11

I was on a LATAM flight a couple days ago, and decided to have a read through their VAMOS magazine. As a lot of airlines do, they had a couple of pages on their fleet, and also a double spread on one specific plane.

This plane was the 777, which was a little bit surprising as they are about to get rid of those I think. Anyway, it had all the usual facts and figures, and also a couple of highlights about the plane.

I managed to find a web copy of the magazine, and the picture is below:

enter image description here

https://issuu.com/spafax/docs/vamos_latam_junio_2017_web

One of these highlights is the wheels, and it says (roughly translated):

Each of the wheels is capable of supporting a maximum of 29,294 kg.

My first reaction was "That's pretty impressive!", then "I wonder how much above the MTOW that is?" The MTOW is specified at the side there: 351,530 kg, which correspondes to the 777-300ER.

So 29,294 x 6 x 2 = 351,528

Oh. That's under the MTOW. I wonder if there's some rounding off here:

So 351,530 / 2 / 6 = 29,294.166 - yes there is.

I was surprised that the maximum each tyre/wheel could take summed up to the MTOW, as I thought that they would experience larger forces on landing. Also, what would happen if a tyre blew?

So, does the maximum load for a tyre/wheel have anything to do with the MTOW? Or is this just marketing guys jumping the gun?

I have seen this question How is Maximum Take Off Weight determined? and that says that the manufacturers determine it, and it is based on the required payload of the aircraft and the design configuration, which doesn't really answer this question.

Edit with extra info mentioned in comments:

When stationary, the wheels will have a total force on them of 351,530 kgs, which according to this magazine is their max. When landing, the wheel like have to bear a weight of 251,290 kgs, plus the force of the landing. In my mind, the extra force of the landing would push the total above the MTOW, but this is just in my mind so I'm probably wrong :P

CalvT
  • 774
  • 2
  • 11
  • 24
  • 1
    Maximum take off weight is often higher than maximum landing weight specifically for that reason. There are quite a few aircraft that can take off but have to burn fuel before it can land again. – Ron Beyer Jun 27 '17 at 12:46
  • @Sanchises right. I get that. But according to this, when the plane is at MTOW and stationary, the wheels are at their max. So surely the impact of landing with 70% of that weight would exceed that max? – CalvT Jun 27 '17 at 12:53
  • 1
    "surely"? have you run the formula? or is "surely I am right in thinking this and I can't be wrong?" :P If you run the formula you'll see that one element is the weight, the other the speed of impact. You could land with 1% of the MTOW and still completely destroy your aircraft if the speed is excessive. – Federico Jun 27 '17 at 12:56
  • 1
    @Federico I'm asking you guys for the formula :P – CalvT Jun 27 '17 at 12:56
  • ok, point taken :D (but be wary, one thing is the MTOW, another is the maximum landing weight) – Federico Jun 27 '17 at 12:57
  • 3
    Seat back brochures aren't an accurate representation of the engineering behind this. The wheels are not at the maximum loading at MTOW. The weight is more determined by stopping the aircraft prior to V1 during take off given the configuration of the aircraft. It isn't really about how much the tires hold. There is no one "formula" to cover this. – Ron Beyer Jun 27 '17 at 13:02
  • @RonBeyer so, to answer my question, Marketing People jumped the gun? – CalvT Jun 27 '17 at 13:04
  • 1
    @CalvT븃 Not so much "jumped the gun", but trying to impress passengers with numbers that they probably don't fully understand. The gear system itself (tires included) can withstand landing pressures significantly higher than that. – Ron Beyer Jun 27 '17 at 14:06
  • @RonBeyer that's what I thought. Do you have any numbers on that? – CalvT Jun 27 '17 at 14:07
  • 14 CFR 25.301: Loads, 14 CFR 25.303: Factor of Safety, 14 CFR 25.473: Landing Load Conditions, 14 CFR 25.479: Level Landing Conditions, Basically anything in 25.471-25.519 – Ron Beyer Jun 27 '17 at 14:12
  • Let me rephrase that: any numbers specific to the 777 which help answer this question? – CalvT Jun 27 '17 at 14:14
  • I feel your initial question was whether the figures for the wheel were accurate in the brochure or significantly underestimated by the marketing guy doing a simplistic division MTOW/# of wheels (a good question imho), but the title was a bit remote... and now you are overwhelmed with details and almost asked to justify your question :-) Keep cool! – mins Jun 27 '17 at 14:49
  • @mins I'm calm don't worry :) - my question in a nutshell for you - "are these figures correct, as they don't make sense to me, and if they aren't, what are the correct figures?" – CalvT Jun 27 '17 at 14:52
  • I can't answer, and maybe it's difficult to find the information in the technical specs, but for sure there is a safety margin that may be 100% or more of the static load (the weight on the wheel, which depends on the position of the center of gravity). Effect of wind while on the ground (negative lift) must also be taken into account. – mins Jun 27 '17 at 14:55
  • @CalvT븃, I suspect you are correct. They jumped the gun (assumed) that is max of each tire, because I find it hard to believe that the tires don't have some redundancy built in to them, so that if one blows while taxing, they all wouldn't then blow out because of the increased load. – Devil07 Jun 27 '17 at 15:36
  • 1
    @CalvT븃 I noticed that you divided the MTOW by 12 tires, but there are actually 14 tires. Unfortunately, the nose wheel tires appear to be different size, so it wouldn't be accurate to divide by 14, but since some of the weight is on the nose wheel, the total MTOW will not be on the main wheels. – Devil07 Jun 28 '17 at 00:35
  • English for picture description : https://www.latam.com/vamos/get-inspired/facts-to-learn-about-the-boeing-777 – vasin1987 Jun 28 '17 at 05:17

