[jhb] Re: Pireps
- From: "bones" <bones@xxxxxxx>
- To: <jhb@xxxxxxxxxxxxx>
- Date: Sat, 8 Sep 2007 05:51:41 +0100
Wind and speed - two of the most important factors on the approach and
landing.
A 10kt headwind can reduce landing run by 20% or more and because this sort
of wind is about normal for the UK most pilots get used to the short landing
it produces. A zero wind landing can still come as a shock for the length of
runway it uses up, the apparent lack of deceleration after touchdown and the
sloppy control reactions.
Speed is more critical. I've already mentioned this in respect to Gerry's
farm strips but it applies to all aircraft regardless of size. Excess speed
on the approach is going to result in excessive landing distance. Speed is
mostly relative to weight, which is why airline pilots calculate the
approach speed for EVERY approach. It is also relative to other factors like
flaps, temperature and pressure so a complex graph is used to determine the
right speed (or the FMS on posher aircraft).
For light aircraft the published approach speed is usually a single value
and based on the maximum weight the aircraft is authorised for. Technically
this means you are safe at any weight (you won't stall if heavy) but the
lighter the aircraft is from MAUW the lower the stalling speed and the more
you will float on flaring the aircraft.
A PA28 181 has a flaps down stall of 49kts. Approach speed is 1.3 x Stall so
this is 63.7kts (usually rounded up to 65kts). This book figure is based on
the MAUW of 2550lb. With just one person on the aircraft and 10 gal fuel the
weight is down to 1800lb - a good 750lb lighter - and this drops the stall
speed by 7.8kts to 41.2kts. At this weight an approach could be flown at
55kts safely. The important point is that if the approach is flown at 55kts
at 1800lb it results in the same landing run as flying at 65kts at 2550lb -
because the lift generated on the approach is the same in both cases.
If you don't adjust for weight and fly at 65kts on the approach at 1800lbs
then you are too fast. In flaring the aircraft it has to decelerate from
65kts down to 41kts before the wing no longer generates lift to fly and that
eats up a huge chunk of runway.
It may be hard to apply this learning in FS because you need to know the
MAUW and the flap down stall speed of the aircraft you are flying and this
isn't always available. If it is then, as a rough guide, for every 100lbs
you are below MAUW subtract 1kt from the stall speed. Your approach should
be made at 1.3 x Stall.
Airliners are far more complex and the approach speed can vary hugely with
weight. I doubt many FS pilots use anything other than a nominal book figure
for approach speed but it would never work in real life. Excess speed in an
airliner on landing has far more serious implications.
Even on big jets the same basic rule applies - approach speed is determined
as 1.25 x stall. If you are used to banging in the same fuel amount on a jet
each time you fly you can see from the above that a short flight of 30min is
going to need a faster approach speed (little fuel used so heavier aircraft)
than taking the aircraft across the pond on a 7 hour trip. Which leads to a
nice little exercise in FS for you..
Take your favourite jet and prepare it for flight as you might normally do.
If you never fiddle with fuel amounts so much the better. Before you start
this trip write down the normal speed you use for the approach. Now get
airborne, fly a circuit round to a 10nm final on the ILS and settle on the
approach in normal landing configuration at your normal approach speed.
Set the Alt Hold to 500ft so the aircraft levels off at this height. DON'T
touch anything - keep the gear and flaps in approach settings. Now, slowly
start reducing the speed on AT so that the aircraft slowly slows down - it
has to be slow for a good reading. Forget that you have overflown the
airport as that was only to get you settled into correct landing
configuration with flaps and gear down. As you bring the speed back watch
for one of two things - either the stall warning or the aircraft starting to
lose height. One or the other will happen first - it's a mark of how well
the perf file has been written. Make a note of the speed at which this
occurs.
This exercise should show (roughly) the stall speed of the aircraft at your
normal loaded fuel weight. OK you may have spend 20 min doing this so you
could say it would represent a short flight. If you multiply the observed
speed by 1.25 this should be your approach speed AT THAT WEIGHT - and it
would be interesting to see if it is anywhere near the speed you normally
use.
I could ask you to repeat the exercise except this time you take the
aircraft for a pretty long run so a lot of fuel is burnt off. It may be
easier just to dump fuel using the menu. Regardless, if you repeated the
exercise but with a great deal less fuel on board (and maybe an emptier
cabin) then when you come to reduce throttle to get the aircraft to stall
you may be very surprised to find that the figure is substantially less than
when you flew with a lot more fuel on board.
For 90% of our flying we don't do any of this in FS and that is where the
division starts to break down between FS and real world flying. FS is good
for showing us system management of aircraft but it's pretty much goldfish
when it comes to teaching airmanship. Very few FS pilots learn to appreciate
the relationship between weight, speed, angle of attack and approach
handling.
One thing I may have misread from Mike's post but you should never vary
approach speed with wind strength. The only time you EVER do this is in
gusty conditions when you should add 50% of the difference between Max and
Min wind speeds as given by ATC.
bones
-----Original Message-----
From: jhb-bounce@xxxxxxxxxxxxx [mailto:jhb-bounce@xxxxxxxxxxxxx] On Behalf
Of Mike Lucas
Sent: 07 September 2007 12:31
To: jhb@xxxxxxxxxxxxx
Subject: [jhb] Re: Pireps
Something I discovered landing a F100 at EGNS last night. Bones had
given me "Winds calm"; I had the approach set up perfectly, usual speed,
full flaps ... but it just felt wrong coming over the threshold. I have
never experienced a medium jet going "floaty" on me, but that's the only
way I can describe it. Fortunately I had enough margin to plonk it down
after using rather more of the runway than usual, and I stopped before
the end - but only just; I had to backtrack for Taxiway B. I
could/should have knocked 10 kts off my final approach speed.
Mike L
bones wrote:
> Wind is your friend - with a 10kt headwind your
> landing run is greatly reduced but pilots get used to this. Landing
> with no wind eats up more runway but this is exceptional in the UK and
> so pilots don't realise what the normal landing performance is - they
> become accustomed to windy conditions and it affects their perception
> of landing distances.
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