Equal Time Point (ETP)
The ETP (Equal Time Point)/Critical Point (CP) is the position in the flight where it is the same time to go home as it is to continue on to your destination. The ETP always moves into the wind. The ETP is not dependant on fuel, but wind, giving a change in ground speed out from, and back to the departure aerodrome.
Pilot's normally look at an ETP as their critical point based on an emergency situation. An example would be engine failure, pressurization problems, or perhaps a medical emergency on board the aircraft.
In order to compute an ETP you must determine the following:
- Total distance which is the distance between the selected ETP diversion airfields.
- Groundspeed to return (GS Home) which is the groundspeed for the ETP back to the last diversion airfield based on TAS and wind factors at the selected altitude and the aircraft configuration.
- Groundspeed to continue (GS On) which is the groundspeed from the ETP continuing on to a diversion airfield based on TAS and wind factors at a selected altitude and aircraft configuration.
- True airspeed (TAS) which is determined from the aircraft flight manual.
- Wind factors determined along the proposed flight route at the selected altitude. A first half wind factor reflects turning back; a second half wind factor reflects continuing on.
To find the distance and time to the ETP we use these formulas:
An aircrft has the to find the ETP with the following data:
- Distance out from "A" to destination "B" is 225 nm
- TAS 100 kt
- Wind (Out/On) 25 kt HWC
- Wind (Home) 25 kt TWC
- G/S (Out/On) 75 kt
- G/S (Home) 125 kt
Use the data in the formula:
Develop more than one ETP based on different conditions. Each emergency situation would constitute different airspeeds, wind factors or distances, so be aware of the most critical situation.
Point of No Return (PNR)
For nonstop flights between two definite locations, the PNR is actually beyond the halfway (more exactly, the "equitime") point, since aircraft usually carry more fuel than is necessary to reach the destination. With loss of mass due to fuel consumption, it takes less fuel for an aircraft to cover a given distance. An aircraft might expend, say, 60% of its total fuel load before reaching the PNR. The PNR can be further extended in this manner by dropping unnecessary weight or ordnance.
The point of no return, is the point on a flight at which a plane has just enough fuel, plus any mandatory reserve, to return to the airfield from which it departed.
Beyond this point that option is closed, and the plane must proceed to some other destination. Alternatively, with respect to a large region without airfields, e.g. an ocean, it can mean the point before which it is closer to turn around and after which it is closer to continue.
To calculate cruise fuel for your PNR:
From the Total Fuel Weight at start up subtract:
- Fixed Reserve (as required by CAAP) (20 minutes)
- Taxi allowance (if required)
- Any holding fuel (if required at departure aerodrome)
- Variable Reserve (as required by CAAP) (15%)
After subtracting the above fuel weight, the remainder can be used for calculating PNR:
- Convert fuel weight to minutes.
- Calculate PNR in minutes.
- Convert PNR minutes to Nautical Miles.
An increase of any wind component (head wind or tail wind) causes the PNR to move closer back to the departure aerodrome.