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The Descent
Alexander Burton,
CFI
Pacific Rim
Aviation Academy Inc.
Pitt
Meadows, BC V3Y 2B4
“Here is Edward Bear, coming
downstairs now, bump, bump, bump on the back of his head, behind Christopher
Robin. It is, as far as he knows, the only way of coming downstairs, but
sometimes he feels that there really is another way, If only he could stop
bumping for a moment and think of it.”
-A. A. Milne- Each and
every flight in an aeroplane ends with a descent and re-acquaintance with
the surface of our planet in one way or another. Since descending is an
inevitable part of flight it is in our interest as pilots to execute the
manoeuvre in as skilful and purposeful manner as we can manage. As Albert
Einstein said,
“Things should be made as
simple as possible, but not any simpler.”
While descending, a skill learned very early in the flight training process,
seems simple and straightforward, there are a number of techniques that can
be incorporated into our skill-set and a base of knowledge that can lead to
improvement in our over-all skills as a pilot. Executing a descent at the
appropriate moment and in a skilful manner will improve both our enjoyment
of flight and increase our safety. We make
use of the descent during several phases of flight. While enroute on a cross
country flight, we must at some point, execute a descent either to set up
our approach for landing, to decrease altitude for weather or to achieve a
specified cruising altitude. We use the descent on approach and, either for
practice or in the event of an engine failure we descend with no engine
power to assist us. There are
several, generic factors affecting the way an aircraft descends and it is
useful to understand them so we can maintain precise control of the process.
As with climb, the weight of the aircraft, the location of its centre of
gravity, density altitude and humidity, use of carburetor heat, deployment
of flaps and landing gear, turbulence and the pilot’s accuracy and skill in
maintaining correct angle of attack and airspeed all affect an aircraft’s
descent. An
aircraft climbs proportionally to excess thrust; it descends proportionally
to deficit thrust, the difference between thrust required to maintain level
flight and available thrust. It both climbs and descends in inverse
proportion to weight. Last month
we discussed climb performance and used the formula Rate of Climb is equal
to Excess Thrust Horsepower (ETHP) divided by weight, R/C = ETHP x
33,000/Weight, to determine our rate of climb at any given airspeed (1). A
descent can be thought of as a negative rate of climb so, using essentially
the same formula, an aircraft’s rate of descent
(R/D), how quickly it
loses altitude expressed in feet per minute (fpm), can be derived from the
formula: R/D = Deficit Thrust Horse Power (DTHP) x 33,000/Weight (2). Weight is
an interesting factor in relation to descent. At any given angle of
climb—nose up attitude—a portion of the weight vector, which acts directly
between the centre of gravity of the aeroplane and the centre of gravity of
the planet, acts as drag and must be overcome by thrust. In a descent, a
portion of the weight vector acts as thrust and assists us in our progress
downward in direct proportion to our angle of descent. The steeper our angle
of descent, the more weight provides assistance in increasing the rate of
descent. The
lighter the machine is loaded, the less assistance is derived from weight at
any given angle of descent, resulting in a lower rate of descent. Of course,
at a 90○ angle of descent—an exciting prospect—all of the weight
of the aircraft would be acting as though it were thrust. In a C-172 at
gross weight, with the engine turned off, we would have the equivalent of
2300 lbs thrust acting straight down. We would discover the thrill of
considerable vertical acceleration. Centre of
gravity location affects descent in the sense that an aircraft with a more
forward centre of gravity is, effectively, a heavier aircraft resulting from
the increase in down-force developed by the tail-plane which acts on the
aircraft as though it were weight. Centre of gravity is not something we
normally have much control over during flight unless, perhaps, we have a
load of skydivers, or, as my old buddy Duke Elegant used to say, a load of
lobsters we can toss out the door, so we won’t get too excited about it
right now. We will simply remember that an aircraft with a more forward
centre of gravity will descend at a slightly higher rate with a given power
setting than the same aircraft with a more aft centre of gravity. Density
altitude and humidity affect rate of descent by increasing or decreasing
drag. A high density altitude and high humidity environment—thinner
air—results in reduced drag which, in turn, decreases the deficit thrust
produced at any given power setting. Once we are airborne, of course, there
is not much we can do about the density altitude or humidity factors expect
understand how they affect performance. Deployment
of flap and landing gear increases drag. Increasing drag increases the
amount of deficit thrust at any given power setting which increases our rate
of descent. If getting down at a higher rate of descent is a goal: get those
flaps down and deploy the gear. We can also, of course, also reduce power,
increasing the amount of deficit thrust. Putting the machine into a slip
also assists in increasing drag and thus increasing rate of descent. Use of
carburetor heat decreases power output from the engine reducing available
thrust and increasing our rate of descent.
