23
$\begingroup$

As a means of weight reduction to reduce fuel consumption, could the tires on an aircraft’s landing gear be deflated once airborne then re-inflated just prior to landing? Is that feasible and safe?

$\endgroup$
  • 1
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Nov 8 '17 at 6:43
  • 4
    $\begingroup$ No. Because they would still need to be inflated at take off, when the plane is already at its heaviest. So you'd make zero impact on the weight budget in the one instance where it would truly matter...assuming the intrigue here is considering how to squeeze a few more suitcases on there. $\endgroup$ – elrobis Nov 8 '17 at 20:45
  • 1
    $\begingroup$ Easier to just chuck some landing (taxiing?) gear away on takeoff a-la the U-2's "pogos" $\endgroup$ – Nick T Nov 8 '17 at 23:28
  • 1
    $\begingroup$ Uhm, thought I'd leave a little note for those who are confused as to why this should affect the weight at all (possibly including the OP too, in case he's also confused). It only matters because the tire has a higher air density than 1 atmosphere and because the outside air will be at less than 1 atmosphere. I neither of these were true, the inflation of the tires wouldn't change anything, since the buoyancy from the outside air would counteract the mass increase. $\endgroup$ – Mehrdad Nov 9 '17 at 8:57
  • 1
    $\begingroup$ @Mehrdad Really? Buoyancy is based on the volume of the displaced fluid, a flat tyre's volume is not much less than an inflated one. $\endgroup$ – Koyovis Nov 11 '17 at 3:31
9
$\begingroup$

It's a good question. Estimations of 120 kg weight savings of compressed air in the tyres have been made, that is not inconsiderable. A detailed estimation with dimensions from this link. The B777-300 has tyres size 52 X 21 R 22:

  • 52 is outside diameter in inches = 1.32m
  • 21 is tyre wide in inches = 0.53m
  • 22 is rim diameter in inches = 0.56m
  • Side area of the tyre itself = $\frac {\pi}{4} (1.32^2 - 0.56^2) $ = 1.23 m$^2$.
  • Volume of the tyre = side area * width = 1.23 * 0.53 = 0.65 m$^3$. The B777 has 12 of these wheels so total tyre volume = 7.8 m$^3$.
  • Air at 1 bar pressure = 1.225 kg/m$^3$. Twelve wheels at 16 bar contain 7.8 * 1.225 * 16 = 153 kg of air.

Weight savings in the order of magnitude of 150 kg, every trip for two to three decades, are nothing to sneeze at. Consider it the other way around: how sane would a person be if they would propose to put an extra 150 kg permanently on board because the extra fuel would be inconsequential?

To re-inflate the tyres, all infrastructure is on board already: jet engine final compressor stage pressure is definitely higher than the required 16 bar. From Rollys Royce The Jet Engine issue 5:

enter image description here

After cooling off from 700C - 1000K, to 30C = 300K, that is in the order of magnitude of the required air pressure for re-inflating the tyres. So technically, all is possible. The tyres can be re-inflated with existing infrastructure on board. Of course not with the existing unmodified infrastructure as it is now, it will need to be tweaked, but order of magnitude wise everything is on board already. Tweaking a tyre pressure inflator in the 21st century is a much simpler task than inventing the first autopilot in 1930. I mean come on guys.

Aluminium 1inch tubing weighs about a kilo a meter, and the engines are very close the wheels. 10 m extra tubing would be a lot. The hydraulic oil has a heat exchanger with the fuel in the wing, which is super cold just before landing, having spent all this time at -30C. Let's say that the total re-inflation installation weighs 30 kg tops.

Where the whole idea falls apart is in the safety aspects. We're approaching an airport with low fuel reserves and with flat tyres, too many things can go wrong and Murphy pops his head up from every corner. The undercarriage is a considerable weight and contributes virtually nothing to the flying and economic capabilities of the aircraft, but we better make sure that it pops out every time and can cushion the last stage of flight before all passengers can get safely off board.

