If you’ve done much flying into busier airports—and even
some not so busy but with defined arrival and departure procedures—you know all
about the dreaded “Slam Dunk.” The Slam
Dunk happens when you are held much higher close in to the airport than you’d
like to be, and then are finally given a descent to the altitude you wanted,
only now with an airport right underneath for a visual approach, or with the
localizer right in front of you and the glide slope underneath you. Why do they have to make this so hard?
The most common assumption is that it is for noise
abatement: trying to appease all those people on the ground who knew they were
buying a house next to a busy airport and then complained about all the
noise. So we get The Slam Dunk.
Noise abatement is a factor, but it isn’t a big one. The real reason is air traffic control. Let’s take a look at that.
Separating arrivals and departures at airports that aren’t
very busy isn’t hard. There seldom are
more than one or two aircraft in the local area and they are coming and going
in different directions, so separation can be as simple as assigning vectors to
keep them apart, maybe holding one at one altitude while the other climbs or
descends through it, or a speed adjustment—something simple that doesn’t affect
the descent profile substantially. A lot
of that is done at busy airports too, to get everyone in line and sequenced
properly. But it isn’t enough. Standard departure and arrival patterns have
to be created to organize and simply the controllers duties, and one of the
most important component of those procedures is to hold departures down to one
altitude until they have gotten some distance from the airport, and to keep
arrivals above that altitude until they’ve gotten to the point where they are
past the traffic departing. Once the
departure is out past the arrival, he is allowed to climb and the arrival
(finally) is allowed to descend. Otherwise you’d have traffic climbing and
descending right into each other. In order
to not hold the departure down any longer than necessary, they keep the arrivals
above the departures until that point where the arrival has just enough
airspace to get on down. That’s The Slam
Dunk.
Now you could say, “’Just enough’ according to whom?”
According to Airways Standards personnel who don’t have to actually fly them in
real world conditions? According to standards for turbine powered aircraft,
disregarding the concerns of recip pilots who don’t want to shock cool their
engines? Both good points, but the
situation isn’t so bad if: 1, You know The Slam Dunk is coming and you don’t
try to fight it but are ready for it; and 2, You know you have tools available
and are ready to use them, all of them, as necessary. So let’s go over that.
The first point is the easy one. There isn’t any point in requesting lower
over and over in this situation. Unless
you have an emergency, or are arriving late at night with only a few freight
dogs around, it just isn’t going to happen.
All you’re doing is wasting time that could be spent getting ready for
the descent when it does come. So that
leaves the tools. Let’ talk about engine
shock cooling. Mike Busch, who writes a
maintenance column for the EAA magazine “Sport Flying”, and who runs a business
managing maintenance for GA pilots, someone I respect very much, says there is
no hard evidence that rapid engine cooling damages engines, but plenty of
evidence that hot cylinder head temps does.
Of course, it’s still better to manage engine temps and not subject them
to rapid changes, but pulling an engine that has been coasting along at 50%
power or so back to idle is probably not going to do a lot of harm. So if that’s what it takes, that’s one tool.
But remember, you don’t have to come all the way back to
idle every time. A recip at idle creates
a lot of drag. At idle the propeller has
changed from pulling the aircraft to turning the engine, pumping a lot of air
and overcoming a lot of internal drag, while a turbine at idle is still
generating thrust. (For turbine aircraft, their ace in the hole is speed
brakes, and not being able to generate drag with the engines is why they have
them.) You can duplicate that by
bringing power back to whatever power setting equates to zero thrust. (Listen
for it—the sound changes as the engine starts to “back pedal”.) That’s a good
place to start and won’t shock cool the engine as much as coming all the way
back to idle does.
The other tools are pretty obvious: gear and flaps. Gear should probably go out first—it creates
a lot of drag and has to go down sooner or later—flaps next. Flaps don’t add much drag until you get down
to landing flaps, but you can start with the first notch of flaps once the
speed is below the flap extended limit.
So now you have power at zero thrust, gear down, approach flaps. In many cases that will be enough. If not, reduce the power to idle and when
slow enough go to full flaps. If you
know The Slam Dunk is coming and you start aggressively configuring drag the
second you get your descent clearance, it should work out just about right,
with a level off just before localizer intercept and underneath the glide
slope. Acting immediately and
aggressively, throwing everything at it if you have to—power to idle, full
flaps—is the key.
Once you understand that Slam Dunks are not some evil joke
that controllers like to pull on pilots just because they can, but are an
inevitable part of air traffic control at airports with more than local
traffic, you’re well on your way to dealing with them. And it feels pretty good when you know what to
do, are ready for it, and it all works out, sitting on the porch with the big
dogs, and looking pretty good doing it.
