What is Nose Wheel?

Main cause of Boeing 737-200’s skid in Sultan Thaha (Jambi) Airport (27/8) is predicted by failure of nose wheel.

What is nose wheel?

Nose wheel is one of landing gear components.

Landing Gear :

The landing gear on the NG has been extensively redesigned. The nose gear is 3.5″ longer to relieve higher dynamic loads and the nose-wheelwell has been extended 3″ forward. The main gear is also longer to cater for the increased fuselage lengths of the -8/900 series and is constructed from a one piece titanium gear beam based on 757/767 designs. There is an externally mounted trunnion bearing on the gear, a re-located gas charging valve, and the uplock link is separate from the reaction link. It is fitted with 43.5″ tyres and digital antiskid.

Unfortunately the 737-700 was particularly prone to a dramatic shudder from the main landing gear if you tried to land smoothly. Fortunately Boeing started fitting shimmy dampers to this series from L/N 406 (Nov 1999) and a retrofit was made available.

One of the peculiarities of the 737 is that it invariably appears to crab when taxying. Theories for this include: A slightly castoring main gear to increase the crosswind capability; Play in the scissor link pins; Weather-cocking into any crosswind impinging on the fin; Torque reaction from the anti-collision light !!! Engineers will tell you that is due to the main gear having a couple of degrees of play due to the shimmy dampers.

Tyres

Tyres are tubeless and inflated with nitrogen. Pressures vary with series, maximum taxi weight, temperature and size of tyres. Unfortunately this large variation in tyre pressures makes it difficult to know your aquaplaning speed. The table below should prove helpful, notice how the aquaplaning speeds are all just below the typical landing speeds. Note: Once aquaplaning has started, it will continue to a much lower speed.

Series Main Gear Aquaplaning Speed Nose Gear Aquaplaning Speed
Originals 96 – 183psi 84 – 116Kts 125 – 145psi 96 – 104Kts
Classics 185 – 217psi 118 – 128Kts 163 – 194psi 111 – 121Kts
NG’s 117 – 205psi 93 – 123Kts 123 – 208psi 95 – 124Kts

Another oddity of the 737 is the resonant vibration during taxying that occurs at approx 17kts in classics and 24kts in NG’s. This is due to tyre “cold set”. This is a temporary flat spot that occurs in tyres with nylon chord (ie all Boeing tyres) when hot tyres are parked and they cool to ambient temperature. Hence the reason why the flat spot is most pronounced in cold weather and tends to disappear during taxying as the tyres warm up again.

The speed rating of all tyres is 225mph (195kts).

Gear Seals

Notice that none of the 737 series have ever had full main gear doors. Instead the outer wall of the tyres meet with aerodynamic seals in the wheel well to make a smooth surface along the underside of the aircraft. The first few 737’s had inflatable seals which were inflated by bleed air when the gear was either up or down and deflated during transit. The landing gear panel had a NOT SEALED caption which would illuminate during transit (normal), if it illuminated at any other time you could have a puncture and the seal could be depressurised with the GEAR SEAL SHUTOFF switch to save bleed requirements.

These were soon dropped as being too complicated and a similar drag and noise advantage was achieved with the present fixed rubber seals.

Brakes

The brakes are steel, actually an alloy called “Cerametalix(R) friction brake material” with versions made by either Goodrich or Honeywell. From 2008 the 737 NG will be available with carbon Duracarb(R) brakes which will be supplied by Goodrich and Messier-Bugatti. These will be 300kgs lighter and last twice as long.

The brake pressure gauge merely shows the pressure of the air side of the accumulator and should normally indicate 3000psi. The normal brake system and autobrakes are powered by hydraulic system B. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes which are manual only (ie no autobrake) and the brake pressure returns to 3000psi. Antiskid is available with alternate brakes, but not touchdown or locked wheel protection on series before the NG’s.

If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.

If the brake pressure gauge ever shows zero, this merely indicates that the pre-charge has leaked out, normal and alternate braking are unaffected if you still have the hydraulic systems (see QRH). The accumulator also provides pressure for the parking brake.

Note that on the 737-1/200, hydraulic system A operates the inboard brakes and system B operates the outboard brakes. Both brake pressures are indicated on the single hydraulic brake pressure gauge.

Brake Pressure Indication (psi) Condition
3000 Normal.
3000 No hydraulics, minimum 6 applications of brakes available with accumulator.
1000 No hydraulics, accumulator used up.
Zero No pre-charge, normal braking available with hydraulics.

Brake Accumulator

Brake Wear Pin

Autobrakes

Autobrake Selector

Max Pressure at Brakes (PSI)

Deceleration Rate (ft/sec²)

1

1250

4

2

1500

5

3

2000

7.2

Max

3000

12 (below 80kts)

14 (above 80kts)

RTO

Full

Not Controlled

There is an “on ramp” period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.

Notice from the table above that autobrake Max does not give full brake pressure. For absolute maximum braking on landing, select autobrake Max to assure immediate application after touch down then override with full toe brake pressure.

Using high autobrake settings with idle reverse is particularly hard on the brakes as they will be working for the given deceleration rate without the assistance of full reverse thrust.

To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi (ie less than that required for autobrake 1). This is more difficult on the NG’s because the feedback springs on the brake pedals are stiffer. Autobrake can also be cancelled by putting the speedbrake lever down or by switching the autobrake off. I would advise against the latter in case you accidentally select RTO and get the full 3000psi of braking!

Occasionally you may see the brakes (rather than the cabin crew!) smoking during a turnaround. This may be due to hard braking at high landing weights. But the most common reason is that too much grease is put on the axle at wheel change so that when the wheel is pushed on, the grease is deposited inside the torque tube; when this gets hot, it smokes. It could also be contamination from hydraulic fluid either from bleeding operation or a leak either from the brakes or another source.

(source : http://www.b737.org.uk)

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One response to “What is Nose Wheel?

  1. Pingback: Sriwijaya Air Tergelincir di Bandara Jambi « RizkiBeo’s Transport Reflections

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