What I was getting at, NCYankee, is why have two DME stations when one could be used, say middle of the runway, and you just change the offset for the selected runway. Here in Europe the service contract on a DME is of the order of $10k/year so why have two?
The only reason for having two is for redundancy (but a DME, or an NDB, are dual-redundant anyway, AIUI) or if somebody wants a GPS waypoints which is the DME station, for some reason.
A_and_C wrote:
You illustrated the DME substitution issue so well I find it difficult to understand how some on this forum seem unable to understand the human factors issues of this practice.
Then please explain why American GA pilots (a much larger group than European pilots) have not come to grief despite 18 years of this very risky (per your opinion) practice ? ? ?
Michael wrote:
Then please explain why American GA pilots (a much larger group than European pilots) have not come to grief despite 18 years of this very risky (per your opinion) practice ? ? ?
It’s because it’s not risky in the US, where you don’t have things like offset DMEs not in the GPS database.
Michael’s point is very valid.
To try and explain why the experience in the USA is better (and there are several factors which lead to a superior safety record for GA stateside), perhaps the following may be worth considering:
- Better training on using GPS in the IFR environment – a point Peter has also made. I fear a lot of IR instructors in Europe are not actually that familiar with the rules of using GPS especially when briefing a GNSS RNAV approach.
- Better design of the IFR approach database where loading an ILS approach will provide correct read outs for Glide Slope check distances, using the GPS approach database?
The Dundee accident is actually a good example of poor training in Europe – a US practitioner would have been drilled to only use the GPS for an approach in the database. Using GPS as a DIY solution for DME readout is a practice which US pilots are trained not to do, unless flying an approach using the GPS which is in the database.
In the same vein the European faith in certain quarters in DIY virtual ILS is a practice which am not sure is that prevalent in the US – emphasis on I am not sure.
The Dundee accident is actually a good example of poor training in Europe – a US practitioner would have been drilled to only use the GPS for an approach in the database
Did the Dundee aircraft have an IFR GPS, with a current database?
Flying in Europe one often hears commercial operators declining an RNAV approach in favour of a VOR or NDB one. Their AOC manual presumably authorises the latter two to be flown (using FMS guidance) but not the RNAV one. We have multiple posts here from commercial operators confirming this.
The FMS would load the approach and a standard procedure is to select the LOC/VOR as the NAV source when cleared for the approach. The DME would be linked to the NAV source and the HSI would switch from magenta to green.
Some FMS might produce a warning if you loaded a VLOC approach and still on GPS.
The question is whether in the US databases where you are allowed GPS to substitute for some functions, would an ILS loaded from the IFR database, but without DME, allow the GPS to give the correct read outs (as a waypoint in the approach) for step down and glide slope checks?
The FAF is in the database. I would be very surprised if there were any missing as this point is neccessary for the auto-swirtch to VLOC to work ob the Garmins.
If there are further altitude check fixes which are not in the database. It is highly unlikely the pilot will come to grief.
Only non-precision approaches with step-down fixes not in the database would be problematic
Peter wrote:
What I was getting at, NCYankee, is why have two DME stations when one could be used, say middle of the runway, and you just change the offset for the selected runway. Here in Europe the service contract on a DME is of the order of $10k/year so why have two?The only reason for having two is for redundancy (but a DME, or an NDB, are dual-redundant anyway, AIUI) or if somebody wants a GPS waypoints which is the DME station, for some reason.
The issue is not cost. It is safety. I seriously doubt that we would have GPS substitution if offsets were used in the US, so it is a safety issue.
RobertL18C wrote:
The Dundee accident is actually a good example of poor training in Europe – a US practitioner would have been drilled to only use the GPS for an approach in the database.
Having the approach in the database makes substitution easier, at least when all the DME waypoints are included, but it is not a requirement for flying a ground based approach. For 32 years of my flying career there was no GPS and approaches were not in any database. The approach or the DME station or a named fix must be in the database. This is from AC 90-108 which describes what can be substituted:
7. USES OF SUITABLE RNAV SYSTEMS.
a. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC, operators may use a suitable RNAV system in the following ways.
(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course.
(2) Navigate to or from a VOR, TACAN, NDB, or compass locator.
(3) Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
(4) Fly an arc based upon DME.
b. Specific Allowances. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).
NOTE: For the purpose of this AC, “VOR” includes VOR, VOR/DME, and VORTAC facilities.
NOTE: For the purpose of this AC, compass locator includes locator outer marker and locator middle marker.8. USES OF SUITABLE RNAV SYSTEMS NOT ALLOWED BY THIS AC. An otherwise suitable RNAV system cannot be used for the following:
a. NOTAMed Procedures. Unless otherwise specified, navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
b. Substitution on a Final Approach Segment. Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.
c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses (including LOC back-course guidance) without reference to raw LOC data.
Although it is not in the AC, the current AIM has a modification in the wording to restriction 8 (b) in the AC. The AC will catch up in the next update:
Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.
In effect this is just a clarification. IOW, you may use your GPS and even have it hooked to the autopilot as long as you display and monitor the VOR, TACAN, or NDB signal on a CDI/RMI/HSI in your primary view with the additional restriction that the navigation facility must be working at the time. If there is a significant discrepancy between the GPS and VOR/NDB guidance, you should abandon the approach. This is not equivalent to “OpSpec C300, 14 CFR Part 97 NDB, NDB/DME, VOR, and VOR/DME Instrument Approach Procedures Using Substitute Means of Navigation” which permits certified carriers to use RNAV in lieu of VOR or NDB on the final approach segment without requiring the raw data be displayed or the facility to be operational.
alioth wrote:
It’s because it’s not risky in the US, where you don’t have things like offset DMEs not in the GPS database.
? ? ?
I’m not sure I fully understand this triple negative .
Care to expand please ?
