Keep DME and second NAV/COM?

Discussion in 'Cleared for the Approach' started by JohnAJohnson, Apr 9, 2019.

  1. JohnAJohnson

    JohnAJohnson Cleared for Takeoff

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    I don't know much about IFR flying and will begin training after the aircraft is finished its upgrade. So I'm building it the way I think it might be most useful. The avionics will be as such:

    Stack
    PSA 450b Audio Panel
    IFD540 NAV/COM/GPS
    GNC255A or SL30 or Matrix1 NAV/COM (all w/GS)
    KN64 DME

    Transponder/ADS-B TBD

    PFD/MFD
    Plans are to go with the Dynon 10" and the D10A for a backup (when my plane gets added to the AML), and one other NAV capable display, possibly a G5.

    I think it might be nice to have the second NAV/COM for redundancy and for monitoring any GPS approach if ILS is also available on the same runway and for finding radials and intersections (the old way) if flying VORs. But yea, I know, the Avidyne will do all that for me based on GPS and/or its NAV receiver. Should I go with COM only for the second radio, or would a second NAV/COM be worthwhile?

    I can afford the weight and space, and can freshen up the KN64 with a new faceplate cheaply, but don't really know if DME is still useful. Keep it or delete it?
     
  2. asicer

    asicer En-Route

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    I vote to have a 2nd NAV/COM and ditch the DME. GPS is a legal substitute for DME these days. And you want 2 NAV radios for when they turn on GPS interference testing.
     
  3. brcase

    brcase Pattern Altitude

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    I think DME can be quite useful. Around Boise they have gotten rid of all the approaches that don't required DME or GPS. So if the GPS goes down (interference testing?), DME is the only way you can shoot an approach.
    I probably wouldn't spend extra to get one, but I wouldn't throw one away I already had and was working if I have the space for it.

    If you had a separate nav and GPS box I would say just buy a com. But with the nav built into the GPS I think a 2nd nav would be a good idea for that one time the IFD540 just quits.

    Brian
    CFIIG/ASEL
     
  4. asicer

    asicer En-Route

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    Ok, I'm a little confused. You said:
    PSA 450b Audio Panel
    IFD540 NAV/COM/GPS
    GNC255A or SL30 or Matrix1 NAV/COM (all w/GS)
    KN64 DME

    Transponder/ADS-B TBD

    Do you consider the GNC/SL/MTX to be NAV2 and the IFD to be NAV1? Or do you consider the GNC/SL/MTX to be NAV1?
     
  5. JohnAJohnson

    JohnAJohnson Cleared for Takeoff

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    NAV1: IFD540
    NAV2: GNC255A or SL30 or Matrix1 NAV/COM (all w/GS)
     
  6. Rgbeard

    Rgbeard Cleared for Takeoff

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    I cant imagine not having a second nav/com

    I have a DME which is currently inop in my plane and I wish it was operable. I just like suspenders with my belt.
     
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  7. asicer

    asicer En-Route

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    OK, then my original opinion still stands. DME can go if you need it to go.
     
  8. mondtster

    mondtster En-Route

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    The KN64 DME is pretty old these days. Yours may work now but if the whole panel is getting redone I'd probably skip reinstalling it for fear of it quitting and leaving a dead radio in the panel that you have to deal with. I've been seeing more and more trouble with the older king radios in the last few years, which is causing my bias. The IFD540 can be used in lieu of the DME so you would not be losing any capability by removing it.

    Personally I'd want a good second nav/com radio but a second com would be the minimum I'd accept for flying IFR.
     
  9. Clip4

    Clip4 En-Route

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    I say dump the DME. Your issue with the KN64 is the display goes bad periodically and the expense to fix it.
     
  10. flyingron

    flyingron Touchdown! Greaser!

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    I have a KN64 DME. I've not turned it on in years. I use the second navcom from time to time.
     
  11. luvflyin

    luvflyin Final Approach

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    I know of no scenarios where GPS is not an authorized substitute for DME. That ain't saying it couldn't be useful if your GPS takes a dump. Keeping the NAV/COM could definitely be handy. There was a thread here recently about defining the intersection of two radials where that intersection was not a published 'Fix.' The consensus, if I remember right, took some 'button pushing.' The consensus was also that ATC would never 'dump' that on you. In other words, being told to hold at, or otherwise 'identify' the intersection of two radials that were not a 'Named' fix isn't going to happen.
     
