Calculate Aircraft Descent Rate
What goes up must come down. This is true of aircraft in flight. Bringing an aircraft back to the ground safely is the primary concern, but it also is necessary that the return to the ground occur at the intended landing point. The aircraft cannot fly to the airport and then drop straight down to the runway like a helicopter. The descent from flight altitude must be started well before the intended landing point is reached, and the rate at which the aircraft descends must be calculated so that the aircraft reaches ground level just as it reaches the desired landing point. This controlled drop in altitude is called the descent rate. Use these tips to learn how to calculate aircraft descent rate.
Steps
- Plan the descent in advance. The higher and faster the aircraft is flying, the further away from the field the descent must be started. The weight of the aircraft also will affect the speed of descent. For example, it takes longer and further to descend a fast and heavy high altitude airliner than it does to descend a slow and light 2-seat private aviation aircraft. Descents can easily begin more than 100 miles (160 km) from the field. It must be ascertained in advance that that runway available is long enough to provide the incoming aircraft with enough roll out distance to come to a safe stop. Runways as short as 2000 feet (600 m) long can be used for private aviation at smaller general aviation airports. Runways for larger passenger jets are typically in the range of 10,000-12,000 feet (3050-3650 m) long.
- Determine the start and end altitudes of the descent path. The end altitude of the landing field should be well documented by the Federal Aviation Administration (FAA). The start altitude will be the cruising altitude of the aircraft. All altitudes are referenced to sea level, rather than ground level, so the landing field will rarely have an altitude of 0.
- Define the amount of descent. This is the start altitude of the descent path minus the end altitude of the flight path plus a safety buffer of altitude that will allow the aircraft to properly line up with the landing field. For example, an aircraft with a cruising altitude of 35,000 feet (10,675 m) approaching an airport that is at 1,500 feet (458 m) might choose to descend to 1,000 feet (305 m) above the landing field for final approach. This would make the descent distance 35,000 - (1,500 + 1,000), or 32,500 feet (9913 m).
- Consider the ground speed of the aircraft. This is a combination of the air speed of the aircraft and any head or tail winds that are affecting the aircraft. For example, an aircraft with an air speed of 300 miles per hour (MPH) or 480 kilometers per hour (KPH) would have a ground speed of 250 MPH if flying into a 50 MPH (80 KPH) head wind, and a ground speed of 350 MPH if assisted by a 50 MPH (80 KPH) tail wind.
- Apply descent rate.
- Calculate the starting point of the descent. Normally, the descent rate would define the point at which the descent must be started. Descent rates are usually chosen to be between 333 feet (102 m) per minute and 500 feet (153 m) per minute to give an effective descent without discomforting the passengers due to rising air pressure as the aircraft descends. The descent is left short of the landing field to give room for the aircraft to line up for final approach. For example, an aircraft is at a ground speed of 300 MPH (480 KPH) and an altitude of 35,000 feet (10675 m). The aircraft needs to complete the descent at 2,500 feet (763 m) altitude and 10 miles (16 km) short of the landing field to allow time for the aircraft to line up for final approach. At a chosen descent rate of 500 feet (153 m) per minute, the aircraft will take 32,500 divided by 500, or 65 minutes, to complete the descent. The aircraft will fly 325 miles (520 km) in 65 minutes. To allow 10 more miles (16 km) to the runway for final approach, the descent must be started 335 miles (536 km) from the landing field.
- Back calculate descent rate. By knowing the time taken to execute the descent and the amount of the descent, the descent rate can be back calculated. The aircraft of the example descending 32,500 feet (9913 m) in 65 minutes is using a descent rate of 32,500 divided by 65, or 500 feet (153 m) per minute.
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