PET & PSR

033 Flight PlanningChapter 4

PET and PSR define critical positions along a route for emergency decision-making. Wind shifts both points — but in opposite directions relative to the route.

Point of Equal Time (PET / CP)

Definition — Point of Equal Time (PET)

Also called Critical Point (CP). The point where flying onward to destination takes the same time as returning home. Used for onboard emergencies (medical, fire, depressurisation decisions).

Key fact

PET always moves into wind. Example: wind from the NE pushes PET further NE along the outbound track.

Key fact

For PET: calculate distance using OEI TAS/GS both ways when the question specifies one-engine-inoperative. Calculate time to PET using all-engines TAS/GS outbound. Endurance is not relevant for PET. Descent is overlooked — it does not affect the equal-time point.

Formula — Distance to PET

X = (D × H) ÷ (O + H)

Where:

• X = distance to PET from departure (nm)
• D = total distance departure to destination (nm)
• H = groundspeed home (knots)
• O = groundspeed outbound (knots)

Formula — Time to PET

Time to PET = X ÷ O (using all-engines groundspeed outbound)

Exam trap

PET distance is inversely proportional to (O + H). Stronger combined groundspeeds bring PET closer to departure. Some questions ask for PET from destination — subtract X from D or work the formula from the far end.

Point of Safe Return (PSR / PNR)

Definition — Point of Safe Return (PSR)

Also Point of No Return (PNR). The last position from which the aircraft can return to departure with reserves intact. Used for isolated aerodromes and over-water legs. After passing PSR, the aircraft is committed to continue to destination or divert to an alternate.

Key fact

Any wind decreases PSR distance from departure — PSR moves closer to departure whether wind is headwind or tailwind outbound.

Key fact

Safe endurance = fuel available for PSR ÷ fuel flow = (Max fuel − minimum reserve fuel) ÷ fuel flow. Deduct fuel already used before calculating endurance remaining.

Formula — Distance to PSR (groundspeed method)

X = (E × O × H) ÷ (O + H)

Where:

• X = distance to PSR from departure (nm)
• E = safe endurance (hours, decimal)
• O = GS outbound — typically all engines
• H = GS home — typically OEI when specified

Formula — Time to PSR

Time to PSR = (E × H) ÷ (O + H)

Calculate time using all-engines TAS/GS outbound, then distance = Time × O (AEO outbound) unless question specifies otherwise.

Formula — Distance to PSR (fuel flow method)

X = E ÷ (SFC_OUT + SFC_HOME)

Where SFC_OUT and SFC_HOME are fuel consumption rates (kg/nm or lb/nm) outbound and homebound.

Exam trap

If the question asks for zero reserve, use PSR logic with endurance = all fuel on board. OEI at PNR: outbound to PNR uses all-engine TAS; return from PNR uses OEI TAS. Read the question stem carefully.

OEI PET and PSR summary

Calculation	PET	PSR / PNR
Distance	Use OEI GS both ways (when OEI specified)	Time with AEO out / OEI home; distance often AEO outbound
Time	Use AEO GS outbound	Use AEO GS for time to PSR
Endurance	Not used	Essential — (FOB − reserves) ÷ FF

Groundspeed from wind — effective TAS

When wind correction angle is large, use effective TAS:

Formula — Wind correction and groundspeed

(i) XWC = sin(wind angle) × wind speed

(ii) WCA = XWC × 60 ÷ TAS

If WCA exceeds 10°, use: Effective TAS = TAS × cos(WCA)

(iii) HWC = cos(wind angle) × wind speed

(iv) GS = Effective TAS ± headwind/tailwind component

Exam trap

Apply effective TAS only when WCA exceeds 10°. Below 10°, use plain TAS for GS. Examiners often embed this step inside PET/PSR questions.

Practical comparison

Feature	PET	PSR
Purpose	Equal time forward or back	Last point to return with fuel reserves
Wind effect	Moves into wind	Moves toward departure (any wind)
Endurance	Not required	Required
Typical use	In-flight emergency timing	Isolated aerodrome commitment

Total endurance note

Key fact

Total time airborne using all fuel on board except final reserves defines maximum endurance for planning. PSR uses safe endurance — never plan on consuming final reserve.