Point of No Return

Question 1

PNR Definition:

Answer 1

● The Point of No Return is the greatest distance or time that an aircraft can fly past the last landable airfield, and then return to that airfield with a specified or safe fuel reserve

Question 2

Types of PNR

Answer 2

● For multi-engined pressurized aircraft there are three types of PNR:
○ PNR Normal Ops
○ Engine Inoperative PNR
○ Depressurized PNR

Question 3

PNR Normal Ops

Answer 3

● This assumes that the aircraft operates at normal TAS.
● We would use it in a case where we require a diversion due to reasons not affecting aircraft performance.
● If aircraft is past the PNR, then it simply can’t turn back

Question 4

PNR

Answer 4

T = Endurance x GS back / (GS out + GS back )
● Endurance = Number of hours of fuel on board – Reserve
● GS back = ground speed back to departure.
● GS out = ground speed to destination

● Example:
○ Endurance = 5.0 hrs
○ TAS = 250 kts
○ Tailwind = 50 kts
○ Reserve = 45 minutes

● Calculate ground speeds:
○ GS out = 250 + 50 = 300 kts
○ GS back = 250 – 50 = 200 kts
○ Endurance = 5.0 hrs – 45 min = 4:15 = 4.25 hrs

T = 4.25 x 200 / (200+300) = 1.7 hrs
● Aircraft can fly up to 1.7 hrs past the airport and return with 45 minutes of reserve fuel.
● Convert this to a distance with D = V x T
○ D = 300 x 1.7 = 510 nm

Question 5

One Engine Inoperative PNR

Answer 5

● In this case, it is assumed that everything remains normal up to the PNR.
● Then, one of the engines decides to fail at the PNR.
○ Engine failure = descent to one engine out ceiling.
○ Descent = increased fuel flow.
● This scenario requires a different formula.
● However, we don’t need to account for the descent itself

D = Fuel On Board / (SFF out + SFF back )
● D = distance from departure to PNR
● SFF back = Specific Fuel Flow back to departure.
● SFF out = Specific Fuel Flow to destination.
● Specific Fuel Flow = amount of fuel burned per nm travelled.
● SFF = Fuel Flow/GS

● Example:
○ Fuel on board = 6300 lbs
○ All engine TAS = 250 kts
○ All engine fuel flow = 1000 lbs/hr
○ One engine out TAS = 150kts
○ One engine out fuel flow = 700 lbs/hr
○ Tailwind = 50 kts
○ Reserves = 500 lbs

● Calculate ground speeds:
○ GS out = 250 + 50 = 300 kts
○ GS back = 150 – 50 = 100 kts
● Calculate SFF
○ SFF out = 1000/300 = 3.3333 lbs/NM
○ SFF back = 700/100 = 7.00 lbs/NM

D = 5800 lbs / (3.33lbs/nm+7.0lbs/nm) = 561 nm
● Aircraft can fly up to 561 nm past airport and return with 500 lbs of reserve fuel.
● Convert to time with T = D/V
○ T = 561 nm / 300 knots = 1.87 hrs
○ 60m * 0.87 = 52 minutes so 1.87 h = 1:52

Question 6

Depressurised PNR

Answer 6

● This assumes normal operations up to the PNR.
● We then have a depressurisation event at or prior to the PNR.
○ Depressurisation = descent = increased fuel flow.
● Depressurized flight requires a lower altitude than one engine out flight: this means there will be significantly lowered performance

● To find the depressurized PNR, use the same formula as the one engine out PNR.
● Remember that depressurized PNR will be the most limiting