Knowing your top of descent is a really important factor for flying. What goes up, must come down and by knowing when to descend at a steady rate you can help ensure a smooth landing.
But working out how to do this can be tricky, and leaves many of us in a bath of cold sweat unsure how to proceed.
Well, no more! In this guide, we will look at what the top of descent means, the factors you should consider, how to use degrees of descent, and some quick and easy methods.
What The Top Of Descent Means
The top of descent should be the calculated or computed point that lies between the en-route or cruise phase and the descent phase of your flight.
In a jetliner, you can expect the top of descent to be calculated by the onboard flight management system.
This is largely to ensure the most economical descent to approach altitude while also taking into account the maximum rate and angle.
This can be a little trickier in a small aircraft that does not have such sophisticated technology yet the calculations can be done simply enough. By knowing when to begin your descent, you can help ensure a safe landing.
The Factors To Consider
When you do begin to calculate your top of descent there are a few details you need to factor in.
You should know where in the cockpit to look so you can quickly work it out before your altitude changes. These should include the following.
- How many vertical feet above mean sea level you are above the pattern altitude for your destination airport
- How much ground you are expected to cover per minute based on your current ground speed and
- Your typical rate of descent
To make the overall calculation easier, you should have made a note of your typical rate of descent and the pattern altitude for the destination airport.
Using Degrees Of Descent
The degrees of descent should be different depending on the speed your aircraft is traveling at which should vary wildly between a jetliner and a small aircraft.
For instance, a jetliner traveling at 250 knots should have a descent rate of 1250 feet per minute if it wants to maintain a relatively steady 3-degree path.
This is based on a quick calculation that multiplies your ground speed by 5 to get your descent rate.
Here 250 knots times 5 equals 1250 feet per minute. For a small aircraft traveling at 90 knots, the descent rate should be 450 feet per minute based on 90 times 5 equals 450.
The Quick And Easy Method
Using a descent rate of 500 feet per minute and knowing your altitude on arrival (based on your on-field elevation) and traffic pattern altitude, there is a quick and easy method to determine your top of descent altitude.
The Altitude Method
Once you have worked out your altitude above field elevation, you can simply multiply the number by 3.
This should give you an approximate distance translated into nautical miles from the airport to base your descent. The approximate calculation will also help you arrive at the airport’s traffic pattern altitude.
Your current altitude is 9,000 feet above mean sea level while at your destination airport the field elevation is 1,000 feet, also above mean sea level.
Taking away the field elevation from your current altitude leaves you with 8,000 feet, times by 3 makes 24 miles (based on 24,000 feet and removing the last three zeros to come up with the number of nautical miles).
This is the distance away from the airport from which you should begin your descent.
At a current altitude of 9,000 feet above sea level and a reduced field elevation of just 500 feet, the calculations become a little different but not by much.
The field elevation of 9,000 minus 500 equals 8,500, times that by 3 and you get 25,500 or 25.5 nautical miles.
That means you should begin a typical 500 feet per minute descent when you are 25.5 nautical miles out as the airport is closer to the sea level meaning you have a little further to travel.
Using The Common Standards
Let’s say you are currently flying at 120 knots so you will expect to cover 2 miles every minute. Begin by subtracting your pattern altitude from the cruise altitude then divide that number using the rate of descent.
This should tell you how many minutes from the field you have to start your descent.
Multiply the number of minutes by the number of miles you can expect to cover per minute which should tell you how far from the field is an ideal point to begin your descent.
At a cruise altitude of 10,000 feet above mean sea level and a pattern altitude of 1,000 feet, which is the typical height above the airport elevation, you only have 9,000 feet to lose.
That 9,000 feet, at a typical descent rate of 500 feet per minute gives you a full 18 minutes from the field.
The actual distance from the field can be calculated by multiplying the minutes from the field (18) by the number of miles per minute that you would cover based on your current ground speed.
For instance, at 120 knots you will cover 2 miles per minute so simple math tells us that 18 times 2 equals 36. Therefore, you should begin your descent 36 miles from the field.
The best, most experienced pilots should know just when they should descend based on how far away from the airport they are. Over time, this gets easier though the calculations you use should stay the same.
Primarily, the rate of your descent should depend on your ground speed, though the typical rate is around 500 feet per minute.
However, the faster your current speed, the faster your descent rate to maintain that crucial descent path.
Frequently Asked Questions
What Is The Descent Rate?
The descent rate is known as the rate of gain for vertical height per unit of time. This is typically given in feet per minute though it can also be meters per second.
What Is Considered The Maximum Rate Of Descent?
A general rule in aviation is known as the rule of three that 3 nautical miles of travel should be given for every 1,000 feet of descent.
This should give an effective glide descent rate from cruising altitude and helps with planning.