Some landing gear questions

[MHN_pilot]

Hi guys!
I have some technical questions about landing gears which I haven't been able to answer with my AGK notes::bored:

- Once I was told that in old a/c the procedure before landing gear retaction was to manually push the brakes in order to stop wheels before entering the l/g bay and causing some damage. Is that correct? This procedure is performed automatically in modern a/c such as 320 or 737 NG I suppose, am I right?

-How many wow (weght on wheels) do you guys have in medium or big jetliners? as I have studied in general aviation a/c there's one in one of the main l/g struts or sometimes two. I suppose in bigger aircraft there must be always one in each main strut in order to deploy asimetrical spoilers in croswind landings and stuff like that and also one in the nose gear to activate autobrake when the nose comes down. Am I right? could you guys confirm it with your fcom?

- Does it exist any device in high cruising aircraft which reduces wheel pressure in order to adjust with decrasing ambient pressure at high altitudes?

- What is the landing gear lever middle " OFF" postion for in a/c such as b 737?

-About landing gear pins used to prevent landing gear from retracting accidentaly on ground: are they used only in longer stops or in shorter ones as well? are they used in all size aircraft or only lighter ones? (I've only seen them in dash and some other regional aircraft)


Thanks in advance, :ok:

Happy landigs.;)

[MarkerInbound]

I don't know what damage it would cause, DC-3s have snubber strips in the wheel well to keep the tire from turning, we never tapped the brakes.

The 727 only has an air/ground sensor on the left main and nose gear. The nose gear works with the stearing, everything else is from the left main. 747 has tilt sensors in each truck plus the nose gear air/ground. Once you're on the ground and deploy ground spoilers, everything is going to come up, you're not worried about the crosswind.

Why, you'd just have to air the tire up again.

Pins are used when they're used. Whatever the operator's manual says. 747s have gear pins. The mains on a 747 or 727 won't retract on the ground even if you move the handle to "UP", they'd have to lift the aircraft. I know the nose gear on a 727 will retract, not sure about the 747.

"OFF" just removes pressure from the system. The 747-8 won't have an "OFF" position.

[SNS3Guppy]

Tapping the brakes after takeoff was a common practice in some aircraft (both light, and large), largely to reduce vibration from the wheels spinning in the gear wells. It also reduced gyroscopic forces on the wheel assembly and gear during retraction.

Some aircraft do not permit brake application after takeoff, and many aircraft use snubbers in the gear system which apply brake pressure during retraction to stop the wheels from turning. Dumping brake pressure once airborne helps prevent inadvertant application of brakes during the takeoff or during the landing, and prevents landing with brakes applied.

[MHN_pilot]

Thanks both! :ok:

[rooaaiast]

For the inward retracting mains the autobraking is necessary when gear retracting for gyro forces, but for forward retracting nose, the braking is not technically necessary
However if there is a tyre imbalance you get a lot of yo-yoing in the cockpit. The snubbers strips (A320) slow the nosewheels quickly, but for some reason if there is a bit of a tyre imbalance, there is usually one last-gasp vibration before the wheel stops. Dont know why.

[sb_sfo]

"The mains on a 747 or 727 won't retract on the ground even if you move the handle to "UP", they'd have to lift the aircraft."

The body gear on a 747 will, as UA proved in SFO a couple years ago. Put the aircraft on its tail...

[karl414ac]

I was always told to press the brakes before retracting becauseof centrifugal forces and gyroscopic effects on the wheel will make it harder for the hydraulics to lift the gear if the wheel is still spinning at a fair rpm

Karl414AC

[lunav]

hi-
Question #1:
Brakes are automatically applyed to Main Gear wheels during Gear retraction (B737)
Question #4:
the OFF position remove Hydraulic press in the lines.
STBY to receive Aswers for Q2-3
Regards,

[Intruder]

747 has tilt sensors in each truck plus the nose gear air/ground. Once you're on the ground and deploy ground spoilers, everything is going to come up, you're not worried about the crosswind.
Maybe you're not worried, but as a 747 Captain, I am worried!

There is still significant control authority available after ground spoiler popup in the 747, including ALL the aileron and rudder travel. Heavy crosswinds CAN pick up a wing after landing, so the handling pilot MUST fly the airplane until below 80 KIAS.

[MarkerInbound]

Ok, bad choice of words. How about, after the main landing gear touch down and with all four hydraulic systems powered and the Speedbrakes ARMED, all twelve spoiler panels will be driven to their full travel position.

The spoilers do not compensate for crosswinds in the ground spoiler mode, that is left to the flight crew with aerilons and rudder.

[18-Wheeler]

The body gear on a 747 will, as UA proved in SFO a couple years ago. Put the aircraft on its tail...