In air passenger transport, if safety needs to compete with economics, safety wins every time.

@mikerodent suggests in a deleted post that the tyres could be filled with helium. Not a bad suggestion actually: helium weighs 0.164 kg/m3, weight saving for the B777-300 is 132 kg and the tyre does not need to be re-inflated. The answer deserves to be undeleted and accepted.

$\endgroup$
  • 1
    $\begingroup$ to those 150kg you have to subtract the weight of the extra redundant plumbing and heat exchangers before you can claim "Weight savings in the order of magnitude of 150 kg". You have done all the calculations for one side of the equation, you can do the same for the other. Alternatively, you can check if the fuel saving of those 150 kgs compare to the extra engine losses incurred in bleeding HP air. $\endgroup$ – Federico Nov 9 '17 at 8:59
  • $\begingroup$ @Federico yes more details need to be worked out before it can be implemented, and total weight savings might be less. But that is beside the point: it won't be implemented no matter how attractive, because it imparts on safety. $\endgroup$ – Koyovis Nov 9 '17 at 9:19
  • 1
    $\begingroup$ grainger.com/category/aluminum-tubing/tubing/… resist only up to 400°F, less than the 500°F (and more) found in the HP section of the engine [aviation.stackexchange.com/a/11483/1467 <- note that here we don't even get enough compression] I'm not sure that Aluminium would be acceptable, nor that a non-redundant system would be ok. So 30kg would be the lower limit, not the upper. $\endgroup$ – Federico Nov 9 '17 at 12:14
  • 2
    $\begingroup$ "In air passenger transport, if safety needs to compete with economics, safety wins every time" I believe it's not true, counterexamples easy to list: Aircraft disappear because they aren't tracked over oceans, and then huge amounts of public money are spent to SaR them. Aircraft use achaic data recorders with archaic ULB to (not) retrieve them. Aircraft can legally fly without a system to alert the crew when they are landing with gear up, etc. Airlines shareholders have a strong lobbying power on regulations. $\endgroup$ – mins Nov 24 '17 at 9:51
  • 1
    $\begingroup$ "None of the old stuff you mention has been implemented because it is cheaper than better new stuff": I'm not challenging past actions, but emphasizing that status quo attitude is cheaper than actively improving security. The examples I listed are status quo cases made possible by the influence of airlines facing competition cost challenges. $\endgroup$ – mins Nov 24 '17 at 11:10
103
$\begingroup$

You want to reduce weight by removing a few kilos of air, and adding an equal, if not greater, amount of kilos of pumps needed to reflate the tires (see David Richerby's answer for details), not to speak the additional costs incurred for periodically checking the correct functionality of said pumps, that would be marked a "safety critical" (because a landing with deflated tires would have really bad consequences) and require being checked quite often.

So, feasible maybe. Safe not really. Meaningful I don't see how, particularly when there is the risk of tire burst, as shown in this video linked by March Ho.

A lighter alternative would be to use bleed air from the high pressure section of the engine, but it would still probably(*) be heavier than the air you are pushing out (you need heavy duty pipes to transport high pressure air from the engines to the wheels), and not a good solution safety-wise: what would happen if you have to land without engine power?

(*): as user3528438 mentions in their comment, it would depend on the wheel size.

Additionally, this Goodyear manual provides insight in some other problems: (emphasis mine)

  1. ALLOW 12-HOUR STRETCH AFTER MOUNTING

All tires, particularly bias tires, will stretch (or grow) after initial mounting. This increased volume of the tire results in a pressure drop. Consequently , tires should not be placed in service until they have been inflated a minimum of 12 hours, pressure rechecked, and tires re-inflated if necessary.

It means that the tire needs adjusting after being inflated, possibly removing the possibility of inflating and deflating it continuously (or, alternatively, you have to reinflate it at least 12 hours before landing). Note that this manual is written for current operations, that call for a replacement/inspection of the tire if pressure drops below 80% of nominal value. Procedures should be rewritten for inflation/deflation cycles, and this could be a factor.

