ILS/LOC timing question:

fiveoboy01

fiveoboy01

Pattern Altitude
Joined
Apr 21, 2013
Messages
2,321
Location
Madison, WI
Display Name
Display name:
Dirty B
I am getting back into IFR practice after being without an airplane for a few months. Previously I was ready for the checkride but became rusty so I've been working on foggle work and approaches in preparation for the checkride in perhaps a month or so.

Yesterday I was shooting an ILS, and the safety pilot(who is instrument rated) pointed out my ground speed on the GPS in the neighborhood of 80 knots, stating that FAF to MAP times are based on ground speed and that I should be trying to keep my ground speed at 90 knots.

I've always done it configured for approach at 90 knots indicated, ignoring ground speed, and if there was a huge difference, mentally math it out to add or subtract a few seconds. So...

1. What's the proper way to do this? Airspeed, or groundspeed? I thought it should be the former, but after yesterday I'm questioning myself.

2. Should I even be timing it on an ILS? DA is where the missed is executed, so why time it? I understand timing as a backup in case you lose the glide slope, but I think I'd rather go missed, set it up and take another shot at the LOC approach. There are enough things different between the two that converting an ILS to a LOC could be potentially hazardous.

All that said, I flew for a couple hours, shot several approaches and did several laps around a hold, and considering I haven't worn foggles since late october, I felt less rusty than I thought I would be!
 
90 knots IAS is how I shoot an ILS in a Warrior/Archer/172 and similar. I don't time my ILS. If I lose my GS, I'm going missed anyway.
 
I've always figured it on ground speed. That said, I wouldn't be trying to keep my ground speed at 90. Fly the appropriate speed for your airplane and just adjust the timing. I also don't bother on an ILS, only on a LOC.
 
The times are obviously determined by groundspeed. Yes, I fly the 90 kt airspeed but I interpolate the times based on the estimated groundspeed.

People give the old "hack the time at the FAF so you can convert to LOC" but frankly if things start to go awry in the soup such that I lose the GS, I'm not going to have faith in the LOC at that point either.
 
If you lose glide slope and choose to go missed, how do you know when to start the first turn without knowing the missed approach point from timing? I don`t claim to be an expert, but I understand that when the missed approach procedure starts with a turn, terrain clearance is only insured when the turn is initiated at the missed approach point. Not before or after.

Do I misunderstand?
 
Speed, I fly what's comfy for my plane and myself, and that's higher than 90kts. When I had my fixed/fixed 90 indicated is what I flew at.

I don't time the ILS for the same reasons already mentioned. My DPE follows the same philosophy. The LOC and ILS may be on the same plate, but they aren't the same approach.
 
In theory you should be maintaining a ground speed to be able to properly time from the FAF. This is easier to do with the aid of a GPS of course. I was using 90knots but the DPE on my checkride afterwards said I should probably fly the approach faster-like 100 knots. This was contrary to my instructor's teaching. Either speed is fine for my plane provided I know how to manage the energy to transition to land at the proper airspeed. Slower gives you more time to think and get ready but faster wastes less time. On my home airport approach I'll be flying 100 knots and using cat B mins which happen to be the same.

I always time even on the ils. It's not hard to push one more button and it can't hurt in case you lose glide slope and GPS. Probably never gonna happen but you never know. At least this way you know when to execute the MAP. And yes the timing is based on ground speed. But if you're off by 10knots because you used ias on a 3 minute faf to map you'll be off by half a mile. Maybe it's safe to assume the area is protected at least that far if you are above mins???
 
bkspero said:
If you lose glide slope and choose to go missed, how do you know when to start the first turn without knowing the missed approach point from timing?
Click to expand...
Actually the best reason for timing - if no DME available. However, with practice, you can determine your position approaching and over the approach end of the runway by the LOC needle sensitivity. Much like the VOR needle as you approach the VOR.
 
I try to only fly LPV or other RNAV, if available. If I am forced to fly an ILS (e.g. due to traffic, or lower mins), and if I had to shoot the missed before DA (e.g. due to GS loss), I'd start the initial climb towards the runway, and use the GPS runway threshold waypoint to continue the procedure from there.
 
Ground speed is the speed for a timed approach. As I want to land the plane I usually fly approaches with 70 - 75 kts IAS today and have to interpolate the time for the ground speed (most of the time close to the 60 kts ground speed number).