  12. Juliet Hotel

    Juliet Hotel Cleared for Takeoff PoA Supporter

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    It sure would suck to sink a bunch of money into a panel upgrade and then have the DME go TU 2 weeks later. Working, its worth something on ebay. Once it breaks you couldn't even sell it as a boat anchor.
     
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  13. sonopoa

    sonopoa Pre-Flight

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    Keep it for cross check. Sell it while it's working applies to anything.

    Exactly. Also on an ifr approach the DME keeps counting down to the station. GPS gives you distance to the next waypoint in your loaded flight plan..
     
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  14. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Difference between GPS distance and slant range is insignificantly tiny at the altitudes where it would matter, though, which is probably why the FAA approves of the use of GPS in lieu of DME.
     
  15. mondtster

    mondtster En-Route

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    Not to mention that the fixes on an approach or airway are defined points in space. I have a hard time believing there is any difference in where you’re at when the GPS says you’re at a fix vs. where a DME says you’re at.

    There may be a slight difference in location if you get some odd instruction such as cross the 020 radial at 22 DME but how often are people getting instructions like that? In the years I’ve been flying and instructing I can count the number of times I’ve gotten this sort of instruction on one hand. And in every case, I can’t see being slightly off as being a big deal.
     
  16. Juliet Hotel

    Juliet Hotel Cleared for Takeoff PoA Supporter

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    It does. So does sell it before it will cost more to fix than you can sell it for.
     
  17. PilotRPI

    PilotRPI Line Up and Wait

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    As a new ifr pilot, I cant imagine flying without a second nav source. What if the first nav/com/gps fries? Id be fine with a vor as the second nav, i dont feel the need for a second gps, but Im not flying to minimums or anything right now either.
     
  18. flyingron

    flyingron Touchdown! Greaser!

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    Approaches aren't predicated on slant range anyhow. They can't, they have no clue what altitude you are at in most cases.

    The only time GPS can't be substituted are for two goofy DME ARC approaches (MTN and WAL). One is to an airport you're not allowed to fly into and the other doesn't get used all that much either.
     
  19. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Huh? They know close to what altitude you are at - The minimum altitude that's published on the plate, on up to any reasonable altitude you could actually get down to there from. But at any approach altitudes, slant vs. ground range is close enough that it doesn't matter. Since I just used this approach in another answer, I'll use the ILS 36 into KMSN as an example. The FAF, OZMIX, is 2500 MSL/1638 AGL and 4.6 DME (27,950 feet) from I-MSN. Basically, slant range puts you on the hypoteneuse of the triangle, while GPS range is at the base. With the 3-degree glideslope, the difference between slant range and GPS range is 0.137%. So, in this case, 38 feet, or at a 90-knot approach speed, about 1/4 second. Insignificant.

    Ah, the good old arcing FAC! I don't see any note saying that GPS can't be substituted on those, though? I think all you'd have to do, in the case of the MTN VOR/DME 15 is to punch direct BAL into your GPS and fly it as if the GPS was a DME. But I'm remembering something in the dark, musty, cobweb-infested corners of my brain that says GPS can't be used as a substitute to DME for lateral guidance on the FAC or something like that?
     
  20. flyingron

    flyingron Touchdown! Greaser!

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    There's no requirement to be at the minimum charted altitude. Slant range errors increase with altitude, but the correct answer is that the distances given are "close enough" for what the fixes are.
    You didn't read carefully then. GPS can be substituted for ADF and DME as long as it's not for the FAC guidance. This is in the AIM as well as the same text in advisory circulars and FAA policy letters:

    Pilots may not substitute for the NAVAID providing lateral guidance for the final approach segment.
    It's more of an issue for NDB approaches, as stated, you're unlikely to fly the two approaches that use DME for FAC lateral guidance.

     
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  21. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Right, but at any altitude where the plane is in position to successfully make the approach, the errors are insignificant.

