Both RNAV and ILS perform very similar functions. The mechanics of them are obviously very different, and ILS is considered a precision approach whereas RNAV GPS approach isn’t. But in the end, they both guide your plane down to the ground, so their function is very similar.
One case in point is RWY 27 at Miami Opa Locka (KOPF). It has both an RNAV and ILS approach. Looking at the plates, they share the same altitudes and same waypoints to enter the pattern. You could literally be assigned either an ILS or RNAV approach and use either as their path is identical.
Sometimes I get assigned an ILS approach, sometimes I get RNAV. Either way, I don’t really care. But what would determine whether a controller would assign a plane one or the other? If you already have the more precise ILS, why would one use RNAV?
If you are talking about the sim assigning one or the other then I would guess it’s random. In real life, it would depend on your aircraft’s capability. Not all aircraft have both systems installed.
Ok, that’s what I figured. However, if your aircraft had the capability for either, how would they determine which to assign? Would they prioritize ILS over RNAV?
I have had FSX and MSFS-2020 change my preferred approach. I like to ILS my approach, but once in while the simulator changes my choice. I don’t know how to decline it in MSFS, but I know how in FSX, but usually just take the change and fly it in. At night that really messes me up, but day it’s something I can deal with. I usually fly big iron.
As far as I was aware (could be wrong), ILS has lower ‘minimums’ than RNAV. So a controller may assign ILS if the cloudbase is lower than RNAV requirements.
If you don’t have an ILS equipped aircraft and flew to the airport with a lower cloudbase, then you need to revisit ‘flight planning’
Yes, in general, ILS approaches have the lowest DA (decision altitude).
You see it here:
That being said, in this case there is also an RNAV approach with the same DA:
But this “LPV” approach requires a certain type of RNAV system (the GPS signal alone is not accurate enough, it has to be augmented by special system called SBAS). Of course not every RNAV system is equipped with SBAS and also the SBAS-signals are not available worldwide but only in certain regions (see https://en.wikipedia.org/wiki/GNSS_augmentation).
In reality, the controller would ask you which approach you’d rather take if you’re flying to an airfield without heavy traffic. You can opt to do the advertised approach or take a different one, and depending on system capacity the controller will accept it.
The advertised approach (i.e., given as the active IFR approach in the ATIS) is typically the ILS, followed by an RNAV, followed by the VOR, followed by the NDB. However, some exceptions exist, I think Ottawa has ILS approaches but prioritizes advertising the RNAV approach to the runway.
SBAS = WAAS (usa), from real life airline experience we usually go always for the ILS, since it’s just better with a lower minimums. The only times we do RNAV with LNAV/VNAV (no WAAS on most airliners) is when the ILS is on maintenance, no ILS is available, or GS is out of service.
Thank you all for your replies. This has been very informative and educational. Not being a pilot, my understanding of this is basic at best and really only comes from what I’ve learned in sim. I appreciate you fine folk instructing me.
And of course, it seems like MSFS ATC is out to lunch there, often prioritizing RNAV or VOR over an available ILS on the same runway. I suspected this was the case, but it’s good to have it confirmed.
Thanks for that. It’s an interesting read! Shows us the “other side” so to speak.
But also reinforces my belief that in-game ATC being out to lunch. ATIS advertises ILS 27 in use, and I get assigned RNAV 27 or Visual on 27 with vectors (that I never get unless I request them) instead…
As mentioned above, at busy airports with a constant arrival stream, the standard order is ILS, RNAV, VOR. Almost every IFR aircraft has ILS capability, so it’s easier for ATC to assign everyone the same approach. It makes sequencing aircraft simpler.
However, there are always local variations to these norms. For instance, maybe the RNAV track provides better noise reductions over populated areas.
Where I’m based, arrival and departures are normally assigned different runways. During the evening however, the two are combined, and the airport operates a single runway. Unfortunately departing aircraft interfere with the GS signal as they taxi in front of the antenna. This causes fluctuations that can lead to a go around if the autopilot is left unchecked. Our company recommendation is to request an RNAV approach instead of the ILS, to prevent this.
An RNAV approach is a non-precision approach so has no glideslope. If you’re flying an aircraft without VNAV you have to descend manually in accordance with the chart
In my experience, MSFS ATC will clear you to land no matter how you approach the airport. So I never bother to enter the downwind the order unless it’s the approach I’d actually use. (Assuming you’re talking about VFR)
Not really, LPV has vertical guidance and can be flown with a GNS530W GPS. There are commercial helicopter operators everywhere flying RNAV LPV approaches fully coupled. I don’t know of any helicopters that have VNAV.
You would use whichever one gets you the lowest if the ceiling is low. If the ILS is out the RNAV would be used. That’s the real world answer. In the sim, I would do it the same way.
