ILS - when to descend?

Guest Discussion
21

In reply to:

For an interesting example of how the big boys do it look at the ILS 22R at ORD. There are several fixes along the localizer. GS crossing altitudes are given for FNUCH and NOLAN as well as the outer marker. If you listen to ATC you will often hear aircraft cleared for the approach when they are still NE of FNUCH. The clearance reads “5 from FNUCH, maintain 7000 until FNUCH, cleared ILS 22R approach. Speed 170 to RIDGE, Tower at RIDGE 126.9”.
Does anyone think that the typical 777 driver dives from 7000 at FNUCH to 5000 at NOLAN and then dives again to 2200 to intercept the GS just outside RIDGE? Of course not. You intercept the GS at 7000 at FNUCH and fly it down.

Right … that’s not how I read that chart. I read the 7000’, where the little lightning bolt is, to be THE
GSIA. The 7000, 5000, 2200 and 1220 altitudes I read as stepdowns when you are LOC-only.
So I would expect to intercept the GS at FNUCH at 7000’ and fly it all the way down.

The chart is extra-confusing because it does also note that the 7000 and 5000 feet altitudes
might be 2700 ‘if authorized by ATC’, again I would have only expected they come into play
on a LOC-only approach.

My chart readings are notoriously as inaccurate as Madame ZhaZha’s palm readings and I’m more
than delighted if I’m wrong and learn something.

22

In reply to:

Mabey it’s the word choice, but to me it just doesn’t seem rational to “dive down” while in the clouds relatively close to ground? That said, one benefit that nobody mentioned to going lower is that you may break out of the clouds making the approach safer. -Todd

Good point, but in most cases, you should have an idea if it would help you break out based upon the ATIS/AWOS/reported Wx. Additionally, I have always felt that especially on dive and drive VOR approaches, that if the engine were to fail, there would be no way to make it to the airport. I am pretty big on smooth, gentle changes in pitch and bank when IMC.

23

Scott,
The point is this:
You said:
“The Glide Slope Intercept Altitude (GSIA) is a mandatory altitude. You cannot intercept the GS anywhere else. So the proper course of action is to maintain the last assigned altitude until you are established on the localizer (whatever that means) and then you being (sic) a descent down to 1,300 without using the GS as a primary means to descend, thus intercepting the GS at 1,300.”

On the approach I cited you can intercept the glideslope at several places. Therefore there CANNOT be just one mandatory Glide Slope Intercept Altitude. You could be asked to intercept and maintain 7000 until FNUCH. You could be asked to maintain 2700 and intercept between NOLAN and RIDGE, you could be vectored to the localizer inside FNUCH and outside NOLAN etc.
In each case ATC will turn you on the localizer below the glideslope at that point and give you a minimum altitude until established, or a mandatory crossing altitude. You can hold that altitude until GS interception and fly it down.
In the approach I quoted there can’t be just one point where you must intercept as you suggested many posts ago (quoted above).

24

Jerry,
Don’t disagree with you here for intecepting the GS at an altitude below 7000 ft. But the original question had to do with intercepting the GS outside or above the GSIA which is not authorized. For example, in the approach you stated, you cannot intercept the GS at 8000 since 7000 is defined as the GSIA. You are not authorized to use the GS as primary vertical guidance until you reach 7000 ft.

That’s why I was unsure how your example applied to the original question.

25

Scott,
At the risk of beating a dead horse, Jeppesen defines the Glideslope Intercept altitude as “The minimum altitude to intercept the glide slope/path on a precision approach…”
At least to me that means that one can intercept the glideslope at or above the altitude shown where the vertical profile line signifying the procedure turn altitudes meets the GS.
In Ed’s original question I assume he was somewhat above 1700 feet when he intercepted the localizer. I also assume that the altitude he was at was below the glide slope at that point. If the definition in the Jepp glossary is correct all he has to do is intercept the GS at or above (i.e. not below) 1700 feet. I see no problem with maintaining one’s altitude, intercepting the GS and tracking it down. Clearly one needs to check that the altitude at which you cross the marker of FAF point agrees with the altitude shown on the chart.
Why it should matter whether I dive to 1700 or gradually descend to 1700 using either my brain or the GS to get me to where I want to be is beyond my comprehension.
I also think you said in one post that the GS is not checked outside the GSIA. I believe that all glide slopes are calibrated for accuracy to 10 miles out and that a number of installations are calibrated for an even greater distance.

