102

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 ...


86

(wikimedia.org) High pressure water removal. Runway rubber removal [requires] high pressure water, abrasives, chemicals and/or other mechanical means. ICAO Annex 14 notes: The surface of a paved runway shall be maintained in a condition so as to provide good friction characteristics and low rolling resistance. Snow, slush, ice, standing water, mud, ...


84

Aircraft land on the main wheels. For aircraft with nose wheel it is the back ones, but for aircraft with tail wheel (also called “tail-draggers”) it is the front ones. In either case the main wheels are very close to centre of gravity and carry most of the aircraft's weight, the nose or tail wheel only carries a small fraction of it. The aircraft must land ...


81

Summary The order of magnitude of brake energy after a rejected takeoff (RTO) for a very heavy aircraft is 1 GJ, or 100 MJ per wheel brake. Aircraft brakes are cooled with ambient air, either through passive airflow or (optionally) through forced air ventilation. Cutaway drawings of A380 wheel and brake (Source) Energy Dissipation Takeoff speed for ...


69

It is a sensing wheel that is part of the antiskid braking system. It contains an electronic sensor to measure the true groundspeed of the airplane. See NASA Technical Note D-4836, Landing Loads and Accelerations of the XB-70-1 Airplane (1968), bottom of page 3 of the document (page 5 on the PDF):


63

This has been researched over the years, but it seems that the solutions did not find wide application. The oldest I could find is from 1941 (see page 112 in the September 1941 issue of Popular Science), and there have been several attempts to implement a spin-up turbine. See “Wheels with wings” on NewScientist Blogs or “Spin Wheels Before Landing”, a ...


55

Why does the 747 not have its gear down yet? Based on the image it is hard to say how far these aircraft are from the runway. The 747 could still be be quite far from the runway and thus does not need to have its gear down yet. The 747-400 FCOM (NP.21.47 Normal Procedures) says: At glideslope alive, call: "GEAR DOWN" "FLAPS 20" It looks ...


54

Short answer: Yes. Not one, but several. A taildragger configuration for modern airliners comes with several disadvantages: Visibility during taxiing is much worse. Visibility while taxiing is a safety-relevant issue (Picture source) Braking hard will result in a headstand. And with today's landing speeds, braking hard is needed if you want to fly from ...


53

Hot brakes are normally cooled by natural convection, but in extreme cases the fire fighters will cool overheated brakes. Some designs use brake fans. From the linked page: Brake fans reduce brake cooling times by using wheel mounted electric fans to blow ambient air across the brake and wheel assemblies. Note that the maximum recommended temperature ...


51

There is a weight sensor which senses if the plane is on the ground. This sensor prevents gear retraction while the plane is still on the ground. Failure of this sensor would prevent gear retraction after takeoff. If you note closely, the landing gears (even the non-retractable ones) are not connected using a simple metal pole; rather, there is an oleo ...


49

The light is a tail-strike indicator. Touched Runway system The Touched Runway system provides a warning light in the flight compartment. It illuminates when the aft fuselage makes hard contact with the runway on take off or landing. The system consists of a frangible1 switch located at the bottom of the aft fuselage, activating the ...


48

The Tu-144 uses much smaller wheels than comparably-sized subsonic aircraft. This makes it possible to rotate the gear bogie sideways towards the middle of the aircraft, so it can be stowed vertically inside the wheel well. Your picture already hints at that: Note that the gear strut is offset inside the wheel well, so it would leave more space on one side ...


47

The Tu-144 landing gear rotates 90 degrees when retracted up between the engines. Source: Tu-144 Landing Gear Retraction Source: Tu-144 Landing Gear Retraction


45

Because the tires are momentarily skidding on the pavement as they rapidly spin up from standstill to the jet’s touchdown speed. It’s just a puff of burnt rubber but the tire tread itself becomes quite warm, reaching 600-700°F for an instant. It’s also the reason a pilot does not ride the brakes upon touchdown as this could cause a blowout as the tire is ...


