Why is the primary analog airspeed indicator showing CAS and not IAS?

[Xupicor]

Why is the analog airspeed indicator showing CAS? Is that some quirk of the version of A-10A we have in DCS, a quirk of the FC3 systems modeling or just something that slipped under the radar all those years ago?

 

So, we flew A-10C and the A-10A side by side yesterday and found that our airspeed indicators differ in what they show us a bit. It led me to manual hunting and from what I found, long story short:

- DCS A-10A has both analog gauge and HUD showing CAS (as per DCS A-10A manual),

- DCS A-10C has analog gauge showing IAS, HUD showing IAS by default but there's an ability to change the HUD display to CAS (as per DCS A-10C manual)

In case of A-10A it might not be entirely correct, not as is.

 

A real life A-10A manual I'm looking at references two different versions with different HUD, where one is showing IAS ([61], earlier version) and the other... Well, it's more complicated with the latter one, but that's the one that looks more like our DCS A-10A is using (that's referenced as [62]). Now, most if not all illustrations and written procedures (both normal and emergency) that are in the manual are referencing speeds in IAS (until you come to math exercises where the topic of IAS, CAS, TAS and GS is handled), so it would be reasonable (to me) to have IAS display somewhere in the airplane, and a reliable, primary one. HUD was, according to the manual, not supposed to be relied upon completely -- quoting the manual: "WARNING • The HUD is not a primary flight instrument and should not be used as a substitute for the airspeed indicator and altimeter for takeoff and landing". I would think that the primary airspeed indicator would be the analog airspeed gauge like it is in A-10C, and the following is why I think so.

From the A-10A manual: 

Quote

FLIGHT INSTRUMENTS
PITOT-STATIC SYSTEM
The pitot-static system consists of a pitot-static probe, located in a boom on the leading edge of the right wing. The probe supplies impact air pressure to the airspeed indicator and "Q" sensors. Static pressure is supplied to the airspeed indicator, "Q" sensors, altitude computer on [61], the CADC on [62], and the VVI.
The pitot-static system functions automatically. However, the pitot-static probe contains a heating device controlled by a switch in the cockpit.

(1-69)

Quote

Airspeed Indicator
[...] The airspeed indicator operates from inputs of impact and static pressures. On [61], the airspeed indicator provides speed data to the
HUD, and the HUD airspeed scale should read within 4 knots of the cockpit airspeed indicator.

(1-70)

Again, [61] here references the version with the earlier HUD which has airspeed tape with IAS speeds (look below).

[62] references the latter version with the CADC and the HUD looking closer to our DCS A-10A.

In [61] the HUD mode selector switch in FLT provides...

Quote

a manually depressible pipper, a mil readout, and aircraft pitch, roll, altitude, and indicated airspeed data. Within limits, the flight symbology will follow the pipper depression position . See figure 1-68.

(1-153)

In [62], the version that interests us more, the mode selector switch is a little different. Instead of FLT setting there's a NAV setting, which is...

Quote

Used during normal point-to-point flight. The display is centered in azimuth and the flight path ladder rotates at the 41 mil depression to display roll. There are two NAV mode displays, INS and HARS. Symbols displayed in the INS NAV (normal) mode are flight path ladder, destination
index, TVV, distance to go, time to go, TISL diamond and line, airspeed, altitude, mil depression, pipper, steerpoint, and the heading scale. Setting the function knob to OPT and depressing once (option A) adds vertical velocity to the normal display. Depressing twice (option B) adds vertical velocity, but deletes the destination index, time to go, and distance to go. Symbols displayed in the HARS NAV mode are airspeed, altitude, pipper, TISL diamond and line, pitch angle (boxed), roll bars, and the mil depression. See figure 1-69.

(1-154)

 Then, also in [62] we have a "function selector switch" which features a TAPE option. To quote the manual yet again, with TAPE:

Quote

Depressing the knob once displays a tape of calibrated airspeed and altitude. Depressing again returns airspeed and altitude displays to numerics.

(1-155)

Now, this can be interpreted in a number of ways, but I'd lean on the interpretation that the default numeric display is IAS and the optional tape gives you CAS. A bit later on you can find that the optional tape (as if there were doubts) is CAS for sure, and the deliberate looking omission of any qualifiers like "calibrated" from other mentions of airspeed seem to be indicative that either IAS is meant or the distinction isn't relevant to the context of a sentence in question. In context of describing airspeed indicators, be it HUD or analog gauges, this distinction would be very much relevant, so it's just a little bit strange they didn't clarify it more.

Quote

Airspeed and Altitude Readouts
Airspeed is presented from 50 - 500 knots. Altitude is presented from -2,000 to 38,000 feet. The airspeed and altitude readouts will flash when a master caution signal is received.

Airspeed Tape
The optional airspeed tape, when selected, provides a moving scale and fixed index of CAS. The operational range is from 50 - 500 KCAS. The moving portion of the display represents a total of 100 knots full scale, with increment marks at 10-knot intervals and elongated marks, with numerics in knots, at 50-knot intervals. The airspeed tape flashes when a master caution signal is received.

(1-165)

 

So in progress of making this post I've become less and less convinced that the HUD in DCS A-10A should show CAS, by default anyway, but it's not entirely clear. SME opinion would be nice to have.

In my interpretation:

- in [62] it very well could be IAS in the numeric view (as it's not ever mentioned that it's CAS) and it should be CAS for sure with the optional TAPE switched on, as that's what they outright say it is in that mode. We don't seem to have a function selector available to switch the TAPE on, though.

- as for the earlier HUD in [61] it should be IAS as per what's written, but that doesn't seem to be the one we're flying, so this is just an aside. Adding to the evidence, though, that IAS on the HUD is not out of ordinary for A-10As.

- lastly, and most importantly to the topic, I did not find and indication that the primary airspeed indicator, the analog gauge, is showing anything else than IAS in either A-10A [61], A-10A [62] or A-10C, since it's described as getting static pressure and impact pressure from the pitot tube and isn't described in the manual as being corrected in any way to display a calibrated air speed. Furthermore, in "CAUTION LIGHT ANALYSIS" section you will read that when CADC caution light is on, indicating CADC failure, your corrective action should be to "Monitor pitot-static airspeed indicator and select STBY or PNEU on altimeter. Deselect mode C on IFF." (3-43)

It very much looks like the primary airspeed indicator should be purely IAS, and not CAS as is at the moment.

 

 

I like the new textures on the FC3 aircraft and that work will surely bring more people to fly them if only because of the nostalgic value for the aircraft in it or the time spent in the oldie goldie times of Lock On. It would be nice if some kind of work was put into reviewing the systems modeling in these aircraft as well, at some point. With the amount of work ahead of ED I'm not going to fool myself into thinking that this post will trigger an immediate action, or one being realized this decade, but hey, you have to start somewhere.

If you, dear reader, have made it this far, you deserve a heartfelt thanks. Enjoy your Sunday, folks. :)