Back course question

[n421nj]

I know when flying a back course the needles are reversed but is there a way to make it so the needles aren’t reversed?

[Blitzer]

Fly backwards?

[ilya1502]

On the KI525 indicator this should not be the case if you set your back course as the front course, i. e. if you're approaching runway 36 on the back course, don't set your CRS to 360, but to 180 instead.

[Oracle427]

Is that so? I was under the impression that a localizer does not transmit radials like a VOR, so the OBS has no purpose. Interesting...

[n421nj]

Hmmm I’ll have to try turning the obs to the reciprocal radial and see if it works like a front course. Lol

[AKar]

OBS has no effect here. Localizer is completely unrelated technology to the VOR, and transmits no radials the receiver could identify. One needs to be a little bit careful to follow what's going on here: needle does remain reversed, however, you are free to rotate it 180° so it's left/right movement is once again kind of reversed in your view. I am not an immediate fan of reversing the course indicator, because when challenged in unexpected circumstances, like somewhat loosing my bearings during missed approach, it would take some extra presence of mind to remain constantly aware of that one needle pointing 180° off the expected.

-Esa

[n421nj]

So better to just fly the approach with the needle reversed.

[Oracle427]

Oh I understand, this is an HSI... Yes, that sounds risky to flip the course like that. Just remember to "pull/drag the needle" when on backcourse.

[MrMe85]

While I do understand the argument of being disorientated for something like a missed approach, I would also like to argue that shooting a backcourse may be akin to riding a bicycle with the controls backwards, and is not as easy as it sounds to do, especially if you are fighting gusty winds. Back courses are not all that common, and the day you have to shoot one and having to think in reverse is likely the day you will destabilise your approach.

You can still execute your approach and missed approach safely so long as you have an appropriate plan of action if you do decide to flip the needles, which you should have for any IFR approach anyways.

I think personally, if I don’t have any other guidance other than my needles, I would consider flipping the needles. If I have a GPS with a moving map, or a TRK/DTK output, I’d keep the needles where they are (not flipped) and crosscheck both instruments to keep my track centered on the LOC.

[joediamond]

So better to just fly the approach with the needle reversed.

To do so would negate the benefit of having an HSI altogether. If you are flying a coupled back course approach using the BC mode you need to have the front course set on the CDI.

[AKar]

KFC 200 is apparently designed so that front course should be dialed for back course approach. Never thought about it, as I don't really do back courses except for reversals to the front course, but it makes sense when doing some mental flying over a map. Anyways, as a result, the needle movement appears normal, but actually is reversed and then inverted.

-Esa

[some1 - A2A]

The autopilot does not need course information to follow VOR/Localizer, but it needs VOR/ILS course information to fly the correct intercept profile. This is a general rule for all autopilots.

Depending on the implementation, you should set front course or back course on the HSI, always consult your manual

[AKar]

Yep, that's right, and it actually makes sense to have front course dialed in because it is consistent when arbitrarily overflying the transmitter. However, it requires the HSI system to be able to resolve course datums around 180° off the nose, not just linear for ±30...45° or whatever, and constant signal beyond, which is the default case. This is specifically accounted for in design of KI 525, which provides linear course offset for ±80° from both upper and lower lubber lines.

-Esa

[ilya1502]

Yep, that's right, and it actually makes sense to have front course dialed in because it is consistent when arbitrarily overflying the transmitter. However, it requires the HSI system to be able to resolve course datums around 180° off the nose, not just linear for ±30...45° or whatever, and constant signal beyond, which is the default case. This is specifically accounted for in design of KI 525, which provides linear course offset for ±80° from both upper and lower lubber lines.

Esa, I am a bit confused here. Can you, please, explain the same thing in maybe more simple terms?

[AKar]

Esa, I am a bit confused here. Can you, please, explain the same thing in maybe more simple terms?

There are certain implications on design and construction of these high end avionics. When you set your heading bug, the system don't register your input as 090°, as an example, but as an offset from the nose, or the top lubber line of your instrument. The system don't care about the actual heading you think you command, but simply how far and which way the bug is from the nose. Typically, distance from the nose is sensed linearly only for a limited range off the nose, in case of KI 525 that's apparently maximum of 30° to either side. Beyond that, the bug is simply kind of off-scale far away, and the system only knows which way it is off, and that it is off by more than 30°.

Offset from the course indicator is similarly picked off, however, the linear range is extended to 80° off either side from the nose, again in case of KI 525, probably to allow for steeper intercept angles. Now, this would still be useless for any backcourse stuff, because you'd still be operating beyond the linear range of the course reference when the front course is set up. This is why the course datum pickoff is duplicated symmetrically for the bottom half of the instrument.

The KI 525 uses a set of optoelectronics and shutter cams to pick the heading and course datums set by the user. It is altogether a modestly complex instrument.

-Esa