During training for the IFR rating I was taught the 90 kts ground speed 'rule'. Could I have landed the plane at DA? Yes, but I would have floated down the runway because of excessive speed in the Archer on approach. One of the CFII's I met later asked what the purpose of the approach is... I want to land the plane in IFR conditions - therefore the same approach speed at the IAF as while on a VFR approach...
 
1. Timing is done off of groundspeed.

2. It depends. Do I do it? No. But I also have the ILS approach programmed into the GPS to give me the MAP. Most ILS approaches I fly to have DME associated with them either by an ILS/DME or a VOR/TACAN on the field. Odds of losing G/S and the DME source, slim to none. In the one time out of a billion that happens, I'll just fess up to the controller I lost my DME reference and ask for vectors. Now if you are flying an ILS and your plane doesn't have a GPS, sorry you aren't as fortunate. I'd fly the airspeed I need to fly for the airplane, then use mental math to come up with the new time based on the groundspeed. I still think even a VFR only GPS to identify the MAP is gonna be more accurate than my interpolated time based off a groundspeed that is probably not very constant.
 
Of course the time is based on how much time it takes to cover the distance from the FAF to the MAP on the localizer approach. But the answer for when you need to time a nonprecision approach is to interpolate the time, not try to fly the LOC approach at 55 KIAS because you have a 35 KT headwind.

If you lose GS on an ILS, it depends. Is it at a point transition to LOC-only is feasible given the pilot and the equipment? Not that much different than the VFR decision to save a bad landing or go around. Comfort, conditions, experience, and ability is a much more important factor than a simple "everyone should do it this way."
 
Last edited:
I wholeheartedly agree with the above that says fly the the airspeed and extrapolate the times from the FAF to MAP table. The wing and the laws of aerodynamics don't know nuthin about approach plates and ground speeds. I round the the times in the tables and find the difference. For me, it's the 60 and 90 knot times. Then divide by 3. It comes up as something like 40 seconds more than the 90 knot speed if I'm doing 80.
 
Your needed descent rate will also depend on ground speed, not airspeed. You can adapt that, but it takes time.

At 90 knots, the only way you'll stall is with a massive tailwind. Keep an eye on the airspeed and don't let it get too low. Fly the approach at 120 if you have to. Better yet, go to an alternate where the wind doesn't exceed your stall speed so you can keep the airplane on the ground once you get it there.

Unlike cruise flight, instrument approaches are tied to the ground. The relevant quantities are course and ground speed. Heading and airspeed are a means to get there.
 
MAKG1 said:
Your needed descent rate will also depend on ground speed, not airspeed. You can adapt that, but it takes time.

At 90 knots, the only way you'll stall is with a massive tailwind. Keep an eye on the airspeed and don't let it get too low. Fly the approach at 120 if you have to. Better yet, go to an alternate where the wind doesn't exceed your stall speed so you can keep the airplane on the ground once you get it there.

Unlike cruise flight, instrument approaches are tied to the ground. The relevant quantities are course and ground speed. Heading and airspeed are a means to get there.
Click to expand...
I understood and agree with everything in you post but I don't understand the underlined part. What about a massive tailwind is going produce an AOA on the approach high enough to produce a stall? I can't imagine a normal category airplane even capable of getting a descent rate high enough to do that. Or are you referring to a tailwind shear that size?
 
Not shear, steady state. What he's saying is if you attempt to hold a 90 knot groundspeed with a (let's say) 60 knot tail wind, you're going to be flying a 30 knot TAS which you're not going to be able to maintain that on a constant rate descent without stalling.
 
Thanks for the replies. I knew the times were based on groundspeed, just wasn't sure if I should do the math in my head or try to adjust IAS to get 90 across the ground. Looks like it doesn't matter a lot given lesser winds...

The question about knowing the MAP if not timing is a good one... I assume that absent timing, I can use my DME of the localizer as well as distance information from my GPS... It's a WAAS unit and I always load and activate whatever approach I'm flying anyways, so between those two I'd hope I'd know where the MAP is at.
 
midlifeflyer said:
I understood and agree with everything in you post but I don't understand the underlined part. What about a massive tailwind is going produce an AOA on the approach high enough to produce a stall? I can't imagine a normal category airplane even capable of getting a descent rate high enough to do that. Or are you referring to a tailwind shear that size?
Click to expand...

