AFM 51 40 AIR NAVIGATION PLUS

The oceanic or remote area route of flight should be examined and suitable diversion airports identified based on aircraft requirements, airport capability, and weather.

Fortunately, with the information they've given us, we can come up with a better answer. This makes flight planning and plotting easier but it is not entirely accurate. In our example the vendor decided a medical ETP requires a descent to FL200 to make the ETP about the same as what they call the 1E INOP (Loss of Engine) and DEPRESS (Loss of Level) ETPs. But some vendors do select what appears to be an arbitrary descent and you may need to request a change from your vendor or compute your own. This ETP can also be called 'Medical' or some variation leading to the idea the airplane divert but not need to descend or decelerate.

Maintain Level ETP - In the event of a need to land as soon as possible without the need to descend, such as a medical emergency, the airplane may be required to turn to either Maintain Level ETP.

In either case, it will take around 3 hours at the recommended speed. In the event of loss of pressurization, the aircraft would descend to 15,000' and either turn back or continue east. In our example this ETP occurs just after the Engine-Out ETP, at 1,541 nm along the route of flight from our takeoff point. This ETP can also be called 'Depressurization' or some variation leading to the idea the airplane must descend. A descent to 15,000 feet permits passengers to breathe without the use of supplemental oxygen, requires the flight crew to remain on supplemental oxygen, but provides greater endurance. A descent to 10,000 feet permits all occupants to breathe without the use of supplemental oxygen. Most flight planning programs compute a rapid descent to 10,000 or 15,000 feet, depending on user preferences.

Loss of Level ETP - In the event of the loss of pressurization or other problem requiring a rapid descent without an engine loss, the airplane may be required to turn to either Loss of Level ETP.

In our example flight from KBED to LSGG we have plotted one equal time point around 37° West:

The terms GS O and GS R are for the Point of Safe Return (PSR), which is sometimes called the 'Point of No Return' (PNR).

Regardless, you will often see 'GS R' to denote your groundspeed while returning and 'GS C' to denote your groundspeed while continuing.

In both the navigator and pilot versions the terms 'departure' and 'destination' are used when in fact they should refer to the alternate airports 'ahead' and 'behind.' You will seldom opt return to your departure point or continue to your destination, though it could happen.

GS B = Ground speed to return to 'Behind' airport at altitude to be flown GS A = Ground speed to continue to 'Ahead' airport at altitude to be flown The following formula is used to calculate the ground distance from the departure airport to ETP: Ground Distance to ETP = ( D ) ( GS B ) GS A + GS B = NM The navigator's formula is mathematically pure but hardly usable, something we can fix with a little algebra, below. Determine it by applying the wind factor for the second half of the route to the TAS. GS c is the GS to continue from the ETP to destination. Compute it for the first half of the route by applying the wind factor with the sign reversed to the TAS. GS r is the groundspeed to return to departure from the ETP. Since ETP is most significant for the overwater portion of a flight, the ETP should be determined from coastal departure points and for alternate landing points.

Total distance is the number of nautical miles from departure to destination.

Total Distance GS r + GS c = ETP (in miles from departure) GS r

Use the following formula to compute a ETP:.

When computing ETP, obtain a wind factor for each half of the route.

When the wind factor is a minus value (GS less than TAS), it is called a head wind factor when it is a plus value (GS greater than TAS), it is a tail wind factor. To do this, algebraically subtract the TAS from the GS. A wind factor is a headwind or tailwind component which is computed by comparing the average groundspeed (GS) to the true airspeed (TAS).Its location is somewhere near the midpoint of the route, however, and it dependent upon the wind factor.

The ETP is not necessarily the midpoint in time from departure to destination.It is usually computed when planning long, overwater flights. The equal time point is a point along the route from which it takes the same amount of time to return to departure as it would to continue to destination.

AFM 51 40 AIR NAVIGATION PLUS

Afm 51 40 air navigation