26

In reply to:

I also think you said in one post that the GS is not checked outside the GSIA. I believe that all glide slopes are calibrated for accuracy to 10 miles out and that a number of installations are calibrated for an even greater distance.

Jerry,

This is the point of my concern. I e-mailed one of my buddies that does the FAA’s flight check for instrument approaches (I think he flies a King Air). Here is his response:

In reply to:

The glide slope is routinely flight checked within the operational service volume to ensure it supports the intended use procedurally on the Standard Instrument Approach Procedure (SIAP). Among other checks, on-path evaluations are conducted from GSIA inbound and checks beyond the published GSI are evaluated below the GS for adequate fly-up indication. The GS is NOT checked for crossing restriction compliance at fixes outside the published GSI. Intercepts of the GS at altitudes above the published GSIA may result in deviations from procedural requirements and failure to meet altitude requirements at fixes outside the published point of GS intercept.

Now you can draw whatever you want from this, but he strongly urges all pilots never to use the GS as primary guidance to descend outside the end of the feather on the charts. He would prefer pilots to intercept the localizer at the ATC-assigned altitude, then descend using the dive-and-drive method to the GSIA to intercept the GS. From this altitude you are reasonably assured that you will not receive any weirdness when intercepting and descending on the GS.

He is not saying that a pilot couldn’t be successful intercepting the GS from a higher altitude, he’s saying that in doing so, you are in the area of an unknown and unproven.

27

Scott,
That’s all well and good but going back to the original question there are NO restrictions outside the GSIA that have to be complied with. You simply have to maintain 1700 until established. The next restriction is to cross the OM (QUEEZY) at or above 1300. Clearly, if you intercept the glideslope from below and track it down you should be able to do that. If you cross the OM and your altimeter should read 1230 feet. If it doesn’t you have a problem and need to miss.
I can understand that if you have restrictions outside the marker or GSIA that it may not be a good idea to use the GS as your primary descent aid but that was not the case in the original question. I also would submit that diving to 1300 feet when you are 10 miles out is simply not a great idea when you can either use the GS as an aid OR you can use VNAV to descend to 1300 feet two miles or so from the OM.
Diving and driving ought to be relegated to history. That makes it less likely you will dive and drive into the ground.

28

In reply to:

Diving and driving ought to be relegated to history. That makes it less likely you will dive and drive into the ground.

Good to see we have something to discuss again! We’ve gone way too long…[;)]

Why is dive and drive so bad? I guess you don’t do many non-precision approaches. You have to keep an eye on the altimeter no matter how you do it. Do you think a GS makes that problem go away? I don’t see this as a risk element. By the nature of the IAP you don’t risk hitting anything by descending without the aide of the GS.

I do see using a glideslope as guidance to get down to the GSIA as a problem (or could be problematic). It works most of the time because most of the time it works. How’s that for logic? In other words, you’ve gotten away with it for now. There’s a GS out there waiting to bite you one of these days. It is a risk (although very small) I don’t need to assume…so I eliminate it.

In reply to:

You simply have to maintain 1700 until established.

Established on the localizer, not the GS.

29

Yes Scott, it is good to have a spirited disagreement now and then.
No doubt that the “maintain xxx until established” refers to being established on the localizer NOT the glideslope. Here we agree.
Dive and drive isn’t bad per se but it does require configuration changes during a busy part of the flight and increases cockpit workload. It certainly does provide terrain/obstacle clearance as long as you remember to level off properly but if there is a distraction it can easy to descend below the target altitude. I know that an experienced and competent pilot like you wouldn’t do that, but I think that in general it is best to keep configuration and altitude changes to a minimum. Intercepting the glideslope at the higher altitude and simply keeping everything constant (like speed and rate of descent) from that point makes for a more relaxed, and therefore safer approach.
You’re right. I do very few non-precision approaches because all my flying is done with precision!!! (If I knew how to post a smiley face it would go right here).

