Re: Question: Vertical speed on landing?

Date:         21 Mar 97 02:43:25 
From:         RydellSCB <RydellSCB@prodigy.net>
Organization: Prodigy Internet
References:   1 2 3 4 5
Next article
View raw article
  or MIME structure

Andre Neves wrote:
> All this seems strange to me as I believe that 500 fpm landings
> are quite frequent specially in bad weather or with some "less skilled"
> pilots and aircraft hold just fine (somebody told me an A340 could handle
> 2000 fpm prior to collapse! that's 33fps).
> 10 fps is only 600 fpm, it's not enormous!
> The only explanation I can think of is that these figures grow fast with a
> lighter than MLW ladden aircraft.

Perhaps what you've heard about is the descent rate prior to landing.
Airplanes come in for a landing at a descent rate which can be much higher
than the actual landing rate, and during the flare that energy is bled
off so the airplane can land at a reasonable rate.

I find it very hard to believe that an Airbus (or any airplane) could land
at 33 fps without fusing out the gear.  I could believe a 30 fps descent
rate prior to the flare, though.

Here's why I question the 33 fps:

The kinetic energy is proportional to the square of the velocity
(E=(0.5)mv^2), so the jump from 10 fps (the limit load requirement per
the FAR's) to 33 fps is a 1000% increase in energy.  Not possible for a
jet transport.  The energy is only linearly proportional to the weight,
so although you're right that the touchdown speed can increase with a
decrease in airplane weight, the energy release at touchdown is much more
a function of the rate than the weight.  For example, for a 410K MLW 777
with a limit landing speed of 10fps, a 300 K 777 could land at 11.7 fps
and impart the same energy into the shock strut.  From this example, a
27% reduction in landing weight only results in a 17% increase in touchdown
speed.  (I can't remember the actual MZW of the 777, so the 300K figure
is made up...)

Steve Baier