TurboProp Speeds in Cruise and Descent
 

Howdy, I'm in the process of developing a range of aircraft performance profiles, in particular airspeed information, for use in simulation and generation of predicted flight-paths. Flight Radar 24 is useful for this, especially where mode-S information is available. However, there seems to be great fluctuation in the IAS information that is reported at phases of flight where I'd expected it to be nearly constant. I'd appreciate it if anyone could provide some deeper insight into the cruise and descent speeds that are adopted for Dh8C, BE20, B350, SW4 and SF34 aircraft.

In particular, I;d like to know details on the following:

1. Is the primary reference for cruise speed IAS or TAS? Obviously the FPL is submitted with TAS, but I'd assumed that the reference speed used for cruising is IAS. However, I notice that companies often submit flightplans for the same sector with the same TAS regardless of the planned leve, implying that TAS is the true reference (this may be for administrative efficiency)/. I also note that looking at FR24, the IAS in the cruise often fluctuates more widely than the TAS.

2. If the reference speed is IAS,, is this commonly the same value across a range of levels, given the same external factors. For example, is a DH8C operating at F140 likely to have the same/similar IAS in the cruise as one at F220?

I understand that many factors contribute to the speed schedule adopted by the pilot, and I'm just trying to get a handle on some basic precepts to get started with the modelling. Any help is appreciated.

Rgds PP

1/ We would fly an IAS if ATC asked us, but there would be limited ability to chose cruise speeds. Essentially we were limited by engine performance (usually ITT or Ng, very occasionally torque). You'd set max chat, and accept what speed youd get. For flight planning, you'd generally pick a TAS that you'd use for planning (eg from the performance manual) and use that.

2/ I've not flown the Dash 8, but a similar turboprop with the same engines i remember we usually were flying at the same IAS (+/- 15KTS maybe), but TAS changed with altitude & more importantly temperature.

Hi TImesReader

Thanks, so t sounds like, rather than having a target speed, cruise profile is driven by engine performance and power settings resulting in an IAS/TAS that will fluctuate depending on the pressure/temperature. That's very useful thanks.

Regrds