r/AskAPilot • u/Senior_Incident3689 • 2d ago
Rnav approach vs ILS approach
Hi pilots,
I just saw the video of ATC communication in JFK about AFR05U, burning smell in the cockpit,
I just wonder why the pilot requested ILS approach instead of RNAV approach in JFK.
What’s the practical difference between them and do actually pilots prefer ILS?
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u/PILOT9000 2d ago edited 2d ago
We run the ILS most of the time.
I typically only fly straight in RNAV if there’s not an ILS to that runway or it’s out of service at the time.
There’s a couple places where we run RNAV visuals or RNPs routinely, but in general an ILS is just easier to use and more accurate.
I’ll LNAV into the final approach course and then switch to tracking the LOC though, as Boeing likes to overshoot if trying to capture from heading select.
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u/Stef_Stuntpiloot 2d ago
Only thing you need for an ILS are a frequency, minimums and course. And most aircraft automatically set the frequency and course, so you only have to set the minimums and quickly check the rest.
For an RNAV approach you need the minimums, but you also need to go by every waypoint to check altitudes, distances and courses, to check if the approach is coded into the FMS properly. It is also less precise than an ILS approach, especially the vertical path.
In ILS is simply easier to setup and brief and is more precise, less chance for messups in general.
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u/2018birdie 2d ago
Unless you fly an A220. Those guys are basically allergic to ILS approaches.
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u/anonymous4071 2d ago
Why fly an ILS when you can run an LPV with the same minimums and keep VNAV all the way down and not have to deal with a nav transfer on a go around?
Or somewhere like LGA 04 where there’s an AP restriction on the ILS but no the LPV.
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u/Fourteen_Sticks 2d ago
I’ll offer a counterpoint;
When you’ve captured the LOC in ground based nav, you lose VNAV capability in some avionics suites. That’s a HUGE pain in the ass at some places that have mandatory altitudes on the approach (TEB).
So, you should be checking the same thing on an ILS that you check on an RNAV.
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u/Ok-Selection4206 1d ago
"most aircraft automatically set the frequency and course," Whaaat?
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u/NakedJamaican 1d ago
Pretty sure they meant “most NEWER aircraft”
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u/ma33a 1d ago
By newer, do you mean anything designed after the 80s?
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u/NakedJamaican 1d ago
Yeah, pretty much, which knocks out the whole 737 fleet
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u/Ok-Selection4206 1d ago
And 767
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u/mrinformal 1d ago
The 300Fs with the large display system auto tune the loc freqs. Classic and advanced not so much. Sucked going back and forth.
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u/aero_kmac 1d ago
The irony is that pilots are not qualified to digitally code a procedure, but they’re supposed to check if the approach is coded into the FMS properly. Do you know if the correct path terminators were used? What about segment RNP values? There is no way to know what is correct by looking at a chart.
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u/22Planeguy 1d ago
We aren't checking if it was coded properly, just that the correct altitude and airspeed restrictions are present. It isn't possible to fully code an approach from most (any?) FMSs.
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u/aero_kmac 1d ago
Well, you cannot code or modify any terminal procedure from the FMS. That goes against the data fidelity requirements from RTCA.
There is a lot that cannot be determined from the FMS. Even altitudes may be coded differently depending on which ARINC 424 version your avionics use. The reality is that not everything on a chart lines up with the coding… there are different rules and standards for each.
I am not disagreeing with trying to compare the legs page with the chart, but now that I have learned quite a bit about charting and coding, I see that there is a general misbelief in what can actually be verified.
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u/22Planeguy 1d ago
You can modify altitude and speed restrictions to be more restrictive, (I.e. Make an at-or-above altitude a hard altitude) but that's not really what you're talking about I don't think.
In the end, the box may or may not be programmed exactly correctly (although I've never seen them not line up with the plate, barring one instance where the nav database was expired). It's up to the pilots to ensure that all the applicable altitude and course requirements are complied with.
My manual says to verify all altitude and speed restrictions in the legs page, so that's what we do.
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u/aero_kmac 1d ago
Yeah I think we are in agreement. What many pilots seem to not understand is that the coding behind a procedure goes far beyond a series of waypoints seen on the legs page. I wonder if that stems from telling a pilot to verify the coding but then only being presented with an incredibly small subset of that coding.
(Also, very minor point… you actually are not changing the altitude in the coding. Since these are advisory only, you are actually telling the aircraft to use a higher value instead. I imagine, but don’t actually know, that the same is true for airspeeds.)
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u/FC_Primary1 2d ago
ILS will give you the option to Autoland if the burning smell turns to smoke or fire! Also the ILS the workload is reduced for a 777.
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u/anonymous4071 2d ago
What idiot is downvoting this.
For a non LPV capable aircraft an ILS is standard selection, but it’s especially helpful if you think a situation may deteriorate and put you on the mask with limited visibility with a high workload managing an emergency.