If you're trying to maintain a 90 knot ground speed and you have a 45 knot tailwind, you'll have a problem. If you just fly the LOC at 120 under that circumstance, it's a lot better.
 
fiveoboy01 said:
Thanks for the replies. I knew the times were based on groundspeed, just wasn't sure if I should do the math in my head or try to adjust IAS to get 90 across the ground. Looks like it doesn't matter a lot given lesser winds...

The question about knowing the MAP if not timing is a good one... I assume that absent timing, I can use my DME of the localizer as well as distance information from my GPS... It's a WAAS unit and I always load and activate whatever approach I'm flying anyways, so between those two I'd hope I'd know where the MAP is at.
Click to expand...

If you are flying a LOC/DME, the DME will give you position information. Not all localizer approaches have DME. An IFR certified GPS will also give you the MAP.

Keep in mind that there are still quite a lot of IFR airplanes out there that aren't GPS equipped. Most of the ones I've run across are DME equipped, though. Obviously, they don't have to be.

Occasionally, the DME is not available. Like the KSNS LOC/DME is currently just a LOC (though it also has a coincident ILS, and is NOTAMed NA unless "suitable RNAV equipment" is installed) because SNS VOR (which gives the DME for that approach) is out of service.

See the KWVI (Watsonville) LOC RWY 2 approach for an example where DME will not help you.
 
MAKG1 said:
If you are flying a LOC/DME, the DME will give you position information. Not all localizer approaches have DME. An IFR certified GPS will also give you the MAP.
Click to expand...

A LOC/DME approach is not the same thing as a LOC approach in which the localizer (or another NAVAID that is part of the approach) has DME. In the first one, DME is required. In the second one, it is optional.

MAKG1 said:
Like the KSNS LOC/DME is currently just a LOC (though it also has a coincident ILS, and is NOTAMed NA unless "suitable RNAV equipment" is installed) because SNS VOR (which gives the DME for that approach) is out of service.
Click to expand...

It is still a LOC/DME approach, but RNAV is used in lieu of DME.

MAKG1 said:
See the KWVI (Watsonville) LOC RWY 2 approach for an example where DME will not help you.
Click to expand...

The Watsonville localizer does not have DME.
 
MAKG1 said:
If you're trying to maintain a 90 knot ground speed and you have a 45 knot tailwind, you'll have a problem. If you just fly the LOC at 120 under that circumstance, it's a lot better.
Click to expand...
Ah! I thought you were talking about airspeed. I was trying to figure out why a tailwind would cause a problem flying at 90 KIAS. Maybe adding KIAS or GS would help keep it straight. I guess I'm being picky, but I don't "fly at" groundspeeds. I fly at indicated airspeeds.

Even in this example, with that 45 knot tailwind, I would fly at my "normal" approach airspeed and not try to target a particular groundspeed. I'd obviously have to adjust my descent rate to compensate for the increased groundspeed from the 45-kt tailwind, but I'd much prefer doing that than flying an instrument approach at 70 KIAS just to try to hit a 120 GS target, especially since winds that strong are, in my experience, rarely steady. Combine that 45 KT tailwind with a conservative 20 KT gust factor, and that 70 KIAS can mean a real problem.
 
fiveoboy01 said:
Thanks for the replies. I knew the times were based on groundspeed, just wasn't sure if I should do the math in my head or try to adjust IAS to get 90 across the ground. Looks like it doesn't matter a lot given lesser winds...
Click to expand...
Let's talk practice rather than theory.

In the real world, wind is rarely static, with higher degrees of variation (i.e., gusts) with stronger winds. Wind strength and direction can and do change substantially with altitude. So, during an instrument approach, doing the math means doing the math repeatedly as you descent. Unless you happen to be particularly gifted, that sounds like a lot of extra make-work for what many consider to be the most work-intensive task in aviation - flying an instrument approach.

OTOH, if you maintain a target (your standard) IAS, even in gusty conditions, you have a single target with respect to airspeed. On a precision approach you also have a single target for descent rate - maintaining the glideslope. Practically speaking you also have a descent target on a non-precision approach since most "dive and drive" folks are not particularly interested in the ear-poppping accompanying descent rates greater that 1,000 FPM (their passengers definitely aren't).

Maintaining a single airspeed and descent target in windy conditions will be more than enough work without adding multiple calculations to the mix.
 
midlifeflyer said:
Of course the time is based on how much time it takes to cover the distance from the FAF to the MAP on the localizer approach. But the answer for when you need to time a nonprecision approach is to interpolate the time, not try to fly the LOC approach at 55 KIAS because you have a 35 KT headwind.

