Which DME receiver do you have and where did you see the large number?
King KDM 705, a very large box with 500W transmission power, originally taken from a Learjet. Usually not found in small aircraft but it was there when I bought the plane 
The problem was a dried out capacitor and it surfaced in flat terrain. I had it repaired and it is working fine again but this episode made me aware that one should never trust a single instrument and always cross check. Therefore I have the GPS with distance to threshold as my 2nd distance cross check.
The much worse case I once had was a clogged static port which made me go 200ft below the approach on an RNAV approach. This taught me a lession, I now always have GPS altitude displaced and compare them. If they are different, the lower number counts.
I talked with the DFS today. While it is, of course, not legal in Germany the DME stations in the GPS database will give you the correct distance at all German Airports. Of course the slant range is a bit different but the error is so small on the ILS that the difference is not relevant.
Yes, i know it is not legal, nobody has to repeat that ;-)
If you look at the smallprint of the Jeppesen Chart for the ILS Approach 28 at EDDE (Chart 12-2, issue date 1 JUN 12) it says in bold typeface: “DME required”. Without DME, you can not fly this approach. (Well, you can, but you are not allowed to and it may be unsafe.) You would have to fly the RNAV-GPS approach instead, which is also a published and official procedure.
I would agree that it may not be legal if the State doesn’t permit GPS substitution, but it has been proven to be safe by more than fifteen years of operational use in the US since it was first authorized during July 1998, late in the last century 
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Distance coding in ILS-approaches (either via DME, GPS, marker beacons, locator beacons or cross radials) serves the purpose of making sure to have intercepted the correct glideslope signal (and not a sidelobe) and to verify that your glideslope indication is valid. Without distance checks (or with selfmade ones) you take away the only safety net built into an approach, that will take you down to 200ft above the ground (or even 140ft if your altimeter has the maximum permissible error!). A lot of people get away with it, but quite a few have flown themselves into the ground or into hillsides.
ILS false glidepaths are at multiples of the GS angle, so 6 degrees, 9 degrees, … They alternate with reverse sensing and correct sensing, so the first false GS that has correct sensing is at 9 degrees. I would hope a pilot could figure that out. Flying a false GS is more a function of not intercepting the GS at the charted altitude and intercepting the GS from above. In the US, controllers are not permitted to vector an aircraft to a vertically guided approach at a point that is above the GS and ideally uses the charted intercept altitude. The cross check of the altimeter with on GS indication is a good practice, primarily to verify the altimeter setting is correct. It can point out a grossly miss set altimeter, but does not have the precision of more than +/- 100 feet. This is less of an issue in the US, since we use QNH below 18,000 MSL. It can also point out a badly operating GS indication, more likely to be caused by own ship equipment than an out of specification GS ground system which should have 24/7 monitoring and flag out of tolerance. All this being said, the GPS can handle this function just fine.
They alternate with reverse sensing and correct sensing, so the first false GS that has correct sensing is at 9 degrees. I would hope a pilot could figure that out.
I know somebody who, many years ago and outside Europe tried to fly an ILS with the GS wires reversed, to see what would happen.
The autopilot couples allright and starts going down the GS. The VS starts to go up and continues to go up. I would put the life expectancy at 1-2 minutes.
However that was on the 3 deg GS, not the 6 or the 9. I would expect the 9 to couple and be flown normally albeit with a “very interesting” VS and probably getting close to Vne at the DH
The 6 would be interesting too. The autopilot should make you fly into the ground so eventually you should intercept the 3 deg one. Or maybe an intercept of the 6 deg GS will send you upwards… I would need to think about that. It depends on whether you intercept from above or below but if intercepting the 6 those get reversed too
I would hope a pilot could figure that out.
Obviously not. This report here has been submitted to all commercial operators in Germany (as a warning) recently: http://news.aviation-safety.net/2013/11/22/dutch-safety-board-issues-alert-on-unexpected-autopilot-behaviour-on-ils-approaches/
… I would agree that it may not be legal if the State doesn’t permit GPS substitution, …
Legal or not, GPS substitution is only good, if the coordinates of the DME station are in the Nav database. Or distances are also given to the runway theshold and this is in the database. Otherwise, GPS substitution is not good.
The cross check of the altimeter with on GS indication is a good practice, primarily to verify the altimeter setting is correct.
That’s one one point and the other one is that it prevents a pilot from following a stuck GP needle into the ground (google knows everything about Alitalia flight 404).
Very interesting report, what next.
Like NCYankee, I would have expected the 9 deg GS to be the right way up, but the report says both the 6 and the 9 are upside down:
- A signal reversal was always present at approximately 9 degree glide path.
- A signal reversal was sometimes present at approximately 6 degree glide path.
A related Q is how do the big jets, which try to do continuous descents onto the ILS, work it. Do they tweak the descent profile “just right” so they briefly end up underneath the GS, or do they just try to get around the 3 degree mark and their autopilot’s logic enables an intercept from above?
I should know this but I have a feeling my KFC225 doesn’t intercept from above, and is looking for the bottom GS sidelobe first.
the other one is that it prevents a pilot from following a stuck GP needle into the ground
That’s a reason for an EHSI; non-mechanical.
Very interesting report, what next.
Look also at the link at the bottom of the page, a similar thing happened to an AirFrance A340 at Charles de Gaulle!
I have no needles, and LCD rarely gets stuck!
Of course ALL the DME stations relevant for ILS approaches in Germany are in the database WITH their coordinates.
a similar thing happened to an AirFrance A340 at Charles de Gaulle!
I wonder if the reason all these jet pilots are not watching their airspeed is that jets have so much power, whereas us in piston GA are severely performance limited in most phases of flight.
I have no needles, and LCD rarely gets stuck!
But your Flight Director only shows what has been selected (and captured!). Many flight directors / autopilots will not capture a glideslope from above and leave the vertical mode in whatever was selected before arming the approach, in case of a late descent usually vertical speed. So if you set your vertical speed to -1500ft/min and descend through the glideslope at that rate, your vertical command bar will stay centered (“stuck” so to say) all the way to the point of impact while you think you are following the glideslope. It has captured the localiser, so you must be flying an ILS, must’nt you?
Of course ALL the DME stations relevant for ILS approaches in Germany are in the database WITH their coordinates.
In your database. Ours has been cut short because our ten year old FMS (the memory is not extendable) is short of memory. Therefore, more and more points get removed that the database administrators regard unnecessary. Among them things like DME coordinates, because there are two real DME receivers on board (at least two – it can track six DMEs simultaneausly to navigate in DME/DME mode).
