I will have to admit I never paid any attention to Equal Time Points when flying four-engine aircraft. Our plan in the event of an engine loss was to push the other three up and pretend nothing happened.

Why do we have two engines in Gulfstreams? Because we always want a spare. What if we are down to one engine? Then we want to land as soon as possible. That's why we need ETPs. Your flight planning service provider can do them for you automatically, but you need to know how to compute one manually to make sure they chose the ETP airports wisely and to compute your own if they didn't. You might also need to figure your own ETP in case your flight plan provider uses the wrong assumptions for some cases, as is the case with ours. (More on this in a minute.) Relax, it's easy. . .

The KBED - LSGG example used in International Operations Manual / Oceanic Departure, International Operations Manual / Oceanic En Route, and International Operations Manual / Oceanic Arrival is used here as well, shown in purple.

What follows comes from the references shown below. I've added my own techniques in blue.


Equal Time Points for Navigators

[AFM 51-40, page 24-9.] (This is the manual Air Force navigators used back in the days the Air Force needed navigators to cross an ocean.)


Equal Time Points for Pilots

Figure: Equal Time Point and Point of No Return, from Eddie's notes.

The following formula is used to calculate the ground distance from the departure airport to ETP:

Ground Distance to ETP = ( D ) ( GSB ) GSA + GSB = NM

Where:

D = Total Trip Distance

GSA = Ground speed to continue to "Ahead" airport at altitude to be flown

GSB = Ground speed to return to "Behind" airport at altitude to be flown

A few notes:

  • 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. Regardless, you will often see "GSR" to denote your groundspeed while returning and "GSC" to denote your groundspeed while continuing.

  • The terms GSO and GSR are for the Point of Safe Return (PSR), which is sometimes called the "Point of No Return" (PNR). More about that: International Operations / Point of Safe Return.

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

Figure: Example Plotting Chart, from Eddie's notes.

The single ETP actually reflects three equal time point situations, as covered next.

ETP Types

Equal Time Points provide pilots with decision making aids in the event the airplane needs to proceed to a landing airport as soon as possible. An ETP is a geographic location along the route of flight where the time to return to an airport behind the aircraft is equal to the time to proceed to an airport in front of the aircraft.

For some routes, a series of ETP location sets may be called for. In the example problem, ETPs between EINN/BIKF and BIKF/CYQX are used. In the event of a problem before the EINN/BIKF ETP, the airplane would turn back to EINN. Beyond this point and before the BIKF/EINN ETP, the airplane would turn north to BIKF. Following this point, a diversion to CYQX would be called for.

At least three types of ETPs should be considered for each location set to cover the following three contingencies:

Selecting ETP Airports


Computing ETPs

Computer Flight Plans

Figure: Maintain Level ETP Example, from Eddie's notes.

Computer ETP Computations are generally superior to manually calculated ETPs because they consider a greater number of wind points and will yield more accurate ETPs. But, as we've seen with this vendor's "DIST TO ETP DIVRSN PT," you need to carefully consider what the data means before making any assumptions.

In our example KBED - KLSGG flight plan, we elected to plot the Maintain Level ETP because it was in the middle. Our flight planning service tells us this ETP occurs 1,521 nm from our departure airport, but more importantly 697 from out "behind" airport, CYQX. The TAS going forward or back are very close but the distance covered is dramatically different because of the 62 knot wind from the west.

Manually

As shown above, ETPs can be computed manually if the ground speed ahead and behind are known. To manually calculate an ETP:

Ground Distance to ETP = ( D ) ( GSB ) GSA + GSB = NM

Figure: Flight Plan Example, Top Information, from Eddie's notes.

All you really need is the wind factor, TAS, and distance between the behind and ahead airports. Remember that GSB = TAS + Wind and GSA = TAS + Wind, and the wind factor is positive if behind you and negative if in front of you.

From our plotting chart we see D = 1722

From our flight plan we see the TAS = 451 and the average wind factor is P062. This could be wildly inaccurate for the pertinent portion of the flight so it pays to scan the oceanic portion to make sure.

Figure: Flight Plan Example, Oceanic Legs, from Eddie's notes.

The oceanic legs reveal wind factors of P075, P067, P060, P051, P051, and P040. Rather than use a wind factor of 62 throughout, as the flight plan would suggest, we'll use P067 for our return scenario and P051 for the continue option:

  • GSB = TAS + WF = 451 + (-67) = 384

  • GSA = TAS + WF = 451 + (+51) = 502

Therefore:

Ground Distance to ETP = ( D ) ( GSB ) GSA + GSB = NM

Ground Distance to ETP = ( 1722 ) ( 384 ) 502 + 384 = 746NM

Our manually computed ETP is 50 nm further east than the computerized version because the vendor selected a descent to lower the TAS in an attempt to make the three provided ETPs about the same. If your decision is based on really staying at flight level, the actual ETP is the one we computed, 50 nm east.

Circular Slide Rule

Figure: ETP With a Circular Slide Rule, from Eddie's Notes.

With a circular slide rule, place the Total Distance (D) on the outer scale opposite the added groundspeeds (GSB + GSA), and place the sliding index over the return groundspeed (GSB) on the inner scale. The ETP will appear on the outer scale under the sliding index:

Figure: ETP With a Circular Slide Rule Example, from Eddie's Notes.

The technique works but it introduces an opportunity for error and has been relegated to the "You can, but why would you?" category.


Three Airport Example

There are times when a third airport may become advantageous. (This occurs frequently in the North Atlantic when the route of flight is near Iceland.) For this example, we will compute only the Maintain Level ETP between EINN-BIKF and BIKF-CYQX to illustrate the positioning of the ETP on the route of flight. (The process of checking the Loss of Level and Loss of Engine ETPs is the same, using the appropriate groundspeeds for those scenarios.)


Plot ETPs

If a plotting chart is required the ETPs should be noted along the route of flight. More about: International Operations / Plotting 101.

A perpendicular line pointing to the route and arrows pointing to the ETP airports will allow rapid and easy identification of a course of action if a diversion decision becomes necessary.

If a plotting chart is not used, another means of identifying the ETP should be employed. A pencil mark on the en route chart is normally sufficient. Some FMS units allow an electronic display of a non-flight plan point that allows easy reference.


CAUTION: Do not enter the ETP into the FMS as a waypoint. The FMS will fly a great circle route to and from this point and can result in a significant off-course error.

I wrote this warning in the very first international operations manual I ever wrote and it has appeared in every copy since. I'm not sure how true the word "significant" is but given the accuracy of modern avionics, the intent is certainly true. But there is now a new reason, if your datalink is making your position report for you, the ETP will only confuse the air traffic services unit. More about this: Surveillance / ADS-C / Position Reports.


Book Notes

Portions of this page can be found in the book Flight Lessons 2: Advanced Flight, Chapter 10.

Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 18.


References

Air Force Manual (AFM) 51-40, Air Navigation, Flying Training, 1 July 1973

Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011