    ...but not on the plate, which is why I said I didn't see it. Glad those cobwebs held some useful information.
     
  22. JohnAJohnson

    JohnAJohnson Cleared for Takeoff

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    Thanks all. NAV2 = yes, DME = probably no.
     
  23. flyingron

    flyingron Touchdown! Greaser!

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    It's never on any plates.
     
  24. Ghery

    Ghery Final Approach

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    I remember when the club upgraded the panel in the 182. They were going to ditch the DME, but I asked that they keep it. Shortly after the 650 and other stuff was installed the DME went TU. Darn it! I was so used to using DME for DME arcs (dirt simple, I don't know why people think it's hard) and really didn't like flying the Arrow IFR because it didn't have one (/U). Oh well, the only damage was that the club lost out on the $400 we would have gotten if we had dumped the DME while it still worked.
     
  25. N1120A

    N1120A Line Up and Wait

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    Sell the DME and see if the difference lets you get another GPS
     
  26. apr911

    apr911 Pre-takeoff checklist

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    Echoing what everyone else is saying here...

    Second comms is highly recommended. Your ground communications are that much more critical in an IFR environment. You wouldn't want loss of one comm to leave you without means of communication. I've had the receive side of a GNS430W quit on me in VFR and it was nice to be able to switch over to the standby radio and still get through the Bravo I was trying to get clearance for.

    I also find it helpful in reducing workload. I like to set my ATIS & Ground frequencies in comm2 and my approach & tower frequencies in Comm1. You can continue to monitor approach on Comm1 while getting the ATIS on Comm2 so you dont have to leave frequency. Once you have the ATIS you can swap the channel out for ground and then you only have 2 buttons to press for the remainder of the flight... Frequency swap when Approach hands you off to tower and a tx-radio swap when tower hands you off to ground. No scrambling around for ground frequencies or trying to tune them in while on the approach or runway. This naturally works in reverse too on departure.

    I also wouldn't be surprised if a simple nav/comm radio uses significantly less power than your GPS and screens so if you had an alternator fail in flight and it becomes advisable to conserve power, you can shutdown the screens and still have comms.

    As to NAV, I'm all for flying the VORs when possible and not using the purple line but even when flying GPS, I still like to use my second nav radio to cross check my fixes and radials with enroute. It's also quite valuable on an ILS as, at least in a typical instrument layout of the 6-pack + 2 CDI's, it allows you to keep your focus more on the instrument panel.

    I'm currently flying a Cherokee6 with an instrument student which does not have a second comm/nav radio. Its much more difficult and I've encouraged him to consider adding a second comm/nav radio with CDI to simplify things.

    As to the DME, I would probably have it removed. There are a few situations where DME is truly required and it might be nice to have the backup but generally find it rarely gets used in planes equipped with GPS. Besides, there are other ways to triangulate your position without DME (intersecting radials for example).

    Believe it or not, I feel like in a plane with a suitable GPS substitute for DME, I'd rather have an ADF available than a DME which is saying something. Might not be a whole lot of use anymore here in the US but if you ever took the plane overseas, NDB approaches are still pretty common.

    Cool approaches. KMTN isnt far from me, might have to venture on down there and request the VOR/DME approach.
     
    Last edited: May 30, 2019
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  27. N1120A

    N1120A Line Up and Wait

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    I always do ATIS and ground in COMM 2 and then tower/departure/en route/approach in COMM 1. I view anything else as unnecessary workload increase.
     
  28. chemgeek

    chemgeek Cleared for Takeoff

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    Agree with having secondary NAV/COM as backup. It's also nice to have two radios to keep Approach, Tower, Ground and ATIS cued up for arrivals. Approach and Tower go in #1, and you can monitor ATIS in #2 with Ground ready to go after landing and turning off the runway.

    The DME will be of limited usefulness with WAAS GPS, which can substitute for it. It could be discarded to save weight or make room for something else.

    A GPS/NAVCOM and a NAV/COM will do pretty much everything you need for IFR. And if the GPS crumps or is OTS due to yet another jamming excercise, you can do crossing radials easily with 2 VORs. With one VOR, it's like a one-armed paper-hanger to monitor crossing radials.