ILS or GPS?

BobDDuck

Island Bus Driver
Scenario:

Winds 010@9. 700 OVC 2SM RA

Landing at an uncontrolled field with an ILS to Runway 20 (with 200 and 1/2 mins) and a GPS to Runway 2 (with 600 and 1 1/4 mins).

Pertinent data (but not really legally controlling in this case):

-Max tailwind for the airframe is 10 knots.
-The runway is wet.
-The runway is "shortish" for the airframe in question.
-The GPS approach has a nasty stepdown fix that gives you about .2 miles to get from an intermediate altitude of 800 feet to the VDP (well, PDP actually) at the MDA of 600 feet while flying at a ref speed of 145 knots.
-There is only a REIL on 20 while 2 has full approach lights.

Do you take the headwind or the tailwind approach?

And just for reference this is KPGV.
 

MoMatt

Well-Known Member
Interesting.

Depending on the landing distance required with the tailwind, I'd try the ILS and be darn sure I went around if it didn't look like I was going to get it on in the first 500-1000 feet. Airnav says that it's a grooved runway, so that helps.

Kind of hard to answer without knowing the capabilities of the airplane and the landing distance. IOW, I'd be happy to try the GPS in a Lear 31, not so happy to try it in a Lear 35 simply because the 31 has much nicer equipment.
 

N519AT

Ahh! This is how I change this!
Does the performance data support a landing on runway 20? What's the landing distance for the specific weight on runway 20 with a contaminated runway and 10 knot tailwind?

20 has a displaced threshold too, which brings the landing distance available down to 6100. Put it right on the thousos and you have 5100 to stop in a jet with no leading edge devices and a high ref speed, add a tailwind in with a wet runway makes it a no bueno situation for me. Plus 20 has a very small slope down from the approach end to the departure end.

I'd try the GPS to 2, if you can get it down stablized and put it in the TDZ, great. If not, bug out to the alternate where the wx is beautiful and winds are light right down the pipe.
 

Roger Roger

Paid to sleep, fly for fun
Doing some rough mental math that descent gradient from the step down to the VDP is gonna require something north of 2000 ft/min at your stated Vref. How well does your jet handle that kind of descent on the last segment of an IAP? To me that seems scary fast for a descent to minimums in IMC but then again my brain maxes out about 150 knots right now and much slower than that if doing an approach in weather.
Even so I guess I'd try the GPS approach, as the guy above stated you can always bug out if it's just not looking like its going to work out. It seems like runway overruns are one of the more common RJ accidents and going off some of what 519AT posted (sounds like he knows the airport and the aircraft) I'd rather not mess with it.
 

Hacker15e

Blue Eyed Murder in a Size 46L Uniform
That GPS stepdown isn't that big of a deal, but I'd really be comparing the tailwind landing performance against where your touchdown point is if you are above 600 at the VDP.

Is the VDP one that is published, or is it what you've computed for your own jet's performance. What glideslope is the VDP computed for?

Rarely do I ever see a published VDP that is actually coincident with what I get computing one for 3 degrees and my MDA, but that is also with Cat D and E jets.
 

MikeD

Administrator
Staff member
That GPS stepdown isn't that big of a deal, but I'd really be comparing the tailwind landing performance against where your touchdown point is if you are above 600 at the VDP.

Is the VDP one that is published, or is it what you've computed for your own jet's performance. What glideslope is the VDP computed for?

Rarely do I ever see a published VDP that is actually coincident with what I get computing one for 3 degrees and my MDA, but that is also with Cat D and E jets.
On the plates, I believe that the Lead Radials depicted and oftentimes the VDPs, are for the highest Category of aircraft depicted in the mins, but might not be for a standard 3 degree GP.

Interesting too at the field being discussed, that the ILS runway only has REILs, while the non-precision runway has a full approach lighting system....
 

deadstick

Well-Known Member
Still rounding up on the step-downs and MDA's? At 536 AGL, wouldn't the PDP be at about 1.8? And 2 has PAPI which would help.
 

Hacker15e

Blue Eyed Murder in a Size 46L Uniform
On the plates, I believe that the Lead Radials depicted and oftentimes the VDPs, are for the highest Category of aircraft depicted in the mins, but might not be for a standard 3 degree GP
Yeah, I'm aware that's the 'book answer', but in years of flying Cat E airplanes around, I've had to recompute the VDP more times than not for the 3-deg GP. It is to the point that I usually disregard published lead radials and VDPs and just compute my own as a manner of habit.
 

BobDDuck

Island Bus Driver
I'm going to work backwards through the replies...

Hacker15e - if we have a published VDP, we are required to use it. As Mike said, it's often not a 3.0 degree GP. If there is no VDP (as in this case) we just make up our own PDP, which works out to about a 3 degree slope if you do the math right.

deadstick - Yes, the numbers get rounded up still. I came out with 1.9 doing it in my head, but I think your 1.8 is correct. And the PAPI does help.

