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Aircraft performing below book numbers

Mooney_Driver wrote:

When airlines get new airplanes, what they do mostly is to impose a 10% fuel penalty on them. Then they fly them on schedule and record the actual values until they get a performance factor which is consistent. That factor then is entered into the flight planning system. I recall a fleet of MD11’s I used to dispatch and they had performance factors from 1% up to 6% within the fleet.

Exactly. I deliberately kept experience with turbine aircraft away from the discussion: turbine aircraft, light or heavy have much larger intrinsic performance variations due mainly to engine hot section degradation. And this is mostly factored in planning with the performance factor. So the same airline will use different planning performance numbers for each aircraft of the same type in their fleet at a given time. THis number is reviewed and adjusted periodically. IIRC on the MD-11 this was a particularly sensitive matter due to manufacturer range guarantees being optimistic…

This does not happen with pistons: as long as internal airflow is good and adjustments are fine, then engine power delivery will be nominal given certain power parameters (MP, RPM, FF, ambient…). Given those, airflow (and thus power) is 99% dependant on having an unobstructed leak-free gas path through the engine, typically:

  • Induction and air filter tidy, clean, obstruction and leak free
  • Turbocharger performance and adjustments (if installed)
  • Camshaft lobes having proper lift
  • Exhaust being leak and obstruction free.

If those are good, again with proper adjustments the engine will be able to deliver nominal power 99% of the times, regardless of age, compressions, wear, etc…

Maybe propeller degradation could be a factor too, since at each overhaul cycle metal blades do change dimensions depending on how damaged they were at the time of OH. Also, blades degrade between overhaul cycles depending on their use.

Last Edited by Antonio at 24 Nov 10:00
Antonio
LESB, Spain

Airborne_Again wrote:

I don’t understand the point of the range tables in the POHs. They are completely worthless in practise.

Not necessarily. They can be of some value if you are looking for the “sweet spots” and trends within the performance tables.

They can be used to determine at what regimes the airplane will achieve maximum range for instance, not as absolute numbers but as trend indications. Personally I do perfer numbers for this rather than graphics, even though in graphics trends can be seen faster.

Clearly, with the new FR rules and other stuff, the no wind range tables need rework, but they do anyway. I use some excel formulas to do this and the results are quite different usually than in the POH tables.

Airborne_Again wrote:

This is of course how CAT did it even before and also many private pilots, but it was not compulsory.

Final Reserve in CAT was different, not a fixed endurance but fuel for one go around, a new approach and landing.

Personally i have treated the 45 minutes at plan regime as Final Reserve since ever I’ve been back to flying on the Mooney.

Peter wrote:

You cannot fly safely in that area of operations without a fuel totaliser, which almost nobody in GA has…

With 45 minutes of fuel we are talking of between 6-10 USG remaining which kind of become “unusable fuel” in addition to whatever is unusable from the technical part. This should be well within the capabilities of most fuel indicators.

LSZH, Switzerland

With 45 minutes of fuel we are talking of between 6-10 USG remaining which kind of become “unusable fuel” in addition to whatever is unusable from the technical part. This should be well within the capabilities of most fuel indicators.

I strongly disagree I’ve seen all kinds of horrifying practices in GA. Start with the G-OMAR report.

Antonio is spot on about piston engine performance being repeatable (assuming negligible instrument errors, although leaning – with an EDM type instrument – for peak EGT is good enough) but that only leads to the winds aloft becoming the dominant factor which ensures that one has to fly with huge reserves (big enough to cripple the value of GA travel to short legs) unless one has a fuel totaliser. Even with great tools like windy.com I still find the forecasts to be off by 10-20kt.

Any POH that quotes speeds in km/h is deeply suspect – an old marketing trick especially in the UL business

Administrator
Shoreham EGKA, United Kingdom

Mooney_Driver wrote:

Final Reserve in CAT was different, not a fixed endurance but fuel for one go around, a new approach and landing.

From a quick reading of part-CAT it appears that the FRF required should give 45 minutes of flight with piston engines and 30 minutes with turbine engines. CAT.OP.MPA.181(c)(5).

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

Any POH that quotes speeds in km/h is deeply suspect – an old marketing trick especially in the UL business

Well, that is what I’ve been saying all along. You need to verify that performance.

I also fly with a totalizer but it also has pitfalls, particularly if the actual quantity of fuel on board at the get-go is not correct. Garbage in, Garbage out. And a lot of planes can’t simply be fuelled to the top all the time due to WnB reasons. So determining what to feed into the totalizer as FOB is sometimes quite inaccurate.

What the final reserve has changed now is that while before it was possible to fly down to 45’ of fuel left in tanks to your destination and use the 45 mins to then go elsewhere, with the new regulation that fuel is off limits. This reduces cruise time quite significantly, as right now you need those 45’ in tanks after landing at the alternate, not the destination.

This means that in practice, your fuel calculation looks different now as you need to carry reserves on top of the Final reserve.

Let’s say you got a plane which has 55 USG total usable, flies 150 kts and uses 10 GPH in the process and uses 4.5 USG and 30 NM to reach top of climb and 0.5 USG as taxi fuel..

With the old method, in this scenario you needed 7.5 GPH reserve to your destination, which leaves you with 42.5 USG for cruise. This means 4.25 hours EET and 635 NM cruise distance to a total of 665 NM plannable distance.

With the new method, those 7.5 USG de facto become unusable fuel. So in reality, your plannable fuel goes down from 55 USG to 47.5 USG. In this you need to include reserves for what the 45’ used to cover beforehand, alternate, holding, whatever, so in most cases another 45 mins or 7.5 USG for reserve fuel. That leaves 40 USG for the flight which translates in 35 USG available for cruise. That means 3.25 hrs EET, 525 NM of cruise to a total distance of 565 NM to destination.

