Equitime vs Equifuel

[A.FLOOR]

Would someone explain when would an equitime point not equal an equifuel point?

Is equitime always equal to equifuel in the same configuration?

[greengage22]

How about when, from the critical point, the ISA deviation for the onward route is signifcantly different from the return route? In warm air, TAS is higher and fuel flow lower.

[Intruder]

At the equal time point, the time of flight to the destination and the divert field is the same. Forecast winds are already taken into account. If one assumes the flight will be at the same altitude, the fuel burn will also be the same.

The "same altitude" assumption is valid operationally, because equal time points are usually figured for engine-out and/or loss-of-pressurization situations. In either case, the aircraft will be likely flying at an altitude governed by the situation, not by ATC (yes, an emergency might be delcared to get that altitude).

The "same fuel burn" assumption is also valid in the engine-out case, because the likely configuration is either long-range cruise speed or max continuous thrust on the remaining engines (or both). It is unlikely a significant headwind or tailwind correction will be made.

[PorcoRosso]

What about the weight of the Airplane ?

Since you have burned a specific quantity of fuel, you are lighter, therefore your Fuel Flow should be less ?

[oxford blue]

Intruder speaks the truth. In classical "critical point / equal time point" calculations, you assume the single-engine over-water TAS as declared in the company Flight Ops Manual. So the considerations mentioned by BIK do not apply - if you were a sufficiently sophisticated pilot to adjust your power setting for best range for separate head or tail wind cases, you would only do it with all engines operating.

The only circumstances using the same flight level on or back that I can see when equitime might not be equifuel would be if there were a difference between the density altitude and the pressure altitude, depending on whether you go on or back. You would fly the same flight level whether on or back, but the temperature, and therefore the density altitude would be different.

[Tinstaafl]

As long as all flight conditions to destination & alternate are the same then ETP would equal EFP (same fuel consumption over the same period).

ETP may not equal EFP if there are different holding reserves required for destination & alternate.

If anything changes the flight conditions of one from the other then fuel consumption will change for the two legs.

[Wizofoz]

Intruder is correct ONLY if the same speed is used to calculate onward and divertion fuels. As BIK pointed out, if the aircraft is to be flown at best range or best economy in either case, speeds and therefore fuel burn will be different depending on wind compoment.

If you were manually working out an ETP, you would likely use the same IAS and TAS for both sectors, but I would imagine a sophisticated flight planning program would use LRC or BEC and therefore come up with different speeds.

[LeadSled]

Folks,

CP or equitime points ( in whatever configuration your are in, one out, two out, depressurise etc, calculated with whatever variables are known and applicable) is what it says, the point at which it will take equal time to go to either of two places, on or back or whatever.

Equi-Fuel is the old PNR, point of no return, the point at which you can return or go on ( in whatever configuration you are in, calculated with whatever variables are known and applicable ) where the fuel on and the fuel to return between two places will be the same.

Of course, that really means the fuel burn, it is only exactly equal total fuel on and back, if the reserve requirements at both places are the same.

Tootle pip!!

[Wizofoz]

Hi LeadSled!

I agree with your explanation of equi-fuel, but it is NOT the same as PNR. PNR is the latest point which you have sufficient fuel to return to point of departure or alternate. It has no relationship to distance to destination (except it would be wise to have enough fuel to reach it!). It can be overhead destination, or even past it. It is the last point at which a continue, return/divert decision must be made.

[LeadSled]

Wizo,
Not disagreeing with you, there are many scenarios re. PNR, particularly PNR to a moving base, ie: return to the carrier or another landing point, maybe a carrier or a land base, mil types will know all about some of the combinations and permutations that can cause serious brain fade. In practice, in my experience, PNR beyond destination means no more than somewhere you have passed, or the departure point, is covered as a conventional alternate to the destination.

PNR is not associated with distance, I don’t believe I said that, but in the “traditional” case, where PNR’s have a very very practical application, over very long range ( usually oceanic) operations where an alternate to destination cannot be carried, the calculation defines a place where a decision must be made to proceed, usually to destination or perhaps an en route alternate ahead, or return to an alternate, be it the point of departure, or some suitable alternate behind you, or off route.

Of course, in the off route case, an alternate somewhere abeam, there will be two points that will be equidistant from the off route alternate along your track, and one point where the distance will be minimum. Which is appropriate will depend on the day (or night) and the circumstances.

All this will be double dutch to pilots who only know about national or other rules that always require an alternate beyond the destination, or who operate aircraft over relatively short ranges, where it is all theoretical.

Over the years, I can recall some very long days and nights, doing seemingly never ending in flight calculations, working successive en route PNR3’s ( or PNR2’s really, now I had to re-plan assuming a further engine failure) to minimise the impact of % based variable reserves. These are about the only times I lamented the passing of the now endangered, if not extinct, pencil wielding homoerectusnavigatorous.

Tootle pip !!