Turbo-Prop optimal flight altitude

[leonard17F]

Dear Fellow PPruners,

I was wondering if, for max range, the optimal flight altitude for a (twin) Turbo-Prop airplane is always as high as possible or if there is a lower optimal altitude as a function of weight, possibly ISA, etc... (or even "Climbing Cruise")

[dixi188]

The L188 Electra (ok. 4 eng) Had a max Alt of 30,000 ft but I seem to recall the best range was between 20,000 and 25,000 depending on weight.
If you go for max altitude you will be hanging on the props so not very efficient.

[Tu.114]

Also, do not forget the influence of headwind or tailwind.

On the DH8D in still air and more so in a tailwind, FL250 is not only maximum, but also a good bet for optimum range. Reduce torque to long range cruise and just trundle along. Facing a strong headwind, you may well fare much better with rated cruise power and at a lower level: while you burn more fuel per unit of time, you will often burn much less per mile than higher up.

[pattern_is_full]

Given that "a (twin) turboprop" can mean anything from a Twotter to a Turbolet to a C90 to a Be1900 to a Q400/ATR (not to mention a dozen others), I'd guess "optimal" covers a fairly wide range.

About a decade ago I saw (perhaps on these forums) an extremely rough rule of thumb for turboprops - related soley to most efficient climb/cruise/descent (but ignoring temps, winds, weight, direction of travel, etc.) - to the effect that 1000 feet per 10 miles of total flight distance would be close to the optimal cruise altitude to plan. E.G. - for a 150 mile leg, plan 14K to 16K feet.

Does that even come close to approximating reality anymore?

[DaveReidUK]

Only up to a point.

Air Iceland fly Dash 8s between Aberdeen and Keflavik (720 nm). I suspect they don't stick to the "1000 feet per 10 miles of flight" rule of thumb. :O

[thetimesreader84]

The answer really is "it depends". The light twin I used to fly ( 2x PT6) was certified up to FL310. We used to regularly fly it at FL270, (we were non-RVSM) and it sipped fuel. If we wanted to get there a bit faster, we had a better TAS at FL210-FL240. A decent head / tail wind would remove any of those savings.

The large twin turboprop (PT126's) I also had time on was the same, but about 10,000' lower. Horses for courses.

[avionimc]

For King Air 200 and larger:

The approximate flight level for cruise should be equal to 4 times the flight time in minutes, not to exceed FL310 for the King Air 200, or FL330 for the B200. Or FL350 for the B350 (depending on how heavy you are); FL250 for the B1900D.

Cruise FL = 4 x (Total Minutes of Flight)

The total minutes of flight should be based on an approximate cruise TAS of 260/270 knots with additions for estimated wind component and climb/descent/approach delays.

Examples:

• A 30 minute flight should be flown at about 12 – 13,000 feet above ground level. (30 minutes x 4 = FL120, 12,000 feet).

• FL180 is appropriate for a flight lasting 45 minutes. (45 minutes x 4 = 180).

• For any flight lasting over one hour and twenty minutes (80 minutes), you should climb to the highest realistic altitude available, based on airplane weight and OAT (probably 290 – 330). Of course, without RVSM approval, your highest altitude will be FL280.

For the smaller King Air 90-series, use the 3x figure due to their lower rates of climb and lower maximum Delta P.

Source: Tom Clements, King Air Academy

[stilton]

The answer really is "it depends". The light twin I used to fly ( 2x PT6) was certified up to FL310. We used to regularly fly it at FL270, (we were non-RVSM) and it sipped fuel. If we wanted to get there a bit faster, we had a better TAS at FL210-FL240. A decent head / tail wind would remove any of those savings.

The large twin turboprop (PT126's) I also had time on was the same, but about 10,000' lower. Horses for courses.

Did you take an oath to never reveal the type of these aircraft :confused:

[tescoapp]

Q400 doesn't follow normal rules and is limited to FL250 due oxygen drop down in the back.

Even at MTOW your doing 1000ft/min between Fl240 and FL250 with ISA +10 doing 190 plus knts. Then it will quite happily the accelerate to 230 + knots.

Anything over 30 min sector it wouldn't be unusual to climb to either FL240 or FL250 cruise. At cruise you would burn a rough ton an hour.

[Piltdown Man]

Overall a Fokker 50 burns 600 kgs/hour at almost any level. To minimise fuel burn and flight time you flew for maximum ground speed, whatever the level. Yes we had marvellous fuel burn and cruise tables. They were basically useless. Given still winds, you climbed until your climb rate fell to 500 fpm or so and levelled off. That is your cruising level. If you ever looked like exceeding VNO, you climbed another 2,000'. This could result in a throttled back cruise at 6,000' to avoid a headwind or handling on the props at FL250 to get into a low jetstream.

[FougaMagister]

ATR: max altitude is FL250. Cruising high saves fuel. For the ATR72-200/212, for each 2000ft above FL180, fuel burn decreases by between 16 and 24kg/eng/hr depending on a/c weight and cruise FL ISA deviation. This is valid both at 77% NP (non-icing conditions) and 86% NP (icing conditions). In a nutshell: CRZ fuel burn will be around 600 kg/hour or less above FL210.

A graph in FCOM 3.04.02 shows the max FL that can be reached depending on weight and outside temperature (expressed in terms of deviation from ISA). This is worked out as the max FL the aircraft can reach with a residual rate of climb of 300 ft/min in normal conditions, and 100 ft/min (!) in icing conditions (IAS climb).

That said, choosing a high CRZ FL has to be weighed against sector time and headwind gradient. According to ATR data, should the wind gradient be less than 6 kts per 1000 ft increment, then climbing will save fuel, with very little increase in trip time.

Cheers

[Car RAMROD]

Avionimc, that's fairly good rule of thumb for the King Air. Tom knows his stuff!!

Another one for the B200, at least as an initial guess to what a good crz lvl is:
distance + 40 = FL. Obviosily adjusted for any local cruise rules re transition layer, really hot/heavy, significant winds etc
ie 180nm + 40 = FL220
80nm + 40 = FL120
240nm + 40 = FL280

[rigpiggy]

All depends I found on less than a 90 min flight on the B190 a diff in 30kts gs would actually favor lower levels. YSJ-YUL I went lots of times at 10-14k with the return 210-250. Nomally save .2 or better, and less fuel burns too