$\endgroup$
  • 1
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Nov 7 '17 at 18:44
37
$\begingroup$

Some military transport aircraft can indeed adjust tire pressure from the cockpit for soft field operations. The Antonov An-22 would be one example. A central tire inflation system is a standard feature on Soviet military trucks as well, and also used on some US trucks.

That the feature was removed on later versions of the An-22 should tell you something already.

Yes, a mass saving of a few dozen kg on a large transport aircraft is feasible, but you need to add pumps and plumbing. In order to clear civil certification standards, this system must be redundant, so you need to have most components duplicated. Also, the energy stored in the pressurized tire cannot easily be recuperated, but needs to be subtracted from the calculation of possible fuel savings because it must be mustered for re-inflation.

Without a more detailed investigation I would expect that the mass savings are minimal and do not outweigh the increased complexity and added failure modes.

$\endgroup$
  • 13
    $\begingroup$ I bet An-22 had those to adjust the tires for the runway at hand (paved - high pressure vs. unpaved - low pressure) rather than save on weight. $\endgroup$ – Agent_L Nov 6 '17 at 18:17
  • 6
    $\begingroup$ @Agent_L: Yes of course. That it was not done for weight saving is obvious from the fact that none of the Soviet airliners used this system. $\endgroup$ – Peter Kämpf Nov 6 '17 at 19:01
33
$\begingroup$

Along with the pump (and such) involved, it should be noted that inflating the tires of a large aircraft is a fairly non-trivial undertaking from a safety point of view.

In particular, when you're inflating a tire you normally put it into a tire cage, like this:

enter image description here

Photo Credit: Martins Industries.

That's not the only variety, but you get the idea. It's nearly certain that the cage you'd need around the tire to inflate it safely would, all by itself, be considerably heavier than the air inside the tire.

Just for what it's worth, there are only a few tire cages that can be used without removing the wheel and tire from the aircraft--and if I'm not mistaken, they tend to be even larger and heavier than the more common ones like I've linked above (though in fairness, not drastically larger/heavier).

$\endgroup$
  • 13
    $\begingroup$ I saw the cages and looked for videos of airplane tires exploding.This video makes it pretty clear why cages are necessary $\endgroup$ – March Ho Nov 7 '17 at 2:28
  • 14
    $\begingroup$ And the fact that one needs these cages implies that there's a decent chance that trying to inflate the tyres in flight will lead to you trying to land with fewer tyres than you took off with. $\endgroup$ – David Richerby Nov 7 '17 at 10:26
  • 3
    $\begingroup$ @DavidRicherby I'd be more worried about less fuselage. $\endgroup$ – Nick T Nov 8 '17 at 23:30
28
$\begingroup$

You simply wouldn't save any weight.

One of Goodyear's largest commercial aviation tyres is the 54x21.0–23 Flight Leader,* used on Airbus A330s and A340s. That is, outside diameter 54 in, width 21 in, rim diameter 23 in (137x53x48 cm). Assuming the tyre has rectangular cross-section, each tyre has a gross volume of about 0.7m3. This is an over-estimate of the air volume, since the cross-section is curved and the calculated volume includes the rubber.

These tyres are inflated to 235 psi, which is about 16 bar. The density of air at 1 bar is about 1.2 kg/m3, so the total mass of air in a tyre is about 16 kg. An A330 or A340 has ten tyres (I guess the nose wheel tyres are smaller but let's neglect that), so the total mass of air is at most 160 kg.

The maximum take-off weight of an A330/A340 is in the ballpark of 250,000 kg, so a 160kg weight saving is something in the region of 0.06%. Another way to see the scale of it is that 160 kg is basically two passengers with no checked luggage, or about 500 g per passenger (these planes carry about 300 people).

However, you would also need to install a multiply redundant high-pressure air compression and distribution system. A quick Google suggests that the sort of air compressor one might use in a workshop weighs 30–40 kg, and only does 8 bar (remember, we need 16). Probably most of that weight is the reservoir tank and I'm not sure how this compares to something that would be needed to inflate aircraft tyres. However, you're going to need something pretty beefy to pump seven cubic meters of air at sixteen bar in reasonable time, and even four of those ordinary workshop pumps would eradicate most or all of our possible weight savings.


