The Descent

“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 – Campbell River. I check my CFS and determine airport elevation is 346’. Circuit altitude is 1300’ and I would like to be at circuit altitude at least 2 nm from the aerodrome. I need to lose 5200’ (6500 - 1300 = 5200). I determine I will need to initiate my descent procedure 11 minutes prior to arrival (5.2 times 2 = 10.4). I round up just to keep things simple. If I am descending at 100 knots I will need to initiate descent approximately 20 nm from the aerodrome (11 x 100/60 = 18.3 + 2 = 20) (3).

As I cross the line between CapeLazlo and Harwood Island, I can reduce power approximately 500 RPM or 5” MP, set up my descent profile at 500’/min, get everything all trimmed in and allow the machine to descend to my intended altitude of 1300’ in good time to enter the pattern at Campbell River. No muss, no fuss.

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.