Another 747 freighter in Dubai about six years ago as well. I managed to visit inside the main deck and the engineers had the remains of the body gear hydraulic rams on the floor. Very nasty.

[Dodo56]

I'll just answer the points I know about, as I suspect some others may vary from aircraft to aircraft.

1. No, you do not reduce tyre pressure. There is no need to do so as the difference between seal level and cruise is only about 10 psi and you can get bigger differences in pressure operating between cold and hot airports. There is no mechanism to adjust tyre pressure in flight, up or down. Set the pressure cold before departure, don't bleed off if hot. No further adjustment.

2. Many aircraft have snubber strips in the NLG bay. More than anything else this is to stop the nosewheels rattling the pilots' fillings. They sit directly on top of the gear and any vibration or shimmy is felt by them. On some aircraft types nosewheel vibration is a major cause of wheel rejections even though the tyres are not worn to limits. Mainwheel vibration is not felt by the crew so I doubt there is a pressing need to brake the main wheels, and as Guppy indicated you don't want to risk landing with brakes applied. Depends on your ops manual I guess.

[mustafagander]

MHN,

On the light twins I flew it was "positive rate, toe brakes, gear up" as far as I recall. The Boeings with which I am familiar have a low pressure applied to the brakes when the gear up line is pressurised. Stomping on the brakes just after lift off will likely damage things - think wheels stopped from spinning at 170kt tyre speed to stop in a millisecond. How much stress on the truck snubbers!!!

B747s have "tilt switches" on the main gears. They're actually "untilt switches" and 2 untilts means ground mode. There are also WOW sensors on the NLG for a few systems.

Most jets have an "over centre" lock on their gear, but generally it is not very robust so never move the aircraft unless the gear system hydraulics are powered or the pins are in. There is often a lever latch to prevent the gear lever being moved accidentally.

Boeing like to have gear hydraulics depressurised in flight hence the "OFF" position for the gear selector. This removes pressure from the gear system and allows the gears on most Boeing jets to hang on the up locks provided for the purpose. It makes sense to remove 3000psi whenever possible. If it's not pressurised, it can't leak.

[Volume]

Does it exist any device in high cruising aircraft which reduces wheel pressure in order to adjust with decrasing ambient pressure at high altitudes?
No, and it is not necessary. Tire pressure is not like for cars, it is much higher, 200psi / 15bar to give a rough estimate. Therefore the additional .7 bar / 10 PSI due to the atmospheric pressure change is not relevant (< 5%). The pressure change due to heat from the brakes after landing is worse.

[leewan]

For most modern a/cs, the brakes will be automatically applied once the gear up lever is selected to stop the wheels from spinning, As the nose wheels don't have brakes, the scrubber at the roof of l/g bay takes care of that.

As for the tyre pressures, no such thing. But there is a device to release all pressure if the wheel is too hot.

And for the landing gear pins, they are put on all a/c with a long ground time( nightstop), for maintenance reasons( wheel change) or in a freighter if an unexpectedly heavy load is expected. For the B737, it is put on in the NLG even during a pushback as a precaution. Most a/cs nose gear including a B747 will retract on ground if the nose gear lever is put in the up position. It has happened a couple of times.

[fastcruise]

Most of your questions have been answered however wheels spin is due to friction and reaction due to weight on the wheels (3rd law Newton). So the rotation stops due to lack of friction between the wheel and the airflow and no weight on wheels. Only a few seconds later than a light tap or auto application would suffice rather than a full blooded application reducing the brake disc life. ldg gear selector is selected off for depressurising up lines and in order to maintain fluid levels. In most aircraft it happens automatically when a complete retraction is sensed. I think a WOW switch in both main gears is for integrity of operation in case of a failure of a sensor.

[muduckace]

I believe most modern aircraft do not apply full pressure/duration the the mlg brakes while gear up, it is usually applied during the retraction sequence (simulating a single manual application).

[ampclamp]

many modern rpt jets have at least one wow per gear, some 2 sensors.
I am not licensed on any aircraft with assymetric spoilers for landing.
Once sensed, up the spoilers go.

mains are generally braked during the retract cycle, nose wheels have kevlar snubbers in wheel well.

gears pins used during many maint procedures mostly when towing.Procedures vary between companies and ports.Always used where stated in the manuals plus where company policy dictates.

Our company does not use the nlg lock pin during pushbacks just the steering bypass pin.
Dont know of any that do here in OZ.
Does not mean some companies do but I reckon your are asking for a NLG failed to retract upon selecting UP report!!:O

Forget the tyre pressure thing.

Off means simply that hyd pressure off the retract actuators via electric or mech means.Gears just hang in the uplocks.

