Descent Gradients

VVI = 600 ft/min

3.5.5. Example #3: An aircraft is traveling at 150 KTAS (approximates 2.5 nm/min) in level flight and makes a 2° pitch change, if the aircraft speed remains constant, the resultant VVI will roughly equate to 500 ft/nm.

3.5.5.1. VVI = 2.5 nm/min × 2° × 100 ft/nm VVI = 500 ft/min

3.5.6. Example #4: You are planning to take-off from an airfield with a required departure climb gradient of 330 feet per nm. See paragraph 3.8.3.

3.6. The Gradient. Whether climbing or descending, the gradient (or angle) the pilot chooses

3.7.1.3. Additionally, you may desire to utilize a predetermined descent gradient. By using basic algebra and solving for “distance” in the equation above, you can determine at what distance from a predetermined point, usually an airfield, upon which to start an enroute descent.

Although the final descent from cruise altitude to the final approach fix altitude is normally divided by a variety of level-offs, calculating a predetermined point upon which to start an enroute descent and then updating throughout the arrival phase should increase your situational awareness. This application is very useful when cleared “pilot’s discretion” from the cruise altitude to a lower one by ATC.

3.7.1.4. Example #2: An aircraft is level at 30,000 feet, 130 miles from the airfield and given an initial clearance to descend “at pilot’s discretion” to 6,000 feet. Since a 300 ft/nm ratio or 3°

descent profile is quite common, the pilot/crew can determine the distance from the airport upon which to initiate the descent by using the following formula:

3.7.1.4.1. Distance = Altitude (to lose or gain) ÷ Descent Gradient (DG) Distance = (30,000 feet – 6,000 ft) ÷ 300 ft/nm

Distance = 24,000 feet ÷ 300 ft/nm Distance = 80 nm

3.7.1.5. Therefore, based on the current set of known information, the pilot/crew would wait until instrument indications indicated the aircraft was 80 nm from the field and then start their descent. As a technique, depending on the type of approach planned and where the IAF or FAF was located in relation to the descent point, the pilot/crew may want to add or subtract 5 or 10 miles from the calculated figure to account for the additional distance needed to fly the IAP.

Another factor that should be accounted for in descent planning includes the affects of an average headwind or tailwind component during the descent.

3.7.2. ATC Crossing Restriction.

3.7.2.1. Example #3: An aircraft is heading southbound on the 360 degree radial and established at FL 270. ATC directs the pilot to cross 10 nm north of the ABC VORTAC at 12,000 feet.

Referencing the DME, the pilot determines the aircraft is approximately 35 miles from the ABC VORTAC. The same 60-1 formula used in “Example 1” applies.

3.7.2.1.1. DG = Total Altitude (to lose or gain) ÷ Distance

DG = (27,000feet – 12,000 feet) ÷ (35 nm – 10 nm)

DG = 15,000 feet ÷ 25 nm DG = 600 ft/nm

3.7.2.2. Therefore, to lose 15,000 feet in 25 nm will require a 600 ft/nm gradient and also equates to a 6° pitch change (from level flight). After calculating the required gradient/pitch change, the approximate VVI to expect can also be determined. The VVI required during the

descent will depend on the aircraft’s speed (in TAS, TMN, or groundspeed converted to nm/min) experienced during the descent (or climb). Normally, if you base your VVI calculation (as in the example above) on the initial speed (TAS) at the higher altitude, you will reach the lower altitude prior to the crossing restriction fix. Because TAS increases for a constant IAS while climbing, the reverse is not true. If you use the aircraft’s initial speed to calculate a VVI when vacating a lower altitude and climbing to a relatively higher one, you will reach the higher altitude at a point beyond that which was desired (see 3.8.2).

3.7.2.2.1. NOTE: When traversing through large altitude changes (whether climbing or descending); utilizing an average TAS will increase the accuracy of VVI calculations.

3.7.3. Gradient Determined from a Published Procedure. During the arrival phase, there are many applications that may require you to calculate a descent gradient (e.g., while flying a STAR) or where you may desire to calculate one to increase your situational awareness as the instrument approach progress.

3.7.3.1. Example #4: You are flying a jet enroute to Minneapolis via the EAU CLAIRE EIGHT ARRIVAL, GREEN BAY TRANSITION (see figure 3.1.) Your aircraft is currently established on the 278° radial at FL 240 flying westbound having just passed the GRB VORTAC. In order to plan the descent profile, ATC is queried. ATC informs the pilot/crew to expect a descent upon reaching the EAU VORTAC and cross the TWINZ intersection as depicted on the STAR.

Although ATC may assign a different altitude, for planning purposes (and for this example), a descent gradient (pitch change) is calculated predicated on the published crossing altitude, 11,000 feet.

Figure 3.1. STAR for Minneapolis, Minnesota (EAU CLAIRE EIGHT).

3.7.3.2. Although the scenario is slightly different from the two previous examples, the application of the formula is the same. You must descend from your current altitude to a

predetermined crossing altitude in a certain amount of distance. The calculated descent gradient

also corresponds to a required pitch change. Here are the calculations based on the information given and taken from the STAR (see Figure 3.1.):

3.7.3.2.1. DG = Total Altitude (to lose or gain) ÷ Distance DG = (24,000 feet – 11,000 feet) ÷ 36 nm DG = 13,000 feet ÷ 36 nm

DG = 361 feet/nm

3.7.3.3. Therefore, you should plan to lose 13,000 feet in 36 nm (the total distance from EAU VORTAC to the TWINZ intersection) which equates to a 361 ft/nm descent gradient. It also equals a 3.6° (use 4° for 60-to-1 purposes) pitch change from level flight.

3.7.4. Affect of Maintaining a Constant IAS During a Descent. If you elect to fly a constant IAS throughout the descent, which is a common practice (and often recommended by the MDS performance manual), then the corresponding TAS will decrease as the aircraft descends. Since the VVI calculation is predicated on the aircrafts speed, it too will decrease if a constant pitch is held. Therefore, if you calculate and hold a VVI, particularly one predicated on a groundspeed, versus maintaining a set pitch, the aircraft will reach the descent altitude at a point prior to the fix. Since crossing the fix at the designated altitude is required, calculating and holding a VVI that will enable the aircraft to descend to and reach the required altitude may work to your benefit in this particular situation. If the situation was reversed and a climb gradient was based on meeting a higher crossing fix altitude, a calculated VVI may not work since TAS as it relates to a constant IAS increases with altitude. This concept is expanded upon in the discussion on climb gradients in the following section.

3.7.4.1. NOTE: The most important part of the equation (which remains constant no matter what speed the aircraft is flying) is the gradient. The pilot/crew must descend at the calculated gradient in order to reach the desired descent point, mandated ATC crossing altitude restriction or gradient published within an instrument procedure.

3.8. Climb Gradients. The formulas used to compute climb gradients (CG) and the factors