If you lose GS on an ILS, it depends. Is it at a point transition to LOC-only is feasible given the pilot and the equipment? Not that much different than the VFR decision to save a bad landing or go around. Comfort, conditions, experience, and ability is a much more important factor than a simple "everyone should do it this way."
Click to expand...

No it doesn't. The LOC is a separate approach, and if you are a conscientious pilot you brief each approach before beginning it. If you lose the GS you go missed at the MAP as close as you can estimate it, report the miss to ATC, and either go around for the LOC (having briefed it, of course) or choose an alternative action.

Bob Gardner
 
bobmrg said:
No it doesn't. The LOC is a separate approach, and if you are a conscientious pilot you brief each approach before beginning it. If you lose the GS you go missed at the MAP as close as you can estimate it, report the miss to ATC, and either go around for the LOC (having briefed it, of course) or choose an alternative action.

Bob Gardner
Click to expand...

You are heading into a familiar airport, lets make it your home base. You are in the clouds. ATIS reports a 900' ceiling. The FAF is 2100 AGL. The ILS minimums are 200 AGL. The LOC minimums are 700 AGL.

You cross the FAF and realize the gideslope indication is incorrect.

Only a foolish pilot would continue the approach down to 700 AGL without going missed first?
 
Last edited:
midlifeflyer said:
You are heading into a familiar airport, lets make it your home base. You are in the clouds. ATIS reports a 900' ceiling. The FAF is 2100 AGL. The ILS minimums are 200 AGL. The LOC minimums are 700 AGL.

You cross the FAA and realize the gideslope indication is incorrect.

Only a foolish pilot would continue the approach down to 700 AGL without going missed first?
Click to expand...

This happened on my checkride prep. ILS into San Jose (30L), in VMC but with foggles on. I descend to intercept altitude, and the GS says I'm REAL high, full scale, 12 miles out. CFII says I sure had better descend. I start to do it, but then realize I'm real close to the GS intercept and haven't passed it, so it should be ABOVE me, not below, and only slightly. Level out, wait for 9.6 DME, then descend to 1000. That would have been a checkride bust, as it was more than 100 feet below minimum prior to the FAF. It would have also been a big problem in IMC, as there is a bunch of terrain around there.

I don't know what was wrong with the GS; I suspect it was a reflection of a false GS off terrain. The same receiver worked fine at other times.
 
midlifeflyer said:
Let's talk practice rather than theory.

In the real world, wind is rarely static, with higher degrees of variation (i.e., gusts) with stronger winds. Wind strength and direction can and do change substantially with altitude. So, during an instrument approach, doing the math means doing the math repeatedly as you descent. Unless you happen to be particularly gifted, that sounds like a lot of extra make-work for what many consider to be the most work-intensive task in aviation - flying an instrument approach.

OTOH, if you maintain a target (your standard) IAS, even in gusty conditions, you have a single target with respect to airspeed. On a precision approach you also have a single target for descent rate - maintaining the glideslope. Practically speaking you also have a descent target on a non-precision approach since most "dive and drive" folks are not particularly interested in the ear-poppping accompanying descent rates greater that 1,000 FPM (their passengers definitely aren't).

Maintaining a single airspeed and descent target in windy conditions will be more than enough work without adding multiple calculations to the mix.
Click to expand...

I had a talk with the double I that I'm finishing up with(dumped the other guy) today as he was at his hangar.

His opinion is fly an airspeed, and not worry about what the groundspeed is. Power and trim settings produce a certain speed and descent rate, and trying to always compensate for wind(which as you point out is always different) is detrimental to a stabilized approach.
 
midlifeflyer said:
You are heading into a familiar airport, lets make it your home base. You are in the clouds. ATIS reports a 900' ceiling. The FAF is 2100 AGL. The ILS minimums are 200 AGL. The LOC minimums are 700 AGL.

You cross the FAA and realize the gideslope indication is incorrect.

Only a foolish pilot would continue the approach down to 700 AGL without going missed first?
Click to expand...

FAA?
 
It's what I thought he meant, but just was wondering if I was unaware of an acronym.

To answer his question, might not be foolish to continue to the LOC MDA, but I wouldn't do it. Just go missed, and shoot the other approach.
 
fiveoboy01 said:
It's what I thought he meant, but just was wondering if I was unaware of an acronym.