* Other brands of aircraft tyre are available; I have no links with any manufacturer.

$\endgroup$
  • 1
    $\begingroup$ I'd wager that even the air hoses and fittings capable of 300 psi and moving a large amount of air (3/4" ID at least) would weigh multiples of the weight of the air "saved" by deflating the tires. Also, air compressors get very hot and use a lot of energy. Maybe some energy could be reclaimed from deflating the tires tho... $\endgroup$ – JPhi1618 Nov 7 '17 at 16:53
  • 1
    $\begingroup$ "You simply wouldn't save any weight." And then you compute that 160 kg is saved. There is compressed air on board already,from the engines. Those compressors you find have not been designed for airborne use and are produced with no regard to weight. $\endgroup$ – Koyovis Nov 7 '17 at 22:37
  • 3
    $\begingroup$ @Koyovis What about engine-out landings? And will the engines provide enough pressure? The RR Trent 700 (as used on the A330) has an overall pressure ratio of 36:1, which I guess means you can bleed off air at 36 bar, when you're close to sea level and at full power. But you're going to need to reinflate the tyres as you're coming in to land, so presumably at greatly reduced power. Is there going to be enough pressure available? $\endgroup$ – David Richerby Nov 7 '17 at 22:58
  • 2
    $\begingroup$ @Koyovis Even using engine bleed air, you would have to install extra plumbing from engines to the wheels, that would have to be redundant. That would add weight. In addition, the air would have to go through a fitting that allowed the wheel to rotate 360 without losing air. If a tire exploded in the air during inflation you would probably lose the entire airframe and everybody in it. Pieces of the tire could easily damage the wing sufficiently for it to fail in flight, leading to loss of the entire airframe. $\endgroup$ – Flynn Nov 8 '17 at 18:27
  • 1
    $\begingroup$ @Flynn The rotating inflation gadget already exists. And, while I agree with your objections, they're covered by the other answers. $\endgroup$ – David Richerby Nov 8 '17 at 18:46
18
$\begingroup$

Everyone here is missing one critical fact - airline tires are filled with Nitrogen. The friction of the tire on tarmac could raise the temperature enough for the Oxygen to react with the rubber, causing a fire. Nitrogen is mostly inert, making it much safer.

The entire question is irrelevant because you would be unable to inflate the tires again with only Nitrogen unless you carried that same volume aboard the aircraft in a second tank, reversing the weight savings.

Alternately, you could carry the lab equipment needed for fractional distillation, but I assume that would be even heavier.

$\endgroup$
  • 2
    $\begingroup$ You know that the fuel tanks are also filled with nitrogen enriched air, and that is indeed created onboard, don't you? $\endgroup$ – Peter Kämpf Nov 7 '17 at 20:20
  • 3
    $\begingroup$ @PeterKämpf as already mentioned in other comments on my answer, that system is not really comparable. 88% nitrogen is a bit far from the 99.X% in the tires $\endgroup$ – Federico Nov 7 '17 at 21:51
  • $\begingroup$ @Federico as it is now, yes. Obviously, the tyre inflation system would be re-designed to provide a higher percentage of nitrogen. $\endgroup$ – Koyovis Nov 9 '17 at 4:30
  • 1
    $\begingroup$ I read recently about a proposal to replace the gas in aeroplane tyres with helium to save weight. This would not require re-inflation in flight but would require better gas barrier layer inside tyres to prevent faster gas diffusion. $\endgroup$ – KalleMP Nov 9 '17 at 6:52
  • 1
    $\begingroup$ As Cody already said, "Alternately, you could carry the lab equipment needed for fractional distillation, but I assume that would be even heavier." Couldn't be more straightforward. $\endgroup$ – Fattie Nov 9 '17 at 18:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.