Rate of descent at touchdown

[sasha601]

Dear Pilots,

I am a MS Flight Simulator user, and my favorite plane is Boeing 737. The most enjoyable moment is when I can touchdown at near 0 fpm. This rate of decscent is ideal, but I would guess not realistic to repeat all the time. What is a good rate of descent at touchdown? Could you answer this by filling in blanks below?:

Very good -

Good -

Marginal (yet acceptable) -


Thank you very much for your help.

[Mad (Flt) Scientist]

Actually, a 0fpm descent at touchdown isn't ideal from an aircraft performance view; while the big wigs in the back of a corporate jet might like it, since it won't spill their drinks, they won't like the runway overrun that might result if you have to float along half the runway to achieve the soft landing.

Nominal performance data assumes a fairly decent descent rate, to minimise distance in the air. On-ground handling requires a decent impact too - especially in gusty or high crosswind conditions, perhaps, where the phrase used is sometimes to "assertively" place the aircraft on the runway. You're not trying to emulate a carrier landing, but you want the time near but not on the ground to be minimized, as a sudden gust could roll one wing into the ground, ruining your day.

Incidentally, gear are usually stressed for descent rates of the order of 10-12 fps - say 750fpm. So that's pretty much a defined upper limit; much higher and it could be a "crash you walked away from", not a landing ;)

[Milt]

MS

Could you be more specific on the required design criteria for main landing gears at AUW?

I seem to remember a figure of 9 feet per second for land planes and 12 for those to operate on to a carrier.

Is drop testing still employed or has computation taken over entirely?

Then we operators start to ask questions such as - "What is the definition of a heavy landing?" and -- "What do the engineers look for following a heavy landing report?" and - "Should the oleos bottom out at the maximum descent rate?"

[738Capt]

As long as your at 50ft at threshold, flare the aircraft, which reduces rate of decent to 100ft/per min. On a 3 degree glide slope, about 900ft per min is standard to threshold (50ft).

(I fly 737-800 on X-plane 8.20)

[mutt]

As long as your at 50ft at threshold

If you are at 50 ft crossing the threshold, what indications would you expect to see on the VASI's and ILS?


Mutt

[Halfbaked_Boy]

Under normal circumstances below 300' AGL the VASIs will begin to 'red-out'. Can't comment on ILS.

Cheers, Jack.

[tomcs]

The PAPIs are calibrated to give 2 white 2 red if you are at 50' over the threshold. The ILS is calibrated exactly the same, so that you touch down within the touchdown zone markers. Obviously not designed for light aircraft....unless you want to land in the touchdown zone markers of course!

As for the rate of descent, well its definitely not 0 fpm, and not 750 fpm

Our GPWS callouts are at 10' intervals from 50'-10' and they deafen you at around 1 per 1.5 seconds, and then a slight flare probably reduces it to between 200-100 fpm. I'll have a look next time.

tomcs

[Mad (Flt) Scientist]

Could you be more specific on the required design criteria for main landing gears at AUW?

I seem to remember a figure of 9 feet per second for land planes and 12 for those to operate on to a carrier.

Is drop testing still employed or has computation taken over entirely?

Then we operators start to ask questions such as - "What is the definition of a heavy landing?" and -- "What do the engineers look for following a heavy landing report?" and - "Should the oleos bottom out at the maximum descent rate?"

aargh. I was trying to stay away from "heavy landing" precision because it's a can of worms. I'll look into it and ask some questions tomorrow, hopefully.

(We were looking at a "heavy landing" indicator at one point, as I believe some of the competition have one, but there's a big philosophical issue as to whether it should be a primary system - in which case you'd have to check it and if it said "heavy" you have to do something - or a backup, perhaps only looked at AFTER a heavy landing report by the crew. The former would increase the amount of maintenance but catch more 'damaged' gears; the latter would eliminate uneccessary maintenance following a conservative crew report but would leave some bad gear on if crew didn't report the landing in the first place. No operator's going to willing to pay for an add-on that increases the MX after all......)