That means a whopping 100 NM less than before.

Clearly, most of us have been planning like this before, use the 45 mins as a final reserve and then calculate alternate, holding, route reserve and what not on top of it. I’ve always done it that way. But for VFR in an area with lots of alternatives, going to 45 mins or 1 hr remaining was more the norm than the exception.

But at destination, this leaves you with rougly double the fuel you used to have, so rather than 6-8 USG we are talking 12-16 USG unusable now. No fuel indicator should be so inaccurate to show much below that quantity.

Last Edited by Mooney_Driver at 25 Nov 10:24
LSZH, Switzerland

Mooney_Driver wrote:

What the final reserve has changed now is that while before it was possible to fly down to 45’ of fuel left in tanks to your destination and use the 45 mins to then go elsewhere, with the new regulation that fuel is off limits. This reduces cruise time quite significantly, as right now you need those 45’ in tanks after landing at the alternate, not the destination.

For VFR, yes. For IFR you needed alternate fuel also with the old regs.

Clearly, most of us have been planning like this before, use the 45 mins as a final reserve and then calculate alternate, holding, route reserve and what not on top of it. I’ve always done it that way. But for VFR in an area with lots of alternatives, going to 45 mins or 1 hr remaining was more the norm than the exception.

Yes, that’s what I’ve always done as well. But I do not plan with an alternate for a VFR flight unless there are special circumstances.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

For VFR, yes. For IFR you needed alternate fuel also with the old regs.

Yep.

My normal fuel calc used to be:
- Taxi
- Trip (TFDC-Cruise-TFDD)
- Route reserve (usually 20% up to a predetermined maximum)
- Alternate
- Final Reserve 45mins.
= Minimum Block Fuel
- Additional Congincency (Wx, Hold, e.t.c.)
= Actual Block Fuel

The new one adds Contingency Fuel to the Minimum Block, as FR now is no longer usable fuel. For VFR Alternate and Contingency should be more than 45’ but rather Alternate plus 45 mins.

- Taxi
- Trip (TFDC-Cruise-TFDD)
- Route reserve (usually 20% up to a predetermined maximum)
- Alternate
- Contingency
- Final Reserve.
= Minimum Block Fuel
- Additional
= Actual Block Fuel

LSZH, Switzerland

If that is true then below-book performance has even more importance than before.

VFR is self-evident, but what is the actual change for an IFR flight? I don’t get it.

This is the old version for NCO.OP.125:

Can someone post the new version?

Last Edited by Antonio at 25 Nov 16:42
Antonio
LESB, Spain

Peter wrote:

Any POH that quotes speeds in km/h is deeply suspect – an old marketing trick especially in the UL business

Many are actually sold using SI units. km/h is very normal in the UL world as well as standard for gliders. km/h is Probably more often used than knots. Miler per hour on the other hand. That is marketing gimmick.

ENVA ENOP ENMO, Norway

Antonio wrote:

Can someone post the new version?

This is not the whole of the new version, but the parts relevant to the Final Reserve Fuel discussion.

NCO.OP.125 Fuel/energy and oil supply – aeroplanes and helicopters
(a) The pilot-in-command shall ensure that the quantity of fuel/energy and oil that is carried on board is sufficient, taking into account the meteorological conditions, any element affecting the performance of the aircraft, any delays that are expected in flight, and any contingencies that may reasonably be expected to affect the flight.
(b) The pilot-in-command shall plan a quantity of fuel/energy to be protected as final reserve fuel/energy to ensure a safe landing. The pilot-in-command shall take into account all of the following, and in the following order of priority, to determine the quantity of the final reserve fuel/energy:
..(1) the severity of the hazard to persons or property that may result from an emergency landing after fuel/energy starvation; and
..(2) the likelihood of unexpected circumstances that the final reserve fuel/energy may no longer be protected.
(c) The pilot-in-command shall commence a flight only if the aircraft carries sufficient fuel/energy and oil:
..(1) when no destination alternate is required, to fly to the aerodrome or operating site of intended landing, plus the final reserve fuel/energy; or
..(2) when a destination alternate is required, to fly to the aerodrome or operating site of intended landing, and thereafter, to an alternate aerodrome, plus the final reserve fuel/energy

AMC1 NCO.OP.125(b) Fuel/energy and oil supply — aeroplanes and helicopters
PLANNING CRITERIA — FINAL RESERVE FUEL/ENERGY
The final reserve fuel (FRF)/energy should be no less than the required fuel/energy to fly:
(a) for aeroplanes:
..(1) for 10 minutes at maximum continuous cruise power at 1 500 ft (450 m) above the destination under VFR by day, taking off and landing at the same aerodrome/landing site, and always remaining within sight of that aerodrome/landing site;
..(2) for 30 minutes at holding speed at 1 500 ft (450 m) above the destination under VFR by day; and
..(3) for 45 minutes at holding speed at 1 500 ft (450 m) above the destination or destination alternate aerodrome under VFR flights by night and IF

AMC3 NCO.OP.125(b) Fuel/energy and oil supply — aeroplanes and helicopters
FINAL RESERVE FUEL/ENERGY PROTECTION
The planned FRF/energy should be protected as a reserve in normal operations. If the fuel/energy on board falls below the FRF/energy, the pilot-in-command should consider this to be an emergency. The FRF/energy should not be used as contingency fuel in normal operations.
When the FRF/energy can no longer be protected, then a fuel/energy emergency should be declared and any landing option explored, including deviating from rules, operational procedures, and methods in the interest of safety (as per point CAT.GEN.MPA.105(b))

Last Edited by Airborne_Again at 25 Nov 17:16
ESKC (Uppsala/Sundbro), Sweden
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