Localizer Approaches

[Mark Hargrove]

I'm practicing for I-08 and have tried flying the KSAN LOC RWY 27 approach in a couple of different ways and would like some guidance on which is best (if one way really can be said to be better than the other). The first way I've flown it is to descend pretty rapidly to the "at or above" altitude for each fix, leveling out before actually reaching the fix, then rapidly descending again for the next fix in the sequence. Starting at VYDDA there are four "step down" points including the descent to the MDA. After flying the approach this way a few times (and wondering what the heck the passengers would think was happening if I was flying the approach real-world in this manner), I tried another way: I set up a constant-rate descent to reach the MDA a few miles after REEBO -- about a mile or so from the MAP at I-UBR. This method was WAY easier to fly, though it took me a couple of tries to sort out the right descent rate that accomplished this. I noticed that when I got this right, I was hitting the intermediate fixes at pretty darned close to the indicated at/above altitude. I've got notes now on what power settings to use in my Pilatus PC-12 to make this sort of 'stabilized' approach work at KSAN, but I don't know that I could determine what to do at another airport without practice.

What is the "right" way to fly a localizer approach? Stair-step down, or try to find a stabilized descent rate? (and if the latter, how to estimate what rate to use for an unfamiliar approach?)

BTW, even though I've been a passenger on various airlines flying into KSAN many times and seen it on the right side, I've never quite appreciated exactly how close that building off the right is on short final into runway 27 -- and we're below the top of the building!

Thanks!

-M.

[Orest Skrypuch]

A great question, look forward to the discussion.

Carriers have firmly adopted a constant descent profile approach (CDAP) method, instead of the old chop (throttle) & drop (altitude). Multiple changes in power and attitude on an approach are not as effective or safe. All modern panels can automatically calculate the glidepath required for a CDAP approach (IAN). That includes WAAS enabled panel mounted GPS equipment in small aircraft. If your panel does not, you can do a quick calculation (just a bit of geometry), and come up with the required VS to use to descend to the runway threshold on a constant descent.

Two things to be careful about, first -- before the FAF be sure you respect any charted intermediate altitudes. That goes for precision approaches as well, Teterboro ILS 6 is notorious for having a minimum and maximum at an intermediate point (and not on the glideslope). Second -- after descending through the MDA on a non-precision approach on the "glidepath" you are visual (as you must be), and there is no terrain or obstacle protection on that "glidepath". With a high MDA, that can be for a fair bit of the approach.

Now, in a small aircraft (low inertia) there might yet be a case made for chop & drop on a non-precision approach in the presence of an irregular ragged ceiling that is close to minimums. Getting low as soon as you can might increase your chances of breaking out and seeing the runway, before reaching the MAP and having to go-around. Of course it could also increase your chances of CFIT if not executed precisely.

* Orest

[Orest Skrypuch]

BTW, a quick rule of thumb, to achieve a 3° slope: take your GS and multiply by 5, that is your required VS. Example, if GS =100 knts, you need a VS of 100 x 5 = 500 fpm to descend at 3°.

For a slope of 3.5°, the multiplier is approximately 6, and for 4° the multiplier is approx 7.

The slope will generally be shown on the chart. If not, then use the 1 in 60 rule to calculate it. A drop of 1 ft in 60 ft of ground run, is about 1°. For example, 300ft in 1nm, is 300 ft in 6000ft or 300/6000ths or 3/60ths = 3°.

* Orest

[TomATL]

As a current instrument student, I'm being taught that the proper method for managing a VOR approach is to descend rapidly (700-800 fpm) to the next minimum altitude, for the reasons Orest mentioned above. Chugging along at MDA well before the MAP gives you a better chance to spot the airfield, and you won't be distracted managing a descent at the same time. I can imagine the scenario is different for bigger iron with more sophisticated avionics, but can't speak directly to that point.

[AviatorScott]

Just a thought, as an IFR pilot in real life (GA, not big guys). I will try and get down as quickly as I can on a VOR or Localizer approach, however my experience in the system has been that some controllers will warn you about your altitude. They worry about that CFIT, and have started to expect that we will all fly a controlled descent regardless of what is "allowed" under the rules for that approach. The obvious way to stop those warnings is to fly only ILS or WAAS approaches into very controlled airports. I tend to shot for altitude before the MAP to give myself the best opportunity to get below the gunk (or get my safety pilot to let me take my hood off) and make the landing.

Scott

[Orest Skrypuch]

There is a middle ground here, still utilizing the proven safer CDAP method, but getting a longer look while at MDA, in time yet to land safely.

Consider this, just offset the runway threshold 1nm short. Set up for and fly a CDAP to that point instead of the runway threshold. Of course respect all other minimums and level off at the MDA as usual, but now you will have a good 30 to 40 seconds to look around. You will lose the LNAV+V advisory guidance (if available), but if you really want a longer look, in my view this is a better way to do it.

