit of 2730Ibs.
Therefore Beechcraft introduced the P-58PA ( with an A on the end ) for the U.K. market that had a reduced MAUW of something like 2700lbs, thus being under the requirement for the cheaper service scheme and all that was changed was the MAUW and the badge on the side, plus some paperwork,


You may be thinking in Kgs,

The B55 I flew had a max to weight of 5300lbs, the 58P would be around 6200

[Buster Hyman]

As a Commercial LoCo bloko, weather was a major factor for us. DC-10's doing MEL/HNL at Midday during Summer... we were counting the kids through the gate & waiting for the right number before loading a pallet on one day that I recall. It was a -30 too!

[Herod]

Sue: that was out of Ridgewell I believe. The actual sequence was gear-up to clear the fence, then down to bounce off the field. There is a book, whose title escapes me (possibly Bomber Crew), written by a flight engineer who was on that trip.

[DaveReidUK]

The one that ends 'what's a panoe?' or a different one? :cool:

Different one.

Two rednecks flew to Canada on a hunting trip. : r/Jokes (reddit.com) (https://www.reddit.com/r/Jokes/comments/ao79t3/two_rednecks_flew_to_canada_on_a_hunting_trip/)

[jumpseater]

Is it not overweight when the airframe weighs more than the certification paperwork?

[Gordomac]

Gosh, rubbish New Years Eve bash- got home early. Didn't even stay up for Rick Astley cos we get New Year 2 hours earlier in Cyprus and here I am, clear headed, focused and enjoying my daily stroll around PPrune. ....................Bort.........................blimey. 3 pages on and only one getting close.

The opener asks for overweight. Er, what weight ? TOW. LW, ZFW ?

Of course he/she means TOW. Manufacturer will get certification for that after serious peer review. That is the CERTIFIED maximum Take-off weight. You could probably exceed that at seal level, 15c, Max HWC, and end endless flat runway- don't try it though -. ACTUAL TOW will have to be modified for several reasons and results in a REGULATED TOW. That becomes the "maximum" for that operation. In nearly all cases, it will be Landing Weight plus burn off. Nothing to stop you exceeding that on take-off but you will then be too heavy to land.

Civil regulation charges Air Traffic overflight based on a "declared" max TOW. One Operator I toyed with decided to declare ithree aircraft to be much lighter than the Max cert TOW. in order to get reduced ATC overflight charges.. Looking trendy in their Lone Ranger/Tonto outfits, they then assigned these aircraft to long routes where that weight needed to be, regularly exceeded. The logic(yeah-right) offered to us was that these over declared weights were not exceeding "certified" criteria. Oh dear. I left. Looked silly. in my Tonto outfit anyway.!

Gosh this is sad. Going up to the Lithos Taverna to see if anyone is around. Happy New Year everyone.

[Geriaviator]

Lose a piston engine even at max authorised weight and you've had it. To the immortal "Brevet" thread which has been honoured by being made a sticky at the top of Military Aviation, now active again with a brilliantly vivid account by P/Off Ron Homes describing takeoff in his 101 Sqn Lancaster, overloaded with half a ton of jamming equipment as well as a full load of bombs.

"The flight engineer takes over the throttles and holds them fully forward, “full power skip”. Both hands on the control column now, keep her straight, the aircraft is throbbing, the roar from the four engines is deafening. Airspeed is building, 60, 80, 90mph is called out by the flight engineer. The runway roars past but the massive weight of 2000 gallons of fuel and six tons of bombs makes itself felt through the controls and the end of the runway gets nearer and nearer. If one engine fails now we shall run off the end and the whole lot will blow up and leave a nasty big hole in the ground ... "