To answer his question, might not be foolish to continue to the LOC MDA, but I wouldn't do it. Just go missed, and shoot the other approach.
Click to expand...

What if the "other approach" is the same approach? ILS and LOC (with or without DME) usually share equipment, and sometimes they even appear on the same plate. The San Jose example I gave above has all the LOC/DME information on the same plate. I don't see any reason to go around unless the CDI is way off or the ceiling is below LOC straight in minimums.
 
MAKG1 said:
What if the "other approach" is the same approach? ILS and LOC (with or without DME) usually share equipment, and sometimes they even appear on the same plate. The San Jose example I gave above has all the LOC/DME information on the same plate. I don't see any reason to go around unless the CDI is way off or the ceiling is below LOC straight in minimums.
Click to expand...

I was doing an ILS on my IFR checkride and the controller hung me high. Gave me join the localizer but no approach clearance. By the time he gave it to me I was way above the glideslope. I told the DE I'm continuing with localizer only and announced the MDA. Now this was an ILS with localizer only minimums so there was no need to scramble for another plate. Caught the glideslope from above in time to get stabilized on it, anounced the DH and continued. I don't see a reason to abandon an approach if there is reasonable chance you'll be able to salvage it without unusual or excessive maneuvers.
 
MAKG1 said:
What if the "other approach" is the same approach? ILS and LOC (with or without DME) usually share equipment, and sometimes they even appear on the same plate. The San Jose example I gave above has all the LOC/DME information on the same plate. I don't see any reason to go around unless the CDI is way off or the ceiling is below LOC straight in minimums.
Click to expand...

They're on the same plate to save paper. In many cases theyre not the same approach other than the final approach course being the same. Of course they share equipment but my scenario involved losing the glideslope which is a separate antenna.
 
fiveoboy01 said:
They're on the same plate to save paper. In many cases theyre not the same approach other than the final approach course being the same. Of course they share equipment but my scenario involved losing the glideslope which is a separate antenna.
Click to expand...
I'm not understanding. You don't expect to see a good glideslope signal much before the FAF. What you get might be a false glideslope if you intercept in the wrong place or at the wrong altitude. And you almost always intercept the localizer first. So, you won't know the GS is bad until you're already on the LOC. All you have to do then is switch to LOC altitudes, and follow the missed approach should it be necessary. They are usually the same.
 
MAKG1 said:
I'm not understanding. You don't expect to see a good glideslope signal much before the FAF. What you get might be a false glideslope if you intercept in the wrong place or at the wrong altitude. And you almost always intercept the localizer first. So, you won't know the GS is bad until you're already on the LOC. All you have to do then is switch to LOC altitudes, and follow the missed approach should it be necessary. They are usually the same.
Click to expand...

What?

The altitudes, FAF, MAP, and descent profiles are different between the two approaches. "Just switch to LOC altitudes" sounds simple enough but it's not that easy.

What if you lose the GS and you're below the MDA for the LOC approach(very possible considering the MDA for a localizer approach can be significantly higher than an ILS' DA)? Now what? Level off and climb to the proper altitude for that segment? Have you been keeping track of the localizer segments while sliding down the glideslope so that if you do lose the GS, you know exactly where you're at? I bet you're lower than the LOC plate calls for. What if there's no DME, and the localizer approach uses cross radials to mark your step down fixes? Are you going to quick tune those into the radio if you haven't already? All these things seem like stupid and risky when you could simply go missed, and come back around and do the localizer approach set up correctly and with a clear head.

Finally, is this "switching approaches in the middle of an approach due to failed equipment" an item on the PTS?
 
fiveoboy01 said:
It's what I thought he meant, but just was wondering if I was unaware of an acronym.

To answer his question, might not be foolish to continue to the LOC MDA, but I wouldn't do it. Just go missed, and shoot the other approach.
Click to expand...
...which is exactly why I said, "it depends," which is my answer to all the "what ifs" in your follow up posts. (Recall what my scenario was in response to)

For example, you say, "what if" you lose GS below the LOC MDA? In my scenario, you are already in visual conditions at that point. You would really go missed? OTOH, put me in your scenario - below LOC MDA in the soup with a loss of GS, and you can make a safe bet I'm going missed!
 
Well obviously if broken out, I'm not initiating a missed approach... Being visual I'm probably not even looking at the GS anyways:)
 
You must log in or register to reply here.
Back
Top