30

In reply to:

I do very few non-precision approaches because all my flying is done with precision!!! (If I knew how to post a smiley face it would go right here).

Jerry,

Ha! I know how to do something you don’t! [;)] Don’t anybody tell him how to do it either…make him suffer for his inadequencies. [;)][;)][;)][H][;)][;)][;)][H][;)][;)][;)][H][;)][;)][;)][H][;)][;)][;)][H][;)][;)][;)][H]

31

Scott:
I do not think it is the practice of the airlines to dive and drive. When you look at the string of planes on approach into BWI, arene’t they all on an extended profile descent from 10 or more miles out? Jerry gave the example of ORD which I think is an exception as their GS intercept altitude is 7000 feet. But take ANY large airport in the east here and I think you will see that all of the jet traffic is intercepting the GS above the GSIA only because they are lined up on the localizers so far out that they are already below the glideslope. In that position they are just flying below the glideslope to capture at a distance out furthur than what we typically do. In that configuration, it would not be the best approach to go lower when you are already below the glideslope.

32

Scott,

Too funny! Jerrold, just put the word “smile” between brackets with no spaces and you get yourself a cute little icon. Other things you can create with a word and brackets are listed here. [:)]

33

Brian,

The airlines don’t do PTs and they are always given an altitude that is consistent with the GSIA. So it is normally a non-event.

34

Hi there Brian,

There is a big difference between flying a non-precision approach in a regional jet, and in an SR-22.

Stabilized approaches are certainly a good thing. The concept is fairly ubiquitous in GA. When it comes to NP approaches in little aircraft, however, dive-and-drive is the way to go. Side note, you mentioned glideslopes in your post, but I think you were just referring to non-precision approaches. The dive-and-drive concept has no relevance on an ILS approach.

First, the cons. Depending on the aircraft, configuration and power settings are changing. The pilot may be re-trimming the aircraft during or after the “dive” and prior to the next. Verifying successful deployment of gear (if RG) and flaps also adds to the workload, although it should occur prior to the Final Approach Fix.

Do that with a 40,000 lb. airplane and your hands are full. Turbine engines take longer to spool up. Sweptwing airfoils and their various drag/lift producing devices, flown at low airspeeds, require a different type of piloting technique than what we’re accustomed to in General Aviation. And of course the greater inertia of a transport category turbojet airplane mean that configuration changes are a relatively big deal. A little late here, a second or two behind there… it can add up.

But, back to our little 3,400 lb’er. In the SR-22, the pilot workload increases only with respect to configuring and flying the aircraft. There are no Vref speeds to calculate. The inertia of the aircraft is quite low due to its relatively light weight. Piston engines respond quickly to increases in power demands. Dropping down from one altitude to another is just not a big deal. The fixed-gear configuration of the Cirrus makes it even easier. Flaps 50% prior to the FAF, trim it, and you’re done. Just pull out the power to go down.

So, initiating at 600-700fpm descent from one stepdown fix to another, on a typical approach under typical conditions, should be easily accomplished by the average instrument pilot. Why stabilize? A better question might be, why not stabilize?

The simple answer is, in most general aviation aircraft, we’re not equipped to easily take advantage of a calculated and stabilized descent rate which brings us to the VDP, or some point prior to the VDP, on a NP approach. (It goes without saying that calcuating a descent rate which brings the aircraft to the Minimum Descent Altitude while simultaneously reaching the Missed Approach Point wouldn’t work - we need to be able to meet the requirements of 91.175(c)(1), which states that descent below MDA may only occur if “The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers,” among other requirements.)

When we use a stabilized descent rate across multiple stepdown fixes on a NP approach in small aircraft, we run the risk of running high and fast on the approach, and we may tend to minimize the time spent at MDA looking for the airport. If an anxious pilot spots the airport just as he reaches the MAP, he may feel pressure to chop the power and slam dunk the landing onto the runway, which could lead to a less-than-desirable outcome. I advocate descending to MDA as soon as the IAP allows - why deny yourself the opportunity to break out, find the airport, and start decelerating as soon as able?

