B.O.G.O.F is nearly right. There are three basic reasons. 1. Economics. 2. Economics 3. Economics. The bottom line is the bottom line.
As average stage length decreases, especially below about 400nm, the operating costs of jets increase more rapidly than T/P. Among many reasons jets just don't get to spend enough time up at "their" altitudes on short sectors.
Turbo-props come out in front of current generation jet aircraft in 2 major areas -
(1) Better short runway performance, and
(2) Better economics for short distance flights.
If the airfield has only short runways, there is no choice, the Turbo-prop will win.
For short distance flights, even at the significantly lower cruise speed typical of Turbo-props, the sector time will differ only slightly. In comparing 250 Kt and 450 Kt speeds over a 150 mile sector, there is only a 16 minute difference.
Odd but true. Is it not the case that the art of the jet is to fly fast enough to increase L/D dramatically? The jet at a mere 365ktas obtains for the sake of argument an L/D of 14.5 and burns 0.65lb of fuel per hour per pound of thrust. So if the jet weighs, shall we say again, 34500lb it will cover 240nm per 1000lb of fuel according to Breguet’s formula. A turboprop having the same weight will also cover 240nm per 1000lb of fuel providing that the following stipulations are met. Firstly actual speed in ktas is not specified but is derived from the necessity of maintaining an identical L/D ratio, in this case 14.5. Secondly the ratio of propeller power efficiency to fuel consumption in lbs per horsepower hour must be 1.75:1, which is easily met, say 85% and 0.486lb/bhp-hr typical of the PWC 120A in cruise. The jet Breguet formula gives nm = (V/c)*(L/D)*ln(W1/W2); slightly differently the prop Breguet formula gives nm/lb = 325.65*(e/c)*(L/D)*(1/W). I know that the former is a range and the latter is a range per pound of fuel but only the former explicitly states speed. Also yup the jet sfc is lb/lb-hr and the prop sfc is lb/hp-hr but equating nm at speed V and nm/lb for the same L/D is an easy mathematical problem. With the prop you have to derive speed from the requirement that L/D must have a specified value. But the prop craft finds it harder and harder to improve L/D because the faster it goes the worse becomes its prop efficiency “e” unless it has a very high wing with very long undercart legs to accommodate a very big prop which gets lossy anyway due to the blade tip speed. So by this example at 365ktas the two types are equal in terms of fuel consumption but not speed. The prop is flat-out Charlie but the jet has speed in hand. Should the jet increase speed its L/D gets better and should it climb higher its drag gets lower. To match it the prop can only fly faster and its propeller efficiency gets worse defeating both objects of the game to fly faster and get higher. Meanwhile the jet consumes much the same in lb/hr for each pound of thrust. So the prop cannot fly fast enough nor high enough to compete in the L/D war but is much, much more fun. Not only back every afternoon but back several times in the afternoon! Aaaahhh, I’m getting old and loving it!
Regional flights require usually 50 to 70 seater and if you compare the price for equivalent seating between jets and turboprops this is also one reason. But it is true that some pax do not like propeller aircraft but they will fly in helicopter! Some prefabricated ideas...
Although turboprops are cheaper to run both from flight time perspective as well as from charges perspective I have the impression that from anything larger than an ATR42 it is worth going to jet (particularly as a start up) since it gives more options more destinations larger pax loads etc.
I am not sure if it makes sence but the idea is a 100-seater be it a jungle jet or an arctic jet... it can flow either short distance or longer ones...
Re pax preference a regional airlines in Southeast Europe phase out its ATR-72 in favour of RJ-100. I am unable to confirm if this is due to fleet uniformity (sp) or to gain a marketing perspective of all jet fleet.
The Q400 is very quiet compared to other turboprops. A nice machine all round.
Another operational benefit of a turboprop over a jet is it's ability to slow down alot faster on an approach than a jet, meaning you can hold a higher speed until much closer in. So much so that I was once asked to slow down as we were catching up a Ryanair 73 in front of us, and that was in a Shorts 3-60! Doesn't happen often though!
It's the Q400 which really turns this whole debate on it's head.
-NVS cabin noise suppression system
-same per-seat costs as B737 but with less seats-ideal for thinner routes that a B737 would be too big for
-need only be 1/3-1/2 full to break even
-comfy spacious interior
-STOL like it's predecessors
It really is a revolutionary a/c, and a lot cheaper to run than the CRJ. Bombardier have scaled down production of the CRJ in favour of expanding the Q-series production line!
Answer: as ever - Money! TP's are cheap(er)! Take Fokker 50 vs Fokker 100. Using "ish" figures F50: £1,000/hour, 260 kts, 50 pax. F100: £2,500/hr, 420 kts, 100 pax. So how many punters how far is the question? Small airlines: Few and not very far!