Minimum vectoring alt and Clear for ILS app

[extricate]

Hi there,

Suppose there's a restriction of not flying lower than 3500ft within 10nm from the airport, let's say the aircraft is 12nm away at 3500ft (thus need to abide by the 3500ft till 10), and subsequently got cleared for ILS approach, can the aircraft disregard the min. vectoring alt and descend on glide since it's cleared for approach?

Thanks

[aterpster]

Originally Posted by extricate

Hi there,

Suppose there's a restriction of not flying lower than 3500ft within 10nm from the airport, let's say the aircraft is 12nm away at 3500ft (thus need to abide by the 3500ft till 10), and subsequently got cleared for ILS approach, can the aircraft disregard the min. vectoring alt and descend on glide since it's cleared for approach?

Thanks

In the U.S. not unless on a published segment of the approach. The glide slope is not a published segment of the approach until the PFAF.

[AtoBsafely]

What a question!

1. No! MVA is there for a reason and ATC will not let you go below it. So your clearance would be, " Not below 3500' until 10nm, cleared for ILS".

2. I don't know of any approach anywhere that requires a glide slope intercept from above.

Is there a real place this happens???

[extricate]

Originally Posted by AtoBsafely

What a question!

1. No! MVA is there for a reason and ATC will not let you go below it. So your clearance would be, " Not below 3500' until 10nm, cleared for ILS".

2. I don't know of any approach anywhere that requires a glide slope intercept from above.

Is there a real place this happens???

Take a look at Manila RPLL, minimum vectoring altitude chart, within 10nm not below 3500ft, by then most often than not, ATC would have cleared ILS app rwy 24 which usually ends up high

[AtoBsafely]

OK, that helps.

For RPLL, the MVA to the east is 3500' from 10-20nm. The ILS not below altitude is 2500' at 7.5nm.

ATC normally vectors you onto the approach around 12-15nm, not below 3500', which works fine.

In this case, you can capture the glidepath outside 10nm, as the 3.1 degree glide slope is 3550' at 10nm. If you aren't certain of the math, just stay at 3500' and capture the glide path at 10nm (from 3500', which is what your ATC clearance would be). If you miss the glidepath, the you can still capture it by 2500' at 7.5nm.

Interesting, the procedural approach has you at 4000' until established inbound. That would be challenging!

There is significant terrain there, be aware of it. Always.

[A Squared]

Originally Posted by AtoBsafely

For RPLL, the MVA to the east is 3500' from 10-20nm.

You must be looking at a different MVA chart than mine. To the East it looks like 3500 from 10 to 15 on mine, > 15 nm MVA is 7000 MSL. 3500 at D20 would place you 327 ft above a 3173 MSL mountain peak out there.

Originally Posted by AtoBsafely

In this case, you can capture the glidepath outside 10nm, as the 3.1 degree glide slope is 3550' at 10nm.

Looks to me like 3500 MSL would put you about 180 ft above the GS at 10 IMA. SIN 3.06 X 10 X 6076 = 3243 add 75 ft TDZE = 3316 GS alt at D10 IMA

Originally Posted by AtoBsafely

If you aren't certain of the math, just stay at 3500' and capture the glide path at 10nm (from 3500', which is what your ATC clearance would be).

You may only descend on the GS if you're established on the localizer. You may have assumed that was understood, but it's worth clarifying for those readers for whom it's not obvious.

Anyway, it looks to me like it's not geometrically possible for ATC to issue a vector that complies with the MVAs, and intercepts the localizer at an angle of 30 degrees or less, below the GS. Even if they brought you in from the southeast on a vector that clips the D15/R080 point, aiming directly for D10 on the localizer, (shallowest possible LOC intercept angle within the 3500 MVA sector) you're still going to intercept the localizer at about 50 degrees, (yes, I did the math, I'm bored) as well as be above the glide slope.


There is another place which, IME is like this; Fairbanks, Alaska (PAFA) ILS 19R (now 20R) I used to fly for an airline based there, and we did all the training and checking in the airplane. So, I did a lot of ILS19R procedures, and every single time, approach would put you on the localizer, close in to the FAF, at a steep intercept, and above the GS .... every time. I haven't ever seen an MVA chart for Fairbanks, but I have to assume that they were up against some MVA sectors which didn't really work, geometrically. Either that, or they were just really consistently bad at vectoring, because I don't think I ever got a vector that intercepted the LOC at less than 30 degrees, 2 miles from the FAF and below the GS.