I also believe that calculating and then maintaining a stabilized descent rate with accuracy is difficult in small aircraft. Our lower inertia - a good thing for making rapid attitude or altitude adjustments - also means that we’ll face more harassment from changing winds on the approach. When’s the last time you flew an approach with a truly constant groundspeed, from FAF to MDA? I can’t remember one - probably because I’ve never maintained a perfectly constant groundspeed on an approach. In short, I believe flying a faux glideslope taxes the GA pilot more than diving and driving down to MDA. I also believe the opportunity for errors of the worst magnitude are more likely when stabilizing a NP approach descent due to pilot fixation on factors not pertinent to safety. Just break out and give yourself more time to find the airport.

Best,

-Ryan

In reply to:

Scott:
I do not think it is the practice of the airlines to dive and drive. When you look at the string of planes on approach into BWI, arene’t they all on an extended profile descent from 10 or more miles out? Jerry gave the example of ORD which I think is an exception as their GS intercept altitude is 7000 feet. But take ANY large airport in the east here and I think you will see that all of the jet traffic is intercepting the GS above the GSIA only because they are lined up on the localizers so far out that they are already below the glideslope. In that position they are just flying below the glideslope to capture at a distance out furthur than what we typically do. In that configuration, it would not be the best approach to go lower when you are already below the glideslope.

35

In reply to:

Scott:
I do not think it is the practice of the airlines to dive and drive. When you look at the string of planes on approach into BWI, arene’t they all on an extended profile descent from 10 or more miles out? Jerry gave the example of ORD which I think is an exception as their GS intercept altitude is 7000 feet. But take ANY large airport in the east here and I think you will see that all of the jet traffic is intercepting the GS above the GSIA only because they are lined up on the localizers so far out that they are already below the glideslope. In that position they are just flying below the glideslope to capture at a distance out furthur than what we typically do. In that configuration, it would not be the best approach to go lower when you are already below the glideslope.

Brian,

You are correct, airlines dont dive and drive. But I guess it depends on what you call diving. When I am being vectored onto a ILS and I am outside the GSIA if I am given a lower altitude below the glideslope I will descend no faster than 1500 FT per minute and that is if I really have to get down. Most of the time it is around 1000 FT per minute. This is flying a MD-83 with a maximum landing weight of 130,000 pounds.

Scott is correct. 99% of the time ATC vectors us right at the GSIA so it is really a none event. If we are given a lower altitude (which sometimes happens) the new lower altitude is the new GSIA.

On non-precision approaches the ballgame changes a little bit.
Airlines have to deal with VDP’s and stuff that most GA pilots never think about. But since this really is not part of the discussion I wont even go there.

I must admit however that sometimes if given the opportunity I will fly the glideslope (or above) well outside the GSIA. Why you ask? Mainly becuase of wake turbulence. If anyone reading this has ever been in bad, and I mean really bad wake turbulence coming off a B-777, B-747, MD-11, B-757 or even a B-737-800 you know you want to avoid it like the plague. In that case I will skip the step downs or altitudes and fly the glideslope outside the GSIA a dot high or climb a few hundred feet.

I also must admit that in the past I have joined the glideslope outside the GSIA becuase of shear laziness. 16 hour duty days with 9.5 hours of flying in and out of New York all day can do that to a person.

Just to stir the pot a little why dont you all look the ILS approaches for RWY’s 26 and 27 for George Bush Intercontinental Houston (IAH). Notice that it has MANDATORY written by all the step downs. If this is infact how the FAA views this then why isn’t it on all the ILS approaches? I know why they put it here becuase a buddy of mine almost failed his ATR-42 type ride becuase of this. When I took my type ride the next day I got an earful from the same FAA inspector. Many times the step downs are waived by approach and you can join where ever you want however it is interesting why they put them there.

Can anyone guess?

B

36

Scott,
In the original question there was no procedure turn either. Like non precision approaches I rarely do procedure turns either (remember our discussion a few years back on that one) and, like the airlines, when we’re vectored for an approach we are given altitudes that allow us to intercept the GS outside the GSIA thereby making it a non-event as well. [:)], [:)]. (thanks Andy!!)