[M2dude]

Concorde had a single Ford automotive disc brake on the nose wheell, nose gear up hydraulic pressure applied the brake.

[lynn789]

Ive heard they tyres are inflated with nitrogen as it expands less than air if brake heat gets to the wheels when gear is retracted, there were exploding tyres when air was used

is there a blow out plug in the wheel in case things still get to hot to save tyre from exploding?
also Ive seen 747s climbing with gear down, apparently to cool brakes from passing air

[18-Wheeler]

Ive heard they tyres are inflated with nitrogen as it expands less than air if brake heat gets to the wheels when gear is retracted, there were exploding tyres when air was used

is there a blow out plug in the wheel in case things still get to hot to save tyre from exploding?
also Ive seen 747s climbing with gear down, apparently to cool brakes from passing air

Yes aeroplanes use dry nitrogen to inflate the tyres as when it gets hot its expansion rate is predictable and be allowed for, plain air from the atmosphere has varying amounts of moisture and may expand at a higher rate.
Yes, on all larger aeroplanes there is a plug made from the metal lead in the wheel, and it's made to melt at a certain temperature to let excessive pressure out.
And it's pretty rare to see a 747 climb-out with the gear still down as you would normally not takeoff until the brakes are cool enough, though I guess if you were doing short multiple sectors (I've done four in one day) or circuits then to help cooling you may elect to leave the wheels out for a minute or so.

[Exup]

Nitrogen also used due to inert properties in the event of wheel fire & lack of moisture which can cause internal corrosion of wheel hubs. The wheels on a 747 are left out on T/O if a brake has been deactivated. There are two methods of brake deactivation (747 classic) one uses a lock out tool which maintains inflight braking (approx 400ps) so gear can be retracted normally, the other method is for the hydraulics to be toatally blanked of removing inflight braking & the mel gives a figure for gear to remain down to allow the wheel to stop rotating before entering the gear bay (think its about 2 mins). If this method is used climb out is affected so there can be weight penalities incurred. No T/O should be attempted until the brakes are in the correct temperature range.

[18-Wheeler]

I forgot about the brake lock-out, thanks - I haven't flown them for a couple of years now ...

[galaxy flyer]

Exup


On the C-5, there were "spin down" sensors, so until spindown occurred, the gear would not begin retraction. Is that true on the 747?

GF

[18-Wheeler]

No, no such device fitted.
They have a system that applies a touch of brake pressure to the main gear and the nose gear has a rubbing strip when the strut is in the retracted position.

[galaxy flyer]

We had a braking system that applied 150 psi to the brakes but if it failed, retraction waited for spindown signals. 2 minutes was the time for spindown, if operating with anti-skid inop.

GF

[NSEU]

Ok, bad choice of words. How about, after the main landing gear touch down and with all four hydraulic systems powered and the Speedbrakes ARMED, all twelve spoiler panels will be driven to their full travel position.

The spoilers do not compensate for crosswinds in the ground spoiler mode, that is left to the flight crew with aerilons and rudder.

So you're saying that none of the spoilers will go down if the control wheel is turned on a 747? (747-400?)

From the Boeing 747-400 Maintenance Manual (27-61-00 Item F (7))

"Do the test of the spoiler control system with the speed brake control lever in the UP position:

(a) Put the speed brake lever in the UP position
1) Make sure that the spoilers No.1 thru 12 are in the fully up position (45 degrees)

(b) Turn the aileron control wheel fully clockwise
1) Make sure that the spoilers No. 1 thru 5 are in the fully down position
2) Make sure that spoilers No.6 thru 12 are in the fully up position (45 degrees)...."

Spoilers 6 & 7 are not used for lateral control at any time, hence they are included in the up group of spoilers.

Rgds.
NSEU

[NSEU]

Boeing like to have gear hydraulics depressurised in flight hence the "OFF" position for the gear selector. This removes pressure from the gear system and allows the gears on most Boeing jets to hang on the up locks provided for the purpose. It makes sense to remove 3000psi whenever possible. If it's not pressurised, it can't leak.

True. However, there is at least one 747-400 airline which flies with the gear lever in the UP position. Apparently, at this airline, repeat rigging problems has seen a number of gears drop onto (closed) doors with the lever in the OFF position.

I guess, rather than train their maintenance personnel properly, they chose to modify the operational procedures (The expensive of maintaining hydraulic systems under constant pressure is apparently less than repairing damaged doors?)

Rgds.
NSEU

[mustafagander]

Another reason for nitrogen inflation of wheels is that the fine rubber dust inside the tyre is very flammable and has, in fact, been blamed for a few serious wheel explosions.