3 Answers3

7

Boeing1[pdf] specs for 777-300ER:

  • Max taxi weight: 777,000 lbs (352,442 kg)
  • Max take off weight: 775,000 lbs (351,535 kg)
  • Max landing weight: 554,000 lbs (251,290 kg)

Bridgestone2 specs for approved tires:

  • Main: 66,500 lbs (30,264 kg) x 12 = 798,000 lbs (361,967 kg)
  • Nose: 44,500 lbs (20,185 kg) x 2 = 89,000 lbs (40,370 kg)
  • Total tire rated load = 887,000 lbs (402,337 kg)

So at taxi weight on all 3 gear there's roughly a 110,000 lbs / 50,000 kg margin (about 14%).

At max landing weight, main gear only, there's a 244,000 lbs / 110,677 kg margin (44%).

At MTOW there's about a 112,000 lbs / 51,000 kg margin (14%) on all gear, but only a 23,000 lbs / 10,400 kg (3%) margin after nose gear lift off.

But, when the nose gear lifts off the ground the wings are supporting nearly all of the weight. Their numbers are pretty close to what the manufacturers list. I think you're just not accounting for the nose gear at takeoff and the weight difference at landing.

As far as Is the MTOW determined by the wheel maximum supported weight? It's the other way around. The MTOW determines how many tires of what weight rating are needed.

A more important tire consideration is speed. The tires are rated at 204 kt. This paper gives an example of a 747 at 825,000 lb there is a weight margin of 30,000 lb. But liftoff speed is 199 kt, so there's only a 5 kt speed margin.

CalvT
  • 774
  • 2
  • 11
  • 24
TomMcW
  • 28,459
  • 21
  • 106
  • 227
0

Another factor in MTOW is the ability to maintain control after takeoff. Just getting airborne isn't enough. Northwest 255 managed to lift off with flaps improperly set, but couldn't maintain control and crashed. Air Florida 90 also lifted off, but crashed almost immediately due to icing and less than full power from the engines.

An aircraft too heavy for the design can suffer the same fate. Ernest Gann described an incident where a C87 (B24 derivative) with an accidental overload of fuel he was flying out of India during WW2 almost hit the Taj Mahal, because it would barely stay airborne with no ability to climb or change direction without stalling. Only a last minute application of flaps got it over the towers.

tj1000
  • 8,767
  • 1
  • 23
  • 37
0

An aircraft will have a certified MTOW which it can't exceed, but keep in mind the actual MTOW for a specific flight can be lower (and often is). MTOW will always be the lowest of the following limits:

Field Length (runway length, condition)

Climb

Obstacle Clearance

Tire speed & brake energy

Landing Limit weight (i.e. can't plan to arrive at something greater than MLWD)

PHChilly
  • 387
  • 1
  • 6