42

No They do not continue this practice anymore. This is likely because landing gear are more reliable now than they were in the 70's. There is also the practice of Air Traffic Control prompting military pilots to check their gear is down, by verbally adding "Check Gear Down" to their landing clearance. Source: Checked in with some of the Air Force / Air ...


41

Yes, they do. They are called Anti-Skid and they go from Mark I of 1948, with simply an on-off switch triggered by wheel lock, till Mark V (or at least that's the last I've seen), with quite complex control systems and sensors behind. The full details are a bit long to include them all here (there are entire book sections about them). For a brief overview, ...


40

Just to roll around on "unimproved" airfields (grass/dirt/gravel strips), the biggest thing planes need is simply a decent set of tires: This MiG-29UN trainer shows off the fairly beefy main tires put on these planes specifically to allow them to more easily roll over minor imperfections in the runway/taxiway surface, including dirt/gravel/grass strips (...


39

There is one aircraft I know of that can do exactly what you state, the B-52: This plane has steerable dual-bicycle gear which allows the crew to point the gear along the runway while the fuselage is "crabbed" up to 20* off the runway centerline. AFAIK the gear are slaved to the ILS; you dial in the OBI to a course and the gear will point that way even if ...


37

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 ...


35

The simple fact is that an unintended gear deployment could be catastrophic, as it has a profound impact on the flight dynamics of the aircraft (one of the reasons there are warnings for both retraction and extension). The benefits would be marginal as checking gear down is part of the bread-and-butter procedures for all retractable gear aircraft. Keep in ...


34

With the gear handle in the UP position, the retraction side of the landing gear actuators remain pressurized. The gear will stay in the full up position, however they will not be hanging on the mechanical locks designed to hold them in the up position. They've simply hit the full up mechanical stop. When the handle is placed in the OFF position, hydraulic ...


34

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: Photo Credit: Martins Industries. That's not the only variety, but you get the idea. It's ...


33

As you mentioned, drag is one of the reasons why retractable landing gears are used in the first place. But in order to use it, there are way more considerations than just drag. Scale: Size of the aircraft plays a big role here. Big aircraft have more room to keep the retracted landing gear. This is an issue as planes get smaller. Weight: Retractable ...


32

When UP or DOWN is selected, the hydraulics pressurize in the appropriate direction. Once the gear is up and the mechanical locks are keeping the gear locked in place, OFF can be selected to disconnect the hydraulic power.


32

The FAA covers this in the airplane flying manual quite nicely: Bouncing During Touchdown When the airplane contacts the ground with a sharp impact as the result of an improper attitude or an excessive rate of sink, it tends to bounce back into the air. Though the airplane’s tires and shock struts provide some springing action, the airplane does not ...


31

Shape-memory alloys change their shape with temperature, with control usually achieved with electric heating in one direction, and back with convective heat transfer to the environment. There is one specific temperature where the change in shape occurs. There are two huge problems with this: The operating temperature range for gear actuators is huge. It may ...


30

The biggest single reason for the decline of flying boats was the proliferation of long runways during World War II. The infrastructure advantage of flying boats – the ability to operate heavy aircraft without long runways – was no longer relevant. Large airfields were a result of the long-range heavy bomber campaigns in Europe and Asia. Century of Flight ...


30

Jan Hudec already gave an excellent description of why the gear configurations are designed as they are. Another important thing to note, though, is that because aircraft are designed to land on their main gear, the other gear (nose gear in tricycle configurations or tail wheel in tail draggers) is not designed to be able to take the force of a landing. ...


30

On older (WWII-era) large planes, the crew had access to a manual crank to lower the gear if the automatic extension/retraction system (usually a linear actuator powered electrically) failed. I don't know of any that had a gravity-based manual system; I'm sure a few bomber crews would have appreciated something like that after having to crank the handle to ...


30

It has been tried before. Piper's PA-28R and PA-32R aircraft were equipped with an automatic landing gear extension system which would automatically extend the landing gear below 85kts regardless of the position of the selector switch in the cockpit. At higher airspeeds this system was overridden by a separate pneumatic system which deferred control to the ...


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