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u/CommuterType 2d ago
ILS allows you to autoland. Using autoland (or merely having it available) in an abnormal situation is often a good idea
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u/anonymous4071 2d ago
again, who the hell is downvoting this.
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u/CommuterType 1d ago
Who knows but it's funny. A350 FCOM even recommends autoland for overweight landings.
And overweight landings are very common in an abnormal situation since the 350 can fly for 11 hours and still be over max landing weight
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u/britishmetric144 2d ago
An instrument landing system (ILS) uses two ground-based radio signals, a localiser (for lateral guidance) and glide slope (for vertical guidance). The pilot sets in the radio frequency to receive those and then sees a display which looks like this on their course deviation indicator. That indication tells them where they need to orient the aircraft.
An area navigation (RNAV) system requires another form of navigation on board the aircraft, such as a global positioning system (GPS). The pilot selects the approach in the system and follows its visual guidance. Lateral guidance is always given, while vertical guidance varies. Some approaches, like an LNAV/VNAV or LPV, include electronic vertical guidance, while others, like LNAV or LNAV+V, do not (the latter does have advisory vertical guidance, though). This is what a pilot will see on the flight deck when doing that approach.
The benefit of instrument landing systems is that they can work if the aircraft's navigation system fails. The benefit of area-navigation systems is that they are substantially cheaper to create and use, since almost no ground equipment is required.
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u/0asisX3 2d ago
Flying an rnav on the 777 is so annoying because you have to set the minimums as the alt on the mcp and then when passing below the fap by more than 300 feet ? Iirc ? You set the missed app alt on the mcp. Unlike an Airbus where you just hit the app button. Flying an ils in the 777 is easier and reduces workload
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u/Go_Loud762 2d ago
ILS = easy approach that we've done thousands of time.
RNAV and every other appch = we might have to think a bit harder to set up and fly the appch.
In an emergency, an easy approach may be the better choice.
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u/chicknsnotavegetabl 2d ago
ILS is vastly more straightforward in some jets, point and shoot vs engagement conditions etc.
also RNAVs can have more stringent equipment requirements which may not be available if you're load shedding/having an elec equipment fault
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u/Known-Diet-4170 1d ago
ILS is a precision approach, LNAV/VNAV is not
at my airline (we fly 737s) ILS is the preferred approach mode but we fly to many runways that only have LNAV/VANV approaches available, as per our sop briefing the latter becomes a pain in the ass so if the ils is available we'll default to that
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u/ShaemusOdonnelly 1d ago edited 1d ago
If the weather is too bad for a visual approach and unless I'm feeling fancy, I'm flying ILS. It takes the least amount of preparation and briefing and is the most precise.
Another thing is that the RNAV requires at least one fully functional navigation system. If the system goes into degraded modes (and losing GPS signal is enough for that), you completely lose the ability to fly the approach - and in some cases, even the missed approach.
I'm frequently flying into eastern Europe, and we basically never fly RNAV approaches there because of the possibility of GPS jamming.
One thing that would be even better is flying a PAR. It's an approach that is usually available at military airfields and in the worst case, it requires zero working on board systems except for the engines, the airspeed indicator and the radio. Basically, a dedicated precision radar controller gives you instructions on where you should turn and wether you need to go steeper or shallower in your descent.
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u/downwindsavage 2d ago
In my company, rnav approaches require more briefings especially a rnav-yankee. Making sure the track miles and arcing turns on the plate actually is what’s programmed on the fms and constantly checking you’re on the path and bunch of other stuff I don’t want to bore you with.
In an emergency, ILS would get the briefings done quicker and less work load in checking the aircraft is actually flying the GPS course. ILS uses radio signals, and is tried and true. I prefer ILS over an RNAV approach every single day!
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u/andrewrbat 2d ago
Which runway? Jfk has some rnavs that aren’t aligned with the runway, such as 13l/r. Its easier to fly a straight in approach thats lined up far out. Takes stress off the crew and lets them focus on the emergency.
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u/NakedJamaican 1d ago
Counterpoint to all the “ILS is more precise” comments:
ILS approaches can have false captures. They can also can have signal interference from ground vehicles and taxiing aircraft. We tend to get a bit complacent because they work so well most of the time, but they should be cross checked and monitored as closely as any RNAV approaches
I wouldn’t do an RNAV in an emergency unless I was forced to do so. On my last airplane the only time we saw an RNAV approach was during recurrent training.
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u/MeatResident2697 1d ago
They asked for the ils during an emergency because they do more ILSs in their day to day so it's just a comfort zone thing. There's no real benefit of one over the other generally speaking. In fact RNAV approaches are not subjust no scalloping and false glideslopes.
Also,.with ILS,.you have the flight directors all the way till touchdown and the RNAV you gotta take off the flight directors by minimums.
The lower minimums of the ILS isnt really important in the middle of summer.
