Descent Planning: Strategies for Safe and Smooth Arrivals
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Descent planning is a critical yet often overlooked aspect of managing your flight. And if not planned properly, a poorly executed descent can present challenges and unnecessary risks when transitioning to an approach or the traffic pattern. Pilots must consider a variety of factors including terrain, weather, airspace constraints, and aircraft performance, to ensure a smooth and safe descent.
A well-executed descent plan not only enhances safety but also improves passenger comfort and fuel efficiency. For example, descending too late can lead to steep approaches, excessive speed, strain on the aircraft, or even discomfort for the passengers. Conversely, descending too early can waste fuel and time and may result in a bumpy ride.
Let’s explore a number of strategies to plan your next arrival.
Use Your Existing GPS Navigator
Most modern GPS navigators include a vertical calculation and/or navigation function that will allow you to plan your descent with precision. As an example, the Garmin GTN series offers a vertical calculation (VCALC) function that includes the ability to set a desired rate of descent to calculate the appropriate point to begin your descent based on your aircraft’s current groundspeed.
You can further choose to arrive at a waypoint at a predetermined altitude or set a mileage offset to create a buffer or to allow for space to enter the traffic pattern.
To use the VCALC function, simply enter a target altitude (pattern altitude for example) and be sure the altitude type matches what you have intended (MSL vs. AGL). Then select a comfortable vertical descent rate (VS Profile), any mileage offset (perhaps three-four miles before the target altitude if you are entering the traffic pattern) and the target waypoint (arrival airport).
Finally, you can enable messages to alert you as to when to begin the descent. Consult the flight manual supplement or pilot’s guide for your GPS navigator for additional details on utilizing the vertical navigation functions.
Flight Planning Apps
Make use of the powererful features of your favorite flight planning app to make your descent planning easier. In ForeFlight, for example, the Basic Navlog includes waypoints for the top of climb (TOC) and top of descent (TOD). These waypoints are calculated automatically using the latest weather forecasts and selected aircraft performance profile.
You can view your TOD within on the Flights tab in the Navlog. In our example flight from the Clermont County Airport (I69) the Ohio State Airport (OSU) in a Cessna 182 Skylane at 7,500’, you can see that our TOD has been calculated at 35nm (13 minutes) from our destination airport.
You can also view your TOD from the flight plan profile view on the Maps tab. Long press in the profile view and then move the vertical slider to the left until it intersects with your cruise altitude of 7,500’. As you can see, this point is 45nm from our departure point which is 35nm from our destination as was calculated in the Navlog.
To quickly insert your TOD into your flight plan, go to the Edit view, select your destination waypoint, OSU, then Along-Track Offset. Enter 35 and the TOD waypoint of 35nm from our destination is now entered in your flight plan.
The Rule of Three
The “rule of three” involves calculating the altitude to lose in thousands of feet and multiplying it by three to determine the distance from the destination to begin your descent, essentially creating a 3-degree descent angle.
- Determine altitude to lose:
7,500’ (cruise altitude) – 900’ (OSU airport elevation) = 6,600’
- Apply the “rule of three”:
Multiply the altitude to lose (in thousands of feet, 6,600/1000 (6.6)) by three to find the distance from the destination to start your descent.
6.6 x 3 = 19.8nm
- Determine the descent rate
As a rule of thumb, to determine the appropriate descent rate, divide your groundspeed (the anticipated groundspeed in your desent) by two and then multiply by 10.
In our example:
167kts / 2 = 83.5 x 10 = 835fpm
Use a Standard Descent Rate
Many pilots utilize a standard descent rate that is comfortable for their aircraft and passengers. Using our same example flight from I69 to OSU and a 500fpm descent rate:
- Determine altitude to lose:
7,500’ (cruise altitude) – 900’ (OSU airport elevation) = 6,600’
- Determine descent point in minutes from destination:
Divide the altitude to lose by the desired descent rate to calculate how many minutes from your destination you will need to initiate the descent.
6,600’ / 500fpm = 13.2 minutes from the destination.
- Convert to mileage from destination if desired:
To convert 13.2 minutes into mileage from the airport, you will need to determine how many miles per minute you are traveling by dividing your groundspeed by 60.
167kts / 60 = 2.8 miles per minute
Traveling at 2.8 miles per minute, you will need to initiate a descent 37nm from the airport (2.8 mpm x 13.2 minutes).
Other factors to consider
Terrain and obstacle considerations, airspace constraints, and ATC limitations can all influence your descent planning. Passenger comfort can also come into play. If you are dealing with sensitive ears, you may wish to opt for a shallower descent rate; however, in the warmer months, the lower altitudes may be more turbulent and you may wish to delay your descent in favor of smoother and cooler air.
If you are approaching a non-towered airport, you may wish to reach pattern altitude several miles from the airport to position yourself onto an appropriate entry leg and get appropriately configured.
As you can see, there are multiple methods for determining an appropriate descent profile. The most important takeaway is to have a plan and stay ahead of the aircraft.
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My descent planning:
Example: Flying 2.5 nm/minute GS, and need to lose 4000 feet using 500 fpm
Double GS and multiple by 1000s of feet to lose
2.5 x 2 =5, Then multiple 5 x 4 = 20, then add 5nm to ensure smooth level off
Answer: My descent starts at 25nm out.
Note: Obviously, at higher altitudes, then reduce descent to a lesser fpm when reaching lower altitudes
Notice that I added the doubled GS to the equation for starting distance.
Example: GS 4 nm/minute and 5000 ft to lose
(4 x 2) x 5 + (4 x 2) = 40 +8 = 48
I just keep it simple, use your ETE on your GPS. If you are at 15’000 feet, descend 15 minutes back for 1000 fpm, or double it for a nice 500 fpm decent.