3 2 1 Descent

I’ve recently been taught that using the VNAV (Vertical Navigation) function to meet crossing altitude can get pilots into trouble. This for example, in terms of meeting a crossing restriction while descending on an Instrument Approach Procedure (IAP) or a Standard Terminal Arrival Route (STAR). This isn’t new. Pilots should know never to fully trust technology. There have been many incidents and accidents that are as a result of complacency.

Whether using VNAV or not, stabilized descent planning is important. A 3:1 descent is a good stabilized descent to use. Here’s how to plan one:

In order to establish a 3:1 descent and meet a crossing restriction, proper planning should begin various miles before, depending on the altitude to lose. The altitude to lose should be first determined by finding the difference between the crossing restriction altitude and the aircraft’s cruising altitude. Since 3:1 descent is going to be used, the difference in thousands of feet is multiplied by 3 to determine the distance to start the descent before the crossing restriction.

Speed reduction is an additional factor that needs to be taken into account when calculating the distance to begin the descent. Since transport category aircraft can’t reduce their speed and descend at the same time, pilots must level off to reduce speed. A rule of thumb to use to determine speed reduction distance is 1nm/10kts of speed reduction. This is then added to the distance determined to be used for a 3:1 descent.

In addition to finding the distance to start the descent before the crossing restriction, the Rate of Descent (ROD) that will maintain a 3:1 descent must also be calculated. For this we use True Airspeed (TAS), but since wind is always present, Ground Speed (GS) is used instead. The formula (GS/2) * 10 is the rule of thumb used to calculate the ROD. As the aircraft moves through different altitudes and descends to thicker air, GS will be changing through the descent and so the ROD must be periodically recalculated in order to continue to maintain a 3:1 descent. Below are some examples.

Example 1:

An airplane is being flown at FL200 on an arrival procedure and is 40 nautical miles (nm) from its next fix, FIRST. FIRST is at FL120 and the instructions are to cross it at 280 knots. The airplane is currently flying at 300 knots. There is a 40 knots headwind at FL200.

Distance to descend: 20000ft – 12000ft = 8000ft/1000 = 8 * 3 = 24 nm

Speed reduction distance: 300 knots – 280 knots = 20 knots/10 = 2 nm

Begin speed reduction: 26 nm from FIRST

Start descent: 24 nm from FIRST

3:1 ROD: 280 knots – 40 knots = GS = 240 knots/2 = 120 * 10 = 1200 ft. /min (initially)

Example 2:

Pilots flying an airplane at FL380 are instructed to descend to FL310 before passing its next fix, SECON. The pilots are also instructed to cross SECON at 330 knots. The airplane is currently flying at 360 knots. There is a 75 knots headwind at FL380.

Distance to descend: 38000ft – 31000ft = 7000ft/1000 = 7 * 3 = 21 nm

Speed reduction distance: 360 knots – 330 knots = 30 knots/10 = 3 nm

Begin speed reduction: 24 nm from SECON

Start descent: 21 nm from SECON

3:1 ROD: 360 knots – 75 knots = GS = 285 knots/2 = 142.5 * 10 = 1425 ft. /min (initially)


Quote:

“You are not only responsible for what you say, but also for what you do not say.” Martin Luther