19
$\begingroup$

When an empty airliner is loaded with passengers, cargo and fuel, the extra weight compresses the gear strut and it sinks. Jet bridges are designed with this in mind.

How much height does it actually sink?

For the purpose of the question, I am interested in any typical commercial airliner models.

$\endgroup$
23
$\begingroup$

The value will obviously vary for different aircraft models (and also with the loading, c.g. location etc), but the usual range can be found in the airport planning document for the particular airliner, where the values for ground clearance for the aircraft at the OEW (max. value) and MTW (min. value) would be given. The difference should give you the compression.

For example, for Boeing 737-800, the quoted value is around 15 cm (plus or minus 7cm).

737 Ground clearance

Image from 737 Airplane characteristics for Airport Planning document

Note that the different parts of the aircraft 'sink' different amounts- the fuselage sinks around 15cm, while the engines (mounted on the wings) sink more- by 16cm due to the fuel in the wings.

Another 'typical' aircraft, the A320neo sinks by about 10cm.

A320 Ground clearance

Image from A320 Aircraft characteristics for Airport Planning document

Note that the values are given here for max. ramp weight.

$\endgroup$
  • 1
    $\begingroup$ the engines (mounted on the wings) sink more- by 0.16m due to the fuel in the wings. not only because of the fuel, but mainly because of the cantilever, concentrated load, and deceleration. an empty wing would still "flap" and sink more than the fuselage. $\endgroup$ – Federico Aug 8 '16 at 8:52
  • $\begingroup$ For example, for Boeing 737-800, the quoted value is around 15 cm (plus or minus 7cm). That's a heck of a margin! $\endgroup$ – Jules Aug 8 '16 at 15:16
4
$\begingroup$

The answer provided by @aeroalias is good. I'd add that in addition to loading, it also depends on the nitrogen pressure in the gear struts as well as ambient temperature. My background was military; there we might adjust strut pressures based on mission loadings (fuel, ammo, external stores). While I wouldn't expect airlines to adjust pressures between flights, I would expect them to set them based on typical fuel load and temps for the routes a given airframe is projected to fly.

While the goal of all of this nitrogen pressure adjustment is to maintain a range of strut extension heights considered safe by the manufacturer, the fact is that, when the aircraft is loaded near the upper or lower bounds of the range the nitrogen pressure is set for, or when an aircraft transitions between temperature extremes, you can still get some interesting strut conditions, either low or high.

I'm going to qualify everything I just said with the caveat that newer aircraft might be shipping with autoleveling systems -- someone else might know more about that.

$\endgroup$
  • 3
    $\begingroup$ Quite a few military aircraft can actively control the nitrogen pressure to shorten the landing gear leg for retraction (the Saab 35 Draken would be one fine example), so they can even adjust the pressure during a mission. $\endgroup$ – Peter Kämpf Aug 8 '16 at 8:29
  • $\begingroup$ I was wondering if there was a mechanism to adjust the pressure dynamically. Do you know if this feature is on any civil commercial aircraft? $\endgroup$ – JustWannaFly Aug 8 '16 at 12:45

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.