Turbulence
and pilot skill also affect rate of descent. This is particularly apparent
in a glide as angle of attack is very important. If we want to achieve
minimum sink rate or maximum distance rate in a glide, maintaining the
required airspeed and angle of attack is critical. Increasing or decreasing
airspeed, angle of attack, will reduce our glide performance either in terms
of time or distance. When
flying in turbulence, typically we choose to increase speed somewhat to
ensure positive control of the machine which, unfortunately, also changes
performance. Increased airspeed increases our rate of descent in a glide or
increases the amount of required power to maintain a given rate of descent.
With the aircraft bouncing all about the sky, it may be a bit more
challenging to maintain constant airspeed and angle of attack than when
operating in still air. But what
about my flight tomorrow? For flying
light aircraft there are a few easy Rules of Thumb that can be a big help in
setting up a desired descent profile. We know that attitude plus power gives
us performance. The key factors we want to control in setting up the descent
are those two factors: power and attitude. In level
flight at cruise power, we remember that a change of 100 RPM or 1” MP
results in a change in airspeed of approximately 5 knots. We know that the
same change in RPM or MP, if we maintain the same airspeed, will result in a
climb or descent at approximately 100’/min. So far so good. If I am
flying at 100 knots and would like to establish a 300’/min rate of descent,
I can reduce power by 300 RPM or 3” MP, adjust my attitude as required to
maintain the 100 knots, and the job is done. Some “fine tuning” may be
required in the power setting but the Rule of Thumb works pretty well for
most, small aircraft. If I want
to change airspeed and set up a rate of descent, for example from that 100
knots in cruise I would like to descend at 90 knots and 300’/min, I can
reduce power 200 RPM or 2” MP for the airspeed change and an additional 300
RPM or 3” MP for the rate of descent: a total power reduction of 500 RPM or
5” MP. Once again, some “fine tuning” may be required. Or, I can
simply leave the power setting as is, poke the nose down, increasing
airspeed to, say, 115 knots, and descend at 300’/min. I’ve already paid for
my excess altitude. Why not get some of that cost back through an increase
in airspeed? One of the
questions students always seem to struggle with is, “How do I know when to
start my descent for approach?” For light aircraft there are a couple of
easy Rules of Thumb that can see us through this dilemma. For larger
aircraft, typically people use some form of the 3/6 Rule:
3
times the altitude (in thousands of feet) you have to lose is the distance
back to start the descent; 6 times your groundspeed is your descent rate. If
I need to lose 5000’ I would begin my descent 15 miles back (3 x 5 = 15); my
descent rate at a ground speed of 100 knots would be 600’/min (100 x 6 =
600). This works well, but at higher speeds starts to give somewhat exciting
rates of descent. With light aircraft,
500’/min is a comfortable and
fairly efficient rate of descent. Much faster and our ears start to pop and
our passengers begin to get edgy. Much slower and it seems like forever to
reduce altitude. A 500’/min
rate of descent means two minutes to descend 1000’. If I am approaching my
destination aerodrome at, say, 6500’ and the circuit height is 1500’, I will
need to lose 5000’. I take the number of thousands, in this case 5, and
multiply that number by 2 (5 x 2 = 10). This gives me the number of minutes
back from the circuit I will need to begin my descent (10 minutes). To convert
time to approximate distance we can simply multiply the time by our airspeed
divided by 60. For example, if I am flying at 90 knots and have determined I
need to initiate my descent 10 minutes prior to arrival at a selected point,
I can multiply 10 by 90/60, or, more simply I can drop the extra zeros and
multiply 10 by 9 and divide the result by 6, giving 15 miles. This is not
exactly precise; my airspeed is not necessarily the same as my ground speed
due to both wind factors and slant angle, but unless there is significant
wind, the answer will do pretty nicely as a working solution. So, here
we are flying at 6500’ enroute Pitt Meadows – As I cross
the line between If your
brain doesn’t appreciate math during flight—pretty normal for most
people—make a plan prior to flight. No need to make this difficult. Planning
ahead is always a good idea. It saves all sorts of confusion later. If you
already know what you will need to do, all you have to do in the aircraft is
execute the plan. Keep
things simple. Plan and think ahead. Enjoy.
Notes: 1. Kershner, William K., The Advanced Pilot’s Flight Manual, Iowa State University Press, 1994, page 90. One horsepower equals 550 ft lb/sec or 33,000 foot pounds/min. 2. ibid, pg. 34. If you find these things interesting, a Rate of Descent of 500’/min at constant airspeed will require a reduction in power of approximately 35 hp (500 = 33000 x DTHP/33000; DTHP = 34.85 hp). 3. Using the 3/6 Rule: 5.2 x 3 = 15.6, call it 16nm, + 2 = 17 nm back from destination. 6 x 100 knots = 600’/min for rate of descent.
PRINCIPAL AIR |
