Math Pages

Estimating ETA

Fun Stuff

Get Down

What goes up must come down.

Planning the descent portion of an IFR flight is one of the most challenging tasks for beginners. Experienced pilots sometimes are challenged by it too. This article defines a standard descent and how to establish it. Once a pilot knows how to plan a standard descent it is much easier to plan and execute variations from this standard.

The Standard Descent

The standard descent in aviation is along a three degree descent path. ILS approaches usually are at a three degree angle, and a three degree angle is a good starting point for enroute descents and for non-precision approaches as well. So, we will start by learning to plan a standard (3°) descent.

Descents are measured as an angle from the horizon, as shown to the left. (The diagram shown is not to scale.)

When planning a standard descent the we need to know when to descend and how quickly - i.e. at what vertical speed. Each of these should be figured out separately. We plan when to descend separately from how quickly for two reasons:

When to descend is quite universally applicable to all aircraft. Therefore, what we learn will carry on throughout our flying career.
Even though how quickly to descent varies from airplane to airplane, it doesn't change from day to day. So, we can just remember our standard descent rate and avoid doing math.

How Much Vertical Speed?

Let us deal with the question of what vertical speed we need for a standard descent first.

The diagram to the left shows that vertical speed and TAS must remain in a constant ratio equal to the Sine of 3 degrees. It is important to note that this ratio will be the same for all airplanes.

The equation, shown at the bottom of the diagram, evaluates to:

Vertical speed = 5.3 x TAS. Most pilots just use 5 x TAS and call that good enough for rough figuring.

It is important to note that the formula just developed will only be accurate in zero wind. Therefore, we should really modify our formula to:

Vertical speed = 5 x groundspeed

The formula above is a standard in use is the aviation industry. Get to know it and be proficient in using it. For example, if flying an ILS approach at 100 knots the required vertical speed is 500 fpm. If cruising at 140 knots a standard descent requires 5 x 140 = 700 fpm descent rate.

When Should we Descend?

If you are cruising in your jet at 40,000 feet and need to descend to an arrival fix, to be there at 10,000 feet, how far back should you start down?

The above question is the very sort of thing we must do on every IFR flight. We don't want this to be a big chore, so we will just do a standard descent. That way whatever the answer is today, it will be the same tomorrow, and the next day. Standardization of procedure is one of the best ways to reduce workload and improve safety in flying. (Once we standardize the procedure and find an easy way to solve the standard problem we then have enough mental energy left over to consider whether we need to make a considered exception to the standard procedures.)

The diagram to the left shows that on a standard descent we lose 1000 feet for every 3Nm we go forward.

It is important to realize that this ratio, 1000 feet for 3Nm is for ALL airplanes. A Cessna or a Lear jet both come down at this same ratio, if they are following the standard descent.

Therefore, no matter what type of airplane we are flying the answer to the question above is the same. If we are descending from 40,000 feet to 10,000 feet that is 30,000 feet. At 3 Nm per thousand feet we need 3 x 30 = 90 miles for our descent.

If you intercept the ILS glidepath at 4000 feet above ground level, how far from the airport are you? Even without a DME we know we must be 4 x 3 = 12 miles from the airport.

On an NDB approach, when you are 100 feet above the beacon crossing altitude, how far back from the beacon do you need to be to cross the beacon in a continuous descent? The diagram above shows that 100 feet equals .3 Nm.

Summary

Every pilot should memorize the facts that:

1000 feet = 3Nm
100 feet = .3Nm
Standard vertical speed = 5 x groundspeed

We will use the standard descent formulae above over and over when we do IFR approaches and plan descent from cruise.