Wednesday, 4 February 2009

Point of No Return (PNR)

Similar to Point of Equal Time or Critical Point (CP), the Point of No Return is a straight forward equation easily resolved on your Dalton flight computer.
The Point of No return (PNR) depends on the fuel endurance time. After passing the Point of No Return the remaining fuel will not be sufficient for a return to the point of departure. The flight can only be continued to the destination or to an alternate airfield. It is, of course, also possible to calculate the Point of Safe Return based on the available fuel after deducting the required reserve.
The simple formula for PNR is:
T (PNR) = E X GS Home/[GS Out + GS Home] (Time formula)
Where: T (PNR) = Flying Time to PNR & E = Endurance.
The formula can also be transposed as:
T (PNR)/E = GS Home/[GS Out + GS Home]

Example: Endurance 6h30min. GS Out = 240kt. GS Home = 210kt.
On the flight computer:
Align 450 (240 + 210) on the inner scale with 210 (GS Home) on the outer scale. Read 182min outer scale against 390min (6h30min) on the inner scale.
Result: The PNR will be reached after a flight of 182min (3h02min). If the point is to be located geographically, determine the distance to the PNR by means of GS Out. In this example the distance to PNR will be 728nm. (182 mins @ 240kt.)

Equal Time Point (Critical Point)

It might be useful to briefly state here the formulae and Dalton flight computer solutions to the questions: What is the Critical Point? And what is the Point of No Return? The PNR will be discussed on the next blog page.
If, for example, engine trouble occurs during flight it is important for the pilot to know whether the airport of departure or the airport of destination can be reached sooner. For this the Point of Equal Time (P.E.T.) or Critical Point (C.P.) is determined. This is the point from which the continuation of the flight to the destination would require the same time as the return flight to the point of departure (or between any two points on your flight plan A & B).
The formula is T (P.E.T.) = TF X GS Home/[GS Out + GS Home] (Time formula)
T (P.E.T.) = Flying time to Point of Equal Time (or Critical Point).
TF = Time to fly from base (or A) to destination (or B) [Flight plan time or time A-B].
GS Out = Ground speed outbound.
GS Home = Ground speed inbound.
The distance from the point of departure (or A) to the point of Equal Time (Critical Point) is calculated by means of the following formula:
D (P.E.T.) = DZ X GS Home/[GS Out + GS Home] (Distance formula)
D (P.E.T.) = Distance from base (or A) to P.E.T.
DZ = Distance from base to destination (or A to B).
These equations may be transposed to the more convenient proportion form, as follows:
GS Home/[GS Out + GS Home] = T (P.E.T.)/TF or D (P.E.T.)/DZ 
On your flight computer simply align the sum of GS Out + GS Home on the inner scale with GS Home on the outer scale. Now read CP or Distance to CP on outer scale opposite Flight Plan time (or A-B time) and Distance departure to destination (or A-B distance) resp.
Distance A to B (DZ) = 920nm. GS Out = 240kt. GS Home = 210kt. Flight time A to B = 3h50min.
Required: Flying time to P.E.T. (C.P). & Distance to P.E.T. (C.P).
Intermediate calculation: GS Out + GS Home = 450kt.
On Flight Computer:
210 outer scale aligned with 450 inner scale. Read 107.5min outer scale opposite 230min (3h50min) innner scale & 430nm outer scale opposite 920nm inner scale.
Result: The P.E.T. (or C.P.) will be reached after a flying time of 107.5min. The distance flown will be 430nm.

Monday, 2 February 2009

What's AHEAD? Is this Aerofile Site Useful?

What's ahead? Is this AEROfile site useful? OR is it time to shut it down?

If there is any topic that YOU want published on this site then please say so via the comments facility. Use It Or Lose It!

Meanwhile, remember some of these Dead-Reckoning (DR) 'tips': See PPL Navigation Ex 18(X).

60/TAS x WS = Max Drift. (60 divided by True Air Speed multiplied by Met Office Windspeed = Maximum Drift in Degrees).

The 1/6ths rule:

0/6 of Max Drift for wind from 0-degrees OFF Track.

1/6 of Max Drift for wind from 10-degrees OFF Track.

2/6 of Max Drift for wind from 20-degrees Off Track.

3/6 of Max Drift for wind from 30-degrees Off Track. Etc up to 6/6 then Max Drift for 70, 80 and 90-degree beam wind.

HWC and TWC (Headwind & Tailwind Component): Use the 120 aide memoire:

Wind from 0, 10 & 20-degrees OFF Track: use 100% of Windspeed. (20+100=120 aide memoire)

Wind from 30-degrees OFF Track: use 90% of Windspeed. (30+90=120 aide memoire)

Wind from 40-degrees OFF Track: use 80% of Windspeed. (40+80=120 aide memoire)

Wind from 50-degrees OFF Track: use 60% of Windspeed. (50 use 60!)

Wind from 60-degrees OFF Track: use 50% of Windspeed. (60 use 50!)

Off-Track Correction:

Where, D1=Distance Flown. d1=Distance OFF track. D2=Distance to go to next waypoint.

T.E.1=Track Error in degrees after D1. T.E.1 correction (only) would result in the aircraft paralleling the reqired track to the next waypoint.

Therefore ADD T.E.2 to T.E.1 to fly direct to next waypoint.

60/D1 x d1 = T.E.1 and 60/D2 x d1 = T.E.2. ADD T.E.1 + T.E.2


Flying between waypoints A and B which are 120nm apart. After 40nm the aircraft is 4nm off track. What is the correction angle to fly to B?

D1=40. d1=4. D2=80.

60/40 x4=6-degrees (T.E1). 60/80 x4=3-degrees (T.E.2). Add 6+3=9-degrees correcton angle to fly to B from the OFF-Track position.

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