Slowing down to save money in a turboprop. Does it really work ?

[Blind Billy]

Hi Pruners

I'm wanting to gauge peoples / operators experience on this. Does reducing power really save money on a turboprop. On a larger jet, fine. But lets take the 30 to 80 seat range of turboprops nipping in and out of regional ports, short flights lots of cycles.

There are a number of operators out there who are reducing fuel flows in cruise to a more "efficient" power but in turn shaving a few knots off the speed. The result is obviously increased flight times and supposedly reduced sector fuel burns. However, with increased average flight times added up over say a maintenance cycle, this results in reduced flights and passengers moved per cycle (or per dollar of maintenance spent). Over a large (and aging, maintenance intensive) fleet there can be some big numbers here.

I know of one operator that experimented with slow downs over a year in the early 2000s, saved them bucket loads in fuel but cost them twice the savings,- in maintenance.


So what wins ? A few pounds or kgs of fuel saved added up or the increased cost of maintenance per flight or per person ? Interested to hear your thoughts and experiences.

[KenV]

I flew P-3s for years, which are based on the L-188 commercial turboprop airliner. To use the minimum of fuel for a given distance, we flew max range airspeed. That results in maximum miles per gallon fuel. Slowing down below max range airspeed would result in lower fuel burned per hour, but more fuel burned per mile flown. Another way of looking at it is to maximize range (which is to say maximize miles per gallon) increase speed until the per cent increase in fuel flow equals the per cent increase in airspeed. This assumes a no wind scenario. There are factors to adjust for head or tail wind.

Because we had a patrol mission and needed to stay "on station" for the maximum period, we routinely slowed down to max endurance airspeed which resulted in minimum fuel burn per hour. As we burned down our fuel and got light enough, we'd shut down one engine to save fuel. On some missions we'd shut down two engines. We always used all four engines for the return flight.

So in summary, we used max range airspeed to get to our on-station location, then used max endurance airspeed to stay on station, and then used max range airspeed to get back home.

[Sidestick_n_Rudder]

Hard to give a definite answer without more data and turboprop operating manuals contain quite sparse info.

In most medium/heavy jets this issue is addressed by using cost index - if properly calculated it enables to establish a speed schedule, which will yield the lowest total cost, taking into account both fuel- and time-related costs.

It definitely does not make sense to fly max speed, as the fuel cost will skyrocket, nor does it make sense to fly too slowly.

There surely is some optimum speed to fly, but as I remember from my turboprop days - nobody knew what it was :}

Having said that, turboprops burn quite little fuel compared to heavier jets, so probably slowing down does not provide significant savings. Extending time OTOH could influence maintenance cost.

My guess is that’s why nobody bothers to use cost index and t-props manuals normally only contains performance data calculated with MAX CRZ PWR.

[WHBM]

It needs appropriate calculation. Fuel costs per hour may be saved with reduced speed, but per mile will follow a curve, just like they do if you gun it at maximum speed. And you are going to need more hours for the trip.

Very many of aviation's costs, such as crew, interval-based maintenance, even some overheads such as leasing and insurance, are based at least in part on hours. So to provide the same service can increase these costs. The only approach is to have an accountant work it out.

Losing out on achievable sectors per day is a lesser issue, as many turboprop operations, certainly on a business day, are one worthwhile return trip morning, and one evening, and the rest either marginal stuff or standing. It would be poor if the morning round trip previously 0700-0745/0805-0850 became 0650-0750/0810-0910, as the user perception of a before 0700 departure on the first leg, or an after 0900 arrival on the second, may lose you some custom. Don't think that cheeseparing of costs allows you to maintain the same revenue.

[DaveReidUK]

Long range cruise. The clue's in the name.

[H Peacock]

Often not the case for best range Dave. LRC is invariably the IAS for max range, plus a bit, ie slightly quicker than max range speed with a slightly higher fuel burn. Therefore you get a slightly quicker transit for a small additional fuel burn.

It then gets more complex when you factor the wind. With a head-wind you will get a better range by flying slightly faster than the still-air max range speed, with a tail-wind you go a tad slower. Use the FMS Perf pages to find the altitude/IAS for best SAR (Specific Air Range).

[Sidestick_n_Rudder]

Still, long range cruise DOES NOT equal ECON SPEED! It only gives you max SFC (or actually 99% of max SFC), ie. min fuel consumption per miles flown. It does not take into account maintenance and time related cost. The ECON speed will be somewhat higher than Max Range Cruise. The higher the time-related cost, the higher the ECON speed.

[KayPam]

True.
For instance if you rent an airplane only by the hour, fuel included, your best price will be full power during the entire flight.

[JammedStab]

We used to have our alternate leg based on LRC.....ATR series. I suppose the performance numbers showed it to be better. But I don't think any headwind/tailwind component was used in the calculation. I do remember on two hour diversion to alternate on minimum fuel, we had a very strong headwind for much of the flight, and the performance tables had no consideration for headwind component. All I could do was adjust power settings and see how it affected arrival calculations. Plus the wind was forecast to drop off significantly after one hour making it more difficult. Somehow we landed exactly at our 900 pound reserve fuel minimum.

As far as I know, the modern Boeing FMC's base their LRC calculations on zero wind as well.

Correct me if I am wrong but I believe these performance numbers are giving you max (or 99%) air miles not ground miles which is the important thing to know.

[Intrance]

Lots of interesting talk about speeds, but not much about maintenance cost. If you know (even fairly roughly) what your plane costs in terms of maintenance per time unit, then you can properly make up the balance of fuel cost and maintenance cost.

In a previous company we had some guys always wanting to go as close to service ceiling as possible, because fuel flow would be lower etc. Meanwhile, they were running the engines fairly hot and kept high RPM to get there and anything they saved in fuel was later basically burned up twice by the engine needing heavy maintenance much sooner due to running it hard all the time.

[ShyTorque]

It's a similar issue with helicopters, which are in effect turbo-props, where quite often the major factor is headwind / tailwind. On one type I flew for offshore SAR, we carried out trials to gain more data than the Flight Manuals provided. Our concern wasn't so much getting there, but getting back to landfall again. We found the best way to minimise fuel used was a cruise climb to altitude (less than 10,000 feet in our case because we were unpressurised) and then a slightly reduced cruise speed if there was a tailwind. If there was a headwind, reducing the cruise speed was pointless because it was offset by a longer airborne time. There was a page in the FMS that showed I specific fuel consumption, i.e. Fuel being burned per mile, which was a big help. Obviously, mainly because of the wind factor, it varied from flight to flight.

[Vessbot]

Save fuel, yes. Save money, maybe. Two separate questions, and it's important to be clear about which one you're addressing.

[Piltdown Man]

Only if you are flying exotic aircraft long distances at high(er) altitudes does slowing down possibly make any sense. The best way of saving money is to fly at a level that will give you the best possible groundspeed, whether that is at FL50 or FL250 (or more for the posher types). The time taken for climbs and ATC interference in descents wrecks meaningful fuel savings gained by flying at optimal speeds. The biggest savings are gained by “directs” and short visual approaches at destination.

[krismiler]

Turboprop operators probably wouldn’t have the same flight planning resources as a major airline operating wide body jets long haul. The fligh planning department have access to huge amounts of data and can plan routes accordingly taking advantage of favourable winds and minimising unfavourable ones.

A London - Los Angeles flight would have several possible routes and significant savings can be made by selecting the most economical track, which may not be the shortest distance.

Turboprop routes are generally short and little variation is possible. A few “rules of thumb” would probably cover most operators.