When is an aircraft overweight?
 

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?

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

It depends.
Here's a second hand story from my CO (many years ago) who flew Huey Gunships (HAL-3 in Viet Nam).
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.

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...?

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.

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!!

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....

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??

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

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:

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.

Little 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 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.WWII 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.

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

IIRC had also to carry wing relieving fuel in the outer tanks that couldn't be used once over 120,000lb ZFW.

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

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

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,

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.

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.