Direction indicator on Concorde

[spikeair]

I've watched a very good video about Concorde from ITVV detailing flights from LHR to JFK and back. I have one question concerning the direction indicator, I recall the captain stating that the direction indicator shows the heading in degrees TRUE rather than MAGNETIC. If that is the case, how does it know what the magnetic variation is at any given location in order to work out the TRUE heading?

[spikeair]

Ppreciate that knowing your heading in degrees TRUE is a good thing. I;m just wondering how the equipment works that out.
My understanding is that Degrees TRUE is worked out from Maganetic heading (compass) plus/minus magnetic variation, (which is different depending upon where you are on the planet, and which changes slowly over time)
IS it held in a computer database on board? If the plane knows where it is, does it look up magnetic variation in order to determine what the TRUE heading is?

[Dan Winterland]

Like most steam driven HSIs, I suspect the concorde HSI has a choice of inputs. Either NAV or INS. In NAV, the heading source will be the compass systems, therefore the data will be magnetic heading. In INS, the raw data will be true as the INS works in true. Those HSIs which dispaly mag heading in INS will require a database of magnetic deviation to be able to compute a mag heading. Some have a mag/true select switch.

Glass cockpits have a mag/true select switch. Most company SOPs state the switch is to be left in mag (variation correctin needed as the glass NAV dispaly will derive it's info from the INS) and mag will be displayed until the aircraft flys above 73N or below 60S when true will automaticly be displayed. (this is a bit of a generalisation as some operators change these values).

[Bellerophon]

spikeair

What you called the direction indicator on Concorde is actually more correctly referred to as a Horizontal Situation Indicator, or HSI for short, to show that it displays lots of other useful information as well as the aircraft heading.

The heading information is displayed by a ring shaped compass card rotating against a fixed outer white lubber line.


http://www.concordesst.com/inside/cockpittour/flightcontrols/pictures/c9.jpg

Photo courtesy of, and copyright to, Gordonroxburgh, from his ConcordeSST.Com website.


As others have correctly said, there is a choice between displaying a TRUE heading (derived from the INS, which only calculates in degrees true) and a MAGNETIC heading (obtained from a compass coupler which senses magnetic heading directly).

Either display may be used, switching is by means of a RAD/INS switch mounted near the HSI, however the position of the RAD/INS switch will also determine what sort of tracking information can be displayed inside the compass ring.

To look at VOR or ILS information you must be in RAD, to look at INS information, you must be in INS.

In very general terms, Concorde will be in RAD, using magnetic headings, during the start and end of a flight, and in INS, using true headings, during the ocean crossing in the middle.

Regards

Bellerophon

[twistedenginestarter]

Not that this is relevant but magnetic versus true is not actually such a big deal these days. My Magellan GPS does both. That cost £100 which is a lot cheaper than a Concorde.:O :p :cool: :ok:

[Hand Solo]

Yeah but I bet it doesn't work so well at Mach 2!