Stretching the stretches

Fast turboprops hold their own up to about 350nm, beyond that the Mach 0. 75/78 cruise of jets beats the best turboprops at just over Mach 0. 5 cruise.

The current ‘king of the props’ for outright performance is arguably the 70-seat class Bombardier Q400 with its 360kt cruise and max 1360nm range, faster and further than ATR’s similarly sized ATR 72. Now, a further stretch of the Dash 8 is being openly canvassed, causing ATR to rethink again.

Some very interesting debate has long gone on the merits of the Q400 against the ATR72. The Q400 has advantages in climb and about 80kt higher cruise speed, while ATR counters with the 72’s more frugal fuel burn.

Both have the advantages of being able to use airstairs or steps for passenger loading, and holds can be loaded off the ground, with sill levels up to 1. 5m enabling minimal ground handling equipment.

Stretching either aircraft to 92-seats at 31in pitch – well into larger regional jet territory – would require a stretch of about 8m, requiring longer undercarriage legs to maintain present rotation angles, raising floor and sill levels.

ATR has mooted a speed just under the Q400’s, but going to even 30kt above it saves so little time that it would be wasted.

Like their jet competitors, all taxi at the same speed in queues, go nowhere at holding points and in holding patterns, are slowed or vectored to lose time, or are in long uniform descent/ approach speed streams.

The Q400 has excellent climb performance with most using a constantly reducing climb speed above 15,000ft (the Convair 580 climbs at a constant attitude to achieve steepness).

Q400s, when up against jets on the same runs seek higher speed climbs and still make standard cruising levels in good time, and can rapidly go to 20,000ft or higher on a 30 minute run to beat much weather.

Many cite 150-250nm stage lengths as optimal for turboprops, but QantasLink’s longest domestic Q400 RPT route is Brisbane-Mackay at 430nm, while the type also flies 454nm on the Cairns-Port Moresby route and 800nm on a Brisbane-Cloncurry mine flight. On shorter stages, such as Brisbane-Rockhampton, Sydney-Port Macquarie and Melbourne-Mildura, the jets average only five minutes faster than the turboprops.

On Mackay-Brisbane, where the Q400 beats the old BAe 146’s times, E-Jets average 10-15 minutes faster while ATR-72s would take about 20 minutes longer. On Cairns- Port Moresby the Fokker 100 beats the Q400 by 25 mins.

To equal this matrix, a stretched ATR would need to cruise at 330kt TAS (a common Q400 cruise speed) at 25,000ft on 350nm+ stages.

While an edge of 8-10 minutes each hour does not put the ATR in a bad light against the Q400, the jets have an edge over the current ATR model.

Bear in mind too that routes such as Brisbane-Rockhampton and longer have seen the entry of low cost carriers with 737/320s. This is countered by the better 2+2 seating on E-Jets and Q400s, the latter with full service. But jets then have an ability to cruise above 25,000ft to beat bad weather.

The old chestnut of runway strength is another factor.

Q400 and ATR72 loads reach approximately ACN 20, with ACN a product of weight, bogie configuration, tyre pressure and wheel base.

It would be highly desirable to keep their growth versions in a similar range which is under smaller jets and does not overload many of the airports now getting Q400 services.

That would avoid damaging weaker runways, such as Gladstone’s expensive experience.

In the future, we will surely see Qantas ‘Q500s’ at ports that have grown to support Q400s and on present trends will call for 90-seaters in a few years.

Assuming Rex still operates only Saab 340s, it could hold a tremendous frequency edge, but on longer sectors the speed and seat/km costs of a much bigger prop might be decisive.