B738 Descent Planning

[FR8TDOGii]

The precision solution

The following is from my PPAC [Precise, Predictive, Aircraft Control] training systen for upgrading airline pilots.

FIRST: We want to know where to point the airplane to accomplish a given alitude loss within a specified distance. The simple rule is: ±One degree of pitch change [DP] = ±100 feet/nm. Therefore altitude change in100s of feet [DAlt] divided by NM to go = DP in degrees.

Examples: Alt to lose[DAlt] NM to go DAlt (100s of feet)/NM = DP
9000 30 90/30 3°
12000 40 120/40 3°
16000 40 160/40 4°

Thus if you decrease the pitch 3°, you will lose 9000 feet in 30 miles.If you hold 3° you will lose 300 feet/NM, REGARDLESS of airspeed because it's a matter or Trigonometry NOT aerodynamics. The faster you go the faster you have to descend to remain on your 3° flight path.

Now for the second step - acounting for the wind. If we have a head or tail wind our Air Mass flight path will have to be adjusted to maintain our desired flight path with relation to the ground.

Fortunately, there is a simple formula to correlate speed, Rate of Descent, and pitch change:

Speed(NM/min) X 100 X DP = Rate of descent.

Where speed is Ground speed for ground-based reference and TAS for Air-based reference.

Take the 1st 2 digits of your speed, divide by 6, multiply by 100 then by DP from above to get rate of descent.

Some examples:
GS (NM/min) X 100 X DP = ROD
420 7 700 3° 2100
360 6 600 3° 1800
420 7 700 4° 2800

The most beneficial application is the inflight (air-to-air) pre-computing of level off pitch changes from climbs/descents.

To do this we simply reverse the second step above to determine DP. This method eliminates the BG's [By Guess & By Golly] and provides for VERY precise aircraft control.

Let’s assume we climbing at 1800 ft/min at 180KTAS with a +10° pitch attitude and we're about to level off. What will be our final pitch attitude, assuming we maintain 180KTS (3NM/min)?

DP
= Rate of climb/descent /[KTAS(NM/min) X 100]
= -1800 /(180/60X100)
= -1800 /(3X100)
= -1800 /(300)
= -6°
New Pitch = 10° - 6° = 4°

Now you're thinking just like the autopilot/flight director !

Sorry if the formatting is off