3
$\begingroup$

Several older airliners (e.g. Airbus A300, Boeing 737 until the NG series, Boeing 747 until the -400 series) have landing gear levers with three positions: UP, OFF and DOWN. OFF (or NEUTRAL for Airbus) means the hydraulic pressure is removed from the system because the gear is held in place mechanically. See also Why does the landing gear control have 3 positions (up/down/off) on Boeing 737-800?

The gear lever is typically set to the OFF position after the gear is retracted during the takeoff and the system therefore remains unpressurized during the flight:

Pilot Monitoring After flap retraction is complete:

[...]

  • Set the landing gear lever to OFF after landing gear retraction is complete.

(Boeing 737 NG FCOMv1 NP.21.46 - Normal Procedures - Amplified Procedures)

The gear is then lowered by selecting DOWN before landing, but it is not selected back to OFF after the landing gear is extended. I can understand why this would not be done during the landing phase (redundancy by keeping the gear pressurized down, reduced crew workload during highly critical phase of flight), but why is the lever not selected back to to OFF after landing?

Every procedure I can find lists the gear lever as DOWN on the ground:

Landing Gear Lever:

  • Check the L/G lever DOWN and the EMRG retract pb protected.

(Airbus A300 FCOM - Cockpit Safety Inspection)

LANDING GEAR lever .....................................................................DN
Verify that the green landing gear indicator lights are illuminated.
Verify that the red landing gear indicator lights are extinguished.

(Boeing 737 NG FCOMv1 SP.6.1 - Supplementary Procedures - Electrical Power Up)

Hydraulic pressure is not required to keep the gear down:

Landing Gear Extension

When the LANDING GEAR lever is moved to DN, hydraulic system A pressure is used to release the uplocks. The landing gear extends by hydraulic pressure, gravity and air loads. Overcenter mechanical and hydraulic locks hold the gear at full extension. The nose wheel doors remain open when the gear is down.

(Boeing 737 NG FCOMv2 14.20.2 - Landing Gear - System Description, emphasis mine)

And furthermore, hydraulic pressure is lost anyway once the engines (and electrically powered hydraulic pumps) are switched off on the ground. So why would you want to keep the gear pressurized down on the ground until shutdown and again during startup?

$\endgroup$
4
  • $\begingroup$ Because it has to move through UP to get there $\endgroup$ Aug 24 '20 at 12:25
  • $\begingroup$ @ratchetfreak Why? The OFF or NEUTRAL position is in the center. At least on the A300, it is possible to move from DOWN to NEUTRAL: "On ground, L/G lever can be moved from DOWN to Neutral position and vice versa, but interlocks prevent an inadvertent UP selection." (from here). Is there something in the Boeings that would prevent selecting OFF from DOWN? $\endgroup$
    – Bianfable
    Aug 24 '20 at 12:28
  • $\begingroup$ @ratchetfreak No, after takeoff you can stop at Off on your way to Up -- the detent is there. It's pointless, but possible. $\endgroup$
    – Ralph J
    Aug 24 '20 at 12:34
  • $\begingroup$ The NG 737s have the Off position; the Max's don't. $\endgroup$
    – Ralph J
    Aug 24 '20 at 12:35
4
$\begingroup$

Because there's no advantage to doing so (since the pumps will soon be off anyway, as the question notes), and this would create the risk that the pilot may overshoot the Off position to the Up position, which could, on a bad day, lead to an inadvertent gear retraction and major damage. The over-center locks are good, but do you really want to bet big bucks that the combination of 3,000 psi + a bad bump in the pavement will never produce an illustration of Murphy's Law?

Okay, there is one other safety to prevent overshooting, but that solenoid can fail, and if it does, watch out! There is simply zero need for this.

Also, depending on the plumbing, you might remove hydraulic pressure from the nosewheel steering by doing this (though I'd have to check the diagram to be sure).

$\endgroup$
1
  • 1
    $\begingroup$ OK, I may have over-estimated the advantages of disconnecting the hydraulics. "you might remove hydraulic pressure from the nosewheel steering" -> that would be really good reason not do disconnect on ground. My hydraulics diagram in the FCOM indeed connects nose wheel steering via the landing gear, but it does not say if it is actually shut off when the lever is in OFF. $\endgroup$
    – Bianfable
    Aug 24 '20 at 12:46
3
$\begingroup$

Hydraulics to the landing gear are shut off in flight for a couple reasons.

  1. Reduction of internal leakage of the hydraulic system. Internal leakage increases pump power extraction from the engines very slightly increasing fuel burn. Also internal leakage consumes available hydraulic power that can not be used for other tasks. Available hydraulic power is already reduced at cruise and descent due to lower engine speeds (max flow from engine pumps is linearly proportional to engine speed).
  2. Fault isolation. If a component starts to leak in flight, or there is a tire burst that damages hydraulic lines, it can't drain the entire hydraulic system over a multiple hour flight.

These concerns just aren't very meaningful on the ground. A small leak is unlikely to deplete you hydraulic system during the limited ground time, and a large leak you want to know about while you are still on the ground. Fuel burn over 20 minutes of ramp time from internal leakage is going to be completely negligible, and, outside of speed brakes, there are no other large consumers of hydraulic power on the ground.

While you taxi, bumps can place an unlocking force onto the gear (this is why some aircraft have bungees to add locking force), so having a hydraulics on as a back up isn't a bad thing either, just in case. While parked at the gate all loads are basically straight down, no drag or lateral loads impacting the gear.

The answer by @Ralph J is also true. Why run the risk of accidentally raising the gear on the ground? Why add to the pilot workload during a high workload time? Brakes and steering are also generally disabled when the handle is in the "off" position as well.

That being said, some electronically controlled landing gear do switch their extension/retraction actuators to neutral once down and locked automatically.

$\endgroup$

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.