Copyright 1999, Glenn Carlson, All Rights Reserved.
No reprints without permission form the Author.
I am sure there is not one instrument instructor out there
who does not teach the instrument student the importance
of checking his or her altitude crossing the final approach
fix against the published crossing altitude on the approach plate.
"Yes Captain Carlson I always do that ! I draw pictures
and tell them about the possibility of false glide slopes and
that if we are getting an on glideslope indication and the
altitude does not match up when crossing the outer marker
then we should go around and shoot the approach again"
Then I say…..and?
The resultant look is priceless.
Ladies and Gentlemen there is a better way to check glide
path quality way before the outer marker so you don't have
to go around. As a mater of fact, there are several ways.
The best thing about this lesson is that it can be taught to
primary students as well and help with their descent planning,
although for them a slight modification is in order. Back to
you instrument jocks.
For the most part, ILS glide slopes are 3 degrees. It is
important to use rules of thumb in all aspects of flying and
this is no exception. A 3 degree glide slope means that you
are descending at a rate of 3 for one, meaning that for
every foot traveled down, you have advanced 3 feetJ
For example, I am on approach to LAX. I am on the glide
slope and at an altitude of 10,000 feet above the ground ( that's
right I said above the ground not sea level). I am 30 nautical
miles from the end of the runway. Get it? No? Ok, now I am
5000 feet above the ground and still on glide slope. How many
miles from the airport threshold am I. If you said 15 NM then
you are getting it. 5 times three is 15, THREE FOR ONE! Ok,
here is the tricky one. I am on a three mile final, how high
above the ground should I be if I am on glide slope? 1000 feet
agl is correctJ
The math has to be using altitude above ground level but you in
SJC don't have to deal with it like someone learning in say Denver.
In the bay area MSL and AGL are so close the rule will work just
fine reading straight off of the altimeter.
Now, lets say you are being vectored for an ILS approach that has
DME or you have a vortac on the field and DME in your airplane.
Today you had to dive for the glide slope because ATC couldn't get
you down earlier because of traffic. You catch the glide slope( you
think you have anyway) and in you come. Assume terrain is flat and
MSL is the same as AGL; you are 21 DME from the field, on glide
slope and the altimeter says you are passing through 7000 feet. Are
you on a true glide slope or a false one? You are in great shape 7
times 3 = 21 DME. YEAH! You have just checked glide path quality
some 16 miles from the outer marker. "We don't need no stinking
outer marker" not with DME.
Provided you have not deviated from the glide path all the way
down when you do check your altitude over the marker it will be
perfect. Now in the previous example you are turned in on a 15
mile final, you are indicating on glide path and your altimeter
says 6500. Well how is your day going so far? Answer, bad ,real
bad! You had better do something fast or when you cross the
marker you will be way too high to finish the approach. So you
increase your rate of descent and guess what the glide slope
says you are right on even though your rate of descent is better
than 1000 FPM. That's how false glide slopes react. In this
case, shoot for 9 miles out at 3000 ft. or 12 miles out at 4000 ft.
to get back on the THREE for ONE glide path.
While you are being vectored for the ILS, ATC will give you
safe altitudes to fly. How you descend is none of his concern.
You might want to stay high for a while to stay out of wake
turbulance for instance, but keep in mind the THREE for
ONE RATIO. Somewhere between your position and the
outer marker you have to get on glide slope ….the correct
glide slope.
Let's say you don't have DME or there is no vortac on the
fieldL "Wow, Captain Carlson, what a bummer!"
Not really, there is another favorite one of mine to help check
glide path quality and it too is simple math. If you are on a 3
degree glide path and your ground speed is 120kts then your
rate of descent to stay on the glide slope should be 600 feet per
minute down. The math is simple half of 120 is 60….add another
zero and you get 600. How about a ground speed of 80 knots?
I you said 400 FPM you have got it. So you are screaming
straight in at 100 kts indicated. The wind is right down the
runway at 10 kts. What is the rate of descent expected to be
to stay on glide path? Please say 450 fpm:-\ 100 minus
headwind of 10=90kts ground speed devided by 2=45, add
the zero=450fpm. Under these conditions if you are descending
at say 800 fpm , you either have a hell of a tailwind or you are
on a false glide slope.
As far as your primary students are concerned, parallel the ILS
in from the practice area and help them learn distance from the
airport by descending on glide slope and learning how far out
from the runway they are. Have fun and stay tuned for part two.
Link to Author's web site with information on some of his Books.