MikeD, I think you've got the runways backwards. The ILS to 20 full lights. The Runway 2 only has REILS. I didn't know that about lead radials and VDPs being setup for cat E aircraft. Makes sense though.

Hacker again- The stepdown (in our jet) is a pretty big deal. We are, per FAA word of mouth, allowed to "lead" our descent fixes by .2, so that actually gives us .5 to get from 2.2 to the PDP of 1.9. It's only a loss of 200 feet but by the time the nose comes down, with either the autopilot flying or hand flying, and the plane actually starts descending, it can take a bit of time.

Roger Roger - My mental math sucks... backed up with a calculator... 140 groundspeed means 2.3 miles a minute which means about 1 miles every 26 seconds, which means that .5 to get from the intermediate stepdown fix to MDA at the PDP has to be done in about 13 seconds. If you expand that out to feet per minute you end up with about 925. It's pretty quick, especially when the plane is slow to nose over.

N519AT - We had landing data for either runway, even with the tailwind and wet surface. I think we'd be using something like 5700 of the 6100 feet available. Again... legal, but REALLY close. I may have some pilot skillz (although not after leg 4 and hour 10 of duty) but 400 feet is cutting it close.

My thought was to try the GPS. I'd rather have to go around because we didn't see anything than have to go around low level because it looked like we might be landing 400 feet long. We found the runway just about at the VDP and managed to get in with no problems.

Looking back on it, I just thought it was an interesting case study about decision making. Do you go with the approach that is sure to get you in but may risk an over run if you don't do everything right or the wind increases a few knots, or do you go with the approach that you may not get in on, but will make for a much easier landing? Things to think about I guess.
 

Roger Roger

Paid to sleep, fly for fun
Roger Roger - My mental math sucks... backed up with a calculator... 140 groundspeed means 2.3 miles a minute which means about 1 miles every 26 seconds, which means that .5 to get from the intermediate stepdown fix to MDA at the PDP has to be done in about 13 seconds. If you expand that out to feet per minute you end up with about 925. It's pretty quick, especially when the plane is slow to nose over.
Ah, didn't realize you were allowed to lead the step down fix. Is it possible with the equipment on your jet to set up your descent to the step down altitude such that you're descending at 1000-ish FPM and reaching that 800 feet at the same time you hit the step down fix, that way you're not having to initiate the descent to MDA but rather just continue what you're doing?
 

BobDDuck

Island Bus Driver
Yes. A constant rate descent is preferable to a dive and drive method, but we only have "advisory" VNAV in the jet so all we can do is follow the snowflake down and try to match the actual descent rate commanded by the vertical speed mode of the autopilot to the recommended VNAV number displayed on the screen. The problem is the plane will capture an altitude several hundred feet above it and start leveling off so you'd actually have to set the next (lower) altitude before you got to your intermediate altitude and just make sure you don't go below it until you pass the fix.

Because of all that, the dive and drive method general gets used.
 

MoMatt

Well-Known Member
And do you find that preferable to hand-flying the approach? Some of our airplanes also take their sweet old time to nose over with the autopilot engaged, so I find a non-precision approach to mins (especially where step downs / VDP is tight) easier to hand fly.

What kind of airplane is it?
 

MoMatt

Well-Known Member
ILS Circle to Land?
No thanks. I make it a goal to have to perform circling approaches at mins only in the sim.

Edit: Also note the minimum circling visibility of 2 miles for Category D.

But in general, a circling approach in a jet where you have to maneuver from downwind to base to final leaves too much opportunity to screw up when you're "low and slow".
 

MikeD

Administrator
Staff member
I'm going to work backwards through the replies...

MikeD, I think you've got the runways backwards. The ILS to 20 full lights. The Runway 2 only has REILS. I didn't know that about lead radials and VDPs being setup for cat E aircraft. Makes sense though.
I hadn't looked it up on the plate, I was just going by your original post:

Scenario:

Landing at an uncontrolled field with an ILS to Runway 20 (with 200 and 1/2 mins) and a GPS to Runway 2 (with 600 and 1 1/4 mins).

-There is only a REIL on 20 while 2 has full approach lights..
I should've looked it up! :)
 

BobDDuck

Island Bus Driver
And do you find that preferable to hand-flying the approach? Some of our airplanes also take their sweet old time to nose over with the autopilot engaged, so I find a non-precision approach to mins (especially where step downs / VDP is tight) easier to hand fly.

What kind of airplane is it?
CRJ. It could be hand flown, but it still takes a few seconds to get the nose to pitch over. Otherwise you tend to scare the passengers and they frown on us doing that sort of thing.

ILS Circle to Land?
Not approved. Circling limited to VMC only.
 
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