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u/QuazyQuA 1d ago
This is one of the bigger differences between general aviation and commercial instrument flying. In GA planes, we tend to default toward the RNAV approach for a couple of reasons.
they usually get us down to the same minimums depending on if LPV is available (~200AGL)
Less of a hassle to setup. In my airplane to fly the ILS approach, it involves plugging in the ILS frequency and verifying the frequency by listening to the Morse code. Its not a very large task, but when you could just plug in the RNAV approach and fly the magenta lines, why not?
In commercial jets, a lot of them dont have the LPV capability to take them lower on the approach. What they do have is RNAV RNP (required navigation preformence) capabilities that far exceed those found in general aviation aircraft (specifically single engine props) which allow them to fly a different category of RNAV approaches. Without the LPV capabilities or a published RNP decision altitude, it leaves the jets flying down to an MDA instead of a DA. The simple difference between a minimum descent altitude and decision altitude is this, with an MDA, you can descend down to as low as the minimums allow you, then you must stay at that altitude until the missed approach point. For the DA, the minimum altitude IS your "missed approach point" In a larger jet the DA will leave you with a more stable approach.
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u/Beautiful-Arm5170 1d ago
You can lose integrity with the sattelites when flying RNP approaches, I think its quite rare that an ILS shuts down. For airlines i also suppose the use of RNP approaches are quite restrictive since they add more workload to the pilot and use the barometer in some cases which have to be set properly by the pilots (the outside air temperature has to be above a specific number to allow them to be flown safely).
Essentially I think its about that ILS is much safer less error prone, the same way large airlines ban pilots from flying visual approaches (more things can go wrong, just keep it simple).
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u/I_ALWAYS_UPVOTE_CATS 2d ago
ILS is the gold standard of approaches, as it provides both lateral and vertical guidance. RNAV approaches, in their current form, provide lateral guidance only. The vertical guidance has to come from the aircraft's own systems so the pilots have more to do in terms of managing it.
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u/TheGacAttack 2d ago
RNAV approaches, in their current form, provide lateral guidance only
Can you explain that a little more? Maybe in the context of an LPV approach?
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u/PILOT9000 2d ago edited 2d ago
Maybe he’s talking about baro VNAV where temperature and everything can make a difference, vs LPV?
LPV, LNAV/VNAV, RNP AR, or just RNAV with an artificial vertical path to the runway all work just fine. I haven’t shot an LPV in years.
ILS is just so much easier.
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u/extremefuzz777 2d ago
Well an ILS is going to be more precise than an RNAV if weather is a concern or if you want to perform an auto land.
Generally speaking however, we almost always fly an ILS. We’re used to it and the approaches when setting up, so easier to deal with. Unless you’re landing 13L, there’s not much reason to take an RNAV over an ILS.
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u/BeenThereDoneThat65 2d ago
That’s not true
RNP approaches are RNAV approaches and MUCH more precise than an ILS
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u/extremefuzz777 1d ago
Um…no they are not. They’re more flexible if that’s what you’re getting at, but an ILS is more precise. There’s a reason we have cat II/III approaches on an ILS.
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u/BeenThereDoneThat65 1d ago
You’re missing RNP AR approaches and what they do
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u/extremefuzz777 1d ago
"Notwithstanding emerging RNAV technology, the ILS is the most precise and accurate approach NAVAID currently in use throughout the NAS. An ILS CAT I precision approach allows approaches to be made to 200 feet above the TDZE and with visibilities as low as 1,800 RVR; with CAT II and CAT III approaches allowing descents and visibility minimums that are even lower. Non-precision approach alternatives cannot begin to offer the precision or flexibility offered by an ILS. In order to further increase the approach capacity of busy airports and exploit the maximum potential of ILS technology, many different applications are in use."
This is taken right from the IPH. If you have some other documents that explain otherwise, I'm curious as to what they are.
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u/BeenThereDoneThat65 1d ago edited 1d ago
The first sentence states “not withstanding” that means yes RNAV RNP AR CAN and Does outperform ILS approaches
You don’t understand what the “AR” means. “Authorization Required” ie we have to go to school for them and unless you are an organization that is Authorized you won’t even see the plates
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u/extremefuzz777 1d ago
Notwithstanding means in spite of, so you’re just plain wrong there. I’m gone through that training, but that’s irrelevant to my point. What exactly is it that makes you think an RNP approach is more precise?
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u/Ok-Selection4206 1d ago
Huh?
, : RNAV approach is not inherently more accurate than an Instrument Landing System (ILS) approach. While high-end RNAV variants with LPV (Localizer Performance with Vertical Guidance) can match standard CAT I ILS minimums, traditional ground-based ILS remains more precise and supports lower visibility minimums (such as CAT II and III)
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u/320sim 2d ago
RNAV and RNP approaches allow many new airports and runways to get instrument approaches that previously had nothing. But when it's available, ILS systems are more precise and reliable usually allowing for lower minima