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