נקודת זמן שווה הנה נקודת המרחק על הנתיב, ממנה הזמן הלוך והזמן חזור לנקודת המוצא, שווים - ETP/PET

 EQUAL TIME POINT (ETP)

Equal Time Point is the geographical point in a flight at which the aircraft has the same flying time continuing on to a given airport or turning around and returning to a given airport. Compute an ETP anytime the flight is a long distance between usable airports. 

The ETP is a function of:

• True airspeed, which is a function of configuration and altitude

• Wind factor, which is dependent upon the final altitude selection

• Distance which is dependent on the availability of airports

• Distance is based on usable airports close to the coastline and close to the flight route.

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.

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.

At the ETP, always have enough fuel to reach any of your ETP diversion airports with some reserves. If forced to descend from your cruise altitude and you have insufficient fuel to reach your intended divert airfields, you are operating with a wet footprint.

A wet footprint is defined as an area encompassing a distance each side of an ETP where an airplane, if required to descend, will not have sufficient fuel to make destination or return to departure. The size of the area is determined by the total fuel onboard, descent altitude, length of over-water leg, wind factors and other individual factors.

In order to operate without a wet footprint should a pressurization problem arise, an operator might need to fly at an altitude above 15,000 ft.

 Here, not only is fuel a concern, but also oxygen. With a limited oxygen supply, there must be a trade-off between fuel required and oxygen required

 to safely recover the aircraft.

ETP Computation

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 (GSrtn) 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 (GScont) 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.

The formula for computing an ETP or CP :

 Example

The ETP for an Excel flight to Bermuda from White Plains (HPN) to TXKF, assuming an engine failure, should be computed as follows:

• Given:

- Engine failure average TAS at FL260   285 kts

- First half wind factor                              -45

- Second half wind factor                         +35

- Total Distance                                       680 NM

Insert the numbers into the formula as follows: 

 163200= (680 x 240)    (240 = 285-45)

560= (240 +320)            (320=285+35)

נקודה קריטית –  CP (Critic Point)

נקודה זו שימושית למטוסים רב מנועיים. זו למעשה נקודת זמן שווה (PET) אך נבדלת ממנה בכך שמהירות הטיסה נמוכה יותר כתוצאה מאובדן מנוע ועל כן המטוס מושפע מהרוח יותר.

.אם מנוע כשל  לפני נקודה זו, יחזור לשדה המוצא. אם מנוע כשל אחריה, ימשיך לשדה היעד

:הערה

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.וואריאציות אלו יילמדו תוך כדי התרגול, באשף המבחנים