37

You’re welcome, Jerry. Nice job. [:)]

38

In reply to:

I advocate descending to MDA as soon as the IAP allows - why deny yourself the opportunity to break out, find the airport, and start decelerating as soon as able?

Now you’ve done it Ryan, you’ve opened up that can of worms. This thread may go on forever. [;)]

I agree totally with you here. I am very sensitive to the comparison of an SR22 (or SR20) with a Heavy 767. Just because the “big boyz do it” isn’t a reason we should either. I know this will ruffle some feathers with a few of our rather astute pilots in COPA but an SR22 is not a Boeing 737.

ATC handles the big iron differently than us smaller GA aircraft. Except on the occasional non-precision approach they may conduct in “good weather” conditions, they are vectored at an altitude that is completely consistent with the GSIA. You will rarely see a commercial jet given an altitude that is 1000 ft above the GSIA and told to maintain this altitude until established and cleared for the approach. ATC does better than that and are sensitive to their flight characteristics and limitations.

I’ve done hundreds of approaches into BWI (a Class B primary airport) and have always been given an altitude that is consistent with the GSIA or below when it is authorized by ATC and is charted as such.

Smaller GA aircraft fly into different airports thay may or may not be radar monitored. It is not difficult, nor unsafe, nor inappropriate to step down to the GSIA where the GS has been flight checked to be accurate for intercept. It nearly eliminates any chance of following a false or erratic or incorrect GS. Why? Because that’s the way the approach was designed, intended and flight checked. Distance and temperature differences can accentuate the errors that may be present at the higher altitude.

Does making a drop to the GSIA increase the workload? I think it actually decreases the workload assuming you follow the practice of flying appropriate airspeeds, decent rates and use the proper power settings. This is much easier than chasing a “potentially” erratic or flakey GS.

39

Hi Ryan:
I want to first thank you for your input. You took a lot of time and effort to help us even though you are not even a COPA member. Having had my instrument rating for over 20 years now, I have developed a fair degree of habits. Some of them are good, I guess, because I have not gotten myself into trouble. Other habits may not be so good and I have just been lucky so far.
But here is a scenario that relates to the latest discussion.
Seems the ILS ids one of the easiest approaches to fly because course guidance is available from altitude to DH with a fair degree of precision. The idea is to fly at a safe altitude (above obstacles) until in a position to descend to the point where you break out of the clouds and then see to land. I do not want to be too low too far out from the field as I would like to have some “altitude cushion” in the event of an emergency.
ATC has me descend to an altitude to intercept the localizer. The GSIA altitude is published and is USUALLLY, but not always located near the FAF. BUt in this case, I am vectored onto the localizer 5 miles out from the FAF. My instrument will show localizer centered and GS needle high up to indicate I am BELOW the glideslope at this point on the approach. I have also been told that false glideslopes are all ABOVE the true glideslope. Noe ATC clears me for the approach.
Since I am already below the glideslope, it seems to me that the easier way to fly the ILS in this example is to keep the SAME altitude until you approach the FAF and pick up the glideslope and then fly it down. In this case you will interceot a bit furthur out than at exact GSIA but it seems easier.
Given all that we know about glideslopes, I do not see a problem in doing in this way as long as you are not zillions of miles away (more than 10 miles) from the point you choose to do this technique.
You will then have one airplane configuration change from the straight and level on the localizer to then just ride the GS down to DH verifying the correct ready at FAF passage.
Right or wrong to do it that way?

40

In reply to:

The fixed-gear configuration of the Cirrus makes it even easier. Flaps 50% prior to the FAF, trim it, and you’re done. Just pull out the power to go down.
Best,
-Ryan

Why do you want to add flaps prior to the FAF? I’ve done every variation possible.

It’s kinda hard to add flaps when they tell you to maintain ‘150kts until the marker’, eh?

The last CFII I flew with said not to add any flaps until ‘landing was assured’.

The 2nd to the last CFII said to add one notch at the start of vectors to final.

Is there a ‘right’ answer?

Jerry