Diving and driving, and having a good look around 3nm back from the runway, would rarely be of much additional benefit. Personally, the last thing I want to be is low & slow, a long way from the runway in challenging IMC conditions. BTW the safety payoff of CDAP is because it has been demonstrated that it easier to maintain continuous and constant speed, power & attitude, then to have to manage level offs and descents, perhaps more than once while on approach.


BUT, whatever you do, when the pressure is on, the ceiling is low, and the vis is poor, you DO NOT want to be creating some novel way of flying an approach. Use your fast ball. Whatever you fly most of the time, whatever is most practiced, whatever you are most proficient at, is what you want to fly. That may be a dive & drive, or CDAP, or something else. If you have to miss, you miss, no big deal, it is not a failure, it is not a crime, you fly somewhere else and get in on an ILS.

The other consideration, most pilots flying Part 91 with an instrument rating, very seldom see an approach down to minimums. You will usually plan around bad weather (or should), delay or cancel, you do not have to go, unlike a scheduled carrier. It is probably 1% of non-precision approaches that you will fly down to hard minimums. I fly these with some regularity as I fly a lot and tend to fly destinations within set time frames. In addition, my home airport only has non-precision approaches, and in my area of the country we have serious weather. I always fly LNAV+V guided non-precision approaches, or at least CDAP if that is not available. I have never found that to be limitation, discussions of the ragged ceiling to the contrary.


But, there is no right or wrong here. Once you get your rating, you are the boss, then you decide. The best way, is the best way you yourself can fly it

* Orest

[muzz]

I have flown many non-precision approaches in real life. I have found over the years, that the best way (for me) is to start descent at the final approach point at around 1000 fpm. That will get me to the minimum altitude several miles from the runway, allowing for a good look and time to adjust the flight path if necessary. Of course you have to continue flying the course needle, you can't just go completely visual. The main problem with the 500 fpm gradual descent is that if the viz is low and you break out one side or the other from the final approach course you might not have time to adjust your course once you become visual. The main thing is practice, practice, practice.

[davisd100]

Fascinating discussion. I was taught the "chop and drop" method many years ago, and did not think of a stabilised approach. Anyway, I experimented with my little Cessna 172SP on KSAN, and found that the easiest for me was

1. Stabilise a level flight entry into the procedure at 90KIAS, all trimmed up as usual, which is about 2150 RPM
2. Go to the standard 1800-1850 RPM for a 500 fpm descent. In still air the groundspeed was obviously equivalent to the TAS, which was about 96 kts.
3. Using no more than + or - 50 RPM it was easily possible to manage the altitudes to hit or slightly exceed the safe crossings of the waypoints, FAF etc.

I set it up with minimum visiblilty and ceilings, and without destabilising the descent rate there was plenty of time to see the runway environment and land comfortably.
If not, a very slight acceleration of the descent rate (-50) on the last leg down to MDA would have given more time.

I haven't tried many permutations on wind, but it strikes me as a lot easier than the yoyoing between non-precison descent rates and level flight that my old method required. On the very rough rule of thumb that 100RPM is about 150FPM, pretty easy to modify the stabilised rate for headwinds., eg 10 kts gives about 85 GS which is about 425 which would need about 1900-1950 RPM

[jtek]

BTW, a quick rule of thumb, to achieve a 3° slope: take your GS and multiply by 5, that is your required VS. Example, if GS =100 knts, you need a VS of 100 x 5 = 500 fpm to descend at 3°.

For a slope of 3.5°, the multiplier is approximately 6, and for 4° the multiplier is approx 7.

The slope will generally be shown on the chart. If not, then use the 1 in 60 rule to calculate it. A drop of 1 ft in 60 ft of ground run, is about 1°. For example, 300ft in 1nm, is 300 ft in 6000ft or 300/6000ths or 3/60ths = 3°.

* Orest

The FAA publishes a handy table of these descent rates in the back of the terminal procedures book. You can get it online here:

http://aeronav.faa.gov/d-tpp/1209/frntmatter.pdf (look at the very last page)

[Orest Skrypuch]

The FAA publishes a handy table of these descent rates in the back of the terminal procedures book. You can get it online here:

http://aeronav.faa.gov/d-tpp/1209/frntmatter.pdf (look at the very last page)

Yes, such tables are available in a variety of places, including right on Jepp plates. They are of course trivial to create on your own as well, in a spreadsheet, they are just basic geometry.

But, while in the busy approach phase, you are likely best served with a quick rule of thumb that you can keep in your head. That will get you close to where you need to be, once stabilized you can adjust further as necessary.

* Orest