Differences in engine speed A-10

[Evg85]

A-10S Has two types of sensors indicating engine speed:
Engine Core Speed,
Engine Fan Speed.
Can anyone explain the differences between them and in what situations, what indicators need to be taken into account?

[some1]

This is a turbofan engine, so it has two independently turning sections. High pressure (Core) that burns fuel and Low pressure which uses the power produced by the Core to turn the Fan at the front.

https://en.wikipedia.org/wiki/Turbofan#/media/File:Turbofan_operation.svg

The pilot mostly uses the Fan speed (N1 in other engines), as this is the part that produces most thrust of the engine. The core speed (N2) is important for startup and abnormal situations. But the startup process in the A-10 is automated, so for most of the time there is not much need to look at it in the sim. 

 

[Yurgon]

Personally, I hardly ever look at these instruments outside of engine starts or engine problems, as long as they're "in the green".

Among Hog pilots it's not uncommon to use the Interstage Turbine Temperature (ITT) as a reference, where lead might say "Setting ITT 650" instead of a fixed airspeed. While this is not an exact science and wingmen's engines may produce a little more or a little less thrust at the same ITT setting, it's a good ballpark measurement to let wingmen know roughly what power setting to use in order to keep up with lead.

[ASAP]

Generally core and fan speeds are more used for making sure the engines are healthy and you are getting expected performance. For instance, pilots have to check they have at least a predicted fan speed at a certain point on their takeoff roll.

Additions to what Yurgon said:

Pilots will set a specific ITT for something like an instrument trail departure where it's important that everyone has equal performance. They all set 800 ITT and they control speed with pitch to climb out at 200 knots. That should give everyone roughly equal climb performance, and nobody should be out climbing the other.

In most cases pilots set engine power based off fuel flows because it makes fuel management a lot easier. If you want 250 KIAS 1800 PPH per engine is a good starting point, 230 -> 1500 pph, 200 -> 1200 pph. Those are all ball park estimates to give you a starting point. From there you just adjust the throttles to hold speed.

Its particularly useful if your loitering in a hold while talking to a JTAC, everyone set 1500 PPH per engine and now everyone in the formation is burning gas at the same  rate and (roughly, wingmen will have to adjust from that to keep formation position, but it helps them to know what lead is doing). Also if you are cruising along at FL 230 and you set 1500 PPH per engine it's easy math for fuel burn rates: 1500pph x 2 engines = 3000 lbs per hour. if your destination is an hour and a half away you know you'll need 4,500 lbs of fuel to get there. That works for any fuel flow rate you pick. Again, if everyone in the flight has something similar then the flight lead can calculate how much gas the flight member with the lowest gas will have at the destination and adjust the plan accordingly.