[swh]

The MVA within 10 DME MIA is 1900. Outside 10 DME to the east it rises to 3500. When established on IMA passing 10 DME IMA (ZULU) you can descend to 2500 to the FAP. ZULU is 10.3 DME MIA. The glide path is 3.06 degrees, the distance between MIA and the IMA DME is 1,921 ft.

It should work out at ZULU at 3500 ft depending on the QNH.

[A Squared]

Originally Posted by swh

. ZULU is 10.3 DME MIA. ... the distance between MIA and the IMA DME is 1,921 ft.

On what do you base that? As nearly as I can tell MIA and IMA DME are coincident. On the ILS or LOC 24 profile view, the the LOC MAP is D0.9 IMA, and is 0.7nm from the runway threshold. That indicates the threshold is D0.2IMA. Looking at the VOR Z Rwy 24, the MAP is D1.8 MIA, and 1.6 nm from the runway threshold, indicating that the runway threshold is D0.2 MIA, also. Unless I'm missing something, I don't see any way that there can be 0.3nm difference between MIA and IMA distances.

[Tee Emm]

You may only descend on the GS if you're established on the localizer

Back in the 1980's I was flying a 737-200 radar vectored for Manila ILS 24. If I recall we were initially coming from Guam (PGUM) and over a VOR called Jomalig. The radar vector kept us at 4000 ft until intercepting the Localiser. We were above low cloud. We did not get established on the localiser until around 8 miles and were fully configured at VREF+5 in anticipation of having to descend quite steeply to intercept the glide slope from above.

At 8 DME ILS we should have been around 2400 ft but were at around 1000 feet higher due poor radar vectoring. Yet the ILS glide slope indication showed half a dot high instead of full deflection too high.
The DME versus height simply wasn't working out. We broke visual about 4 miles out and obviously very high yet the glide slope showed on slope. The TVASIS was all over the place and displayed obviously erroneous light signals. We landed safely and I reported to ATC I thought there was a false but flyable glide slope at about five degrees. I also reported the TVASIS lights were useless for guidance.

A few weeks earlier a Air Manila (?) Boeing 707 had crashed short of the runway at a high rate of descent. I think everyone survived but the crew swore they were on glide slope all the way to impact. I wondered if they had followed a false but flyable glide slope to impact. On my return through Manila from Hong Kong a couple of weeks later my inquiries revealed that a calibration flight test on the 24 ILS did indeed find a flyable glide slope at 5.5 degrees. Also the reason for the TVASIS being useless was that there had been earth tremors which displaced the TVASIS installation. I asked in that case why was the TVASIS left switched on when it was obvious erroneous indications were likely and why no NOTAM? The answer came back that it was policy to have the TVASIS on for all jet aircraft landings. Words failed me.

[swh]

Originally Posted by A Squared

On what do you base that?

The MIA VOR/DME is 1912 ft further from the IMA DME, the location of the MIA VOR/DME is on the southern side of the 06/24 opposite E2, 193.66M FM RWY06/24 CL and 117.83M FM extended RWY13/31 CL. The IMA DME is located on the southern side on 06/24 opposite E1.

From the Philippines AIP (RPLL AD 2) the position of
MIA is 14 30 28.7 N 121 01 18.2460 E,
IMA DME 14 30 39.6330 N 121 01 34.2590 E,
ZULU 14 35 31.60 N 121 10 35.44 E

From MIA to ZULU the GPS distance is 62,672 ft, when at a true altitude of 3,500 ft, the calculated slant (DME) distance is 10.33 nm. ZULU is actually 10 GPS from IMA DME, I calculated it to be 10.01 IMA DME.

Be careful with the profile views, 3.01 vs 2.81 degree approach paths.

[AtoBsafely]

A Squared,

The points you made about my original post are all correct. I should read the chart more carefully before posting.

So we do have an approach where you need to intercept the glide slope from above. I'll just add that the normal temperatures for Manila exacerbate the problem, adding about 200' to your true altitude at 10nm.

[A Squared]

Originally Posted by swh

The MIA VOR/DME is 1912 ft further from the IMA DME, the location of the MIA VOR/DME is on the southern side of the 06/24 opposite E2, 193.66M FM RWY06/24 CL and 117.83M FM extended RWY13/31 CL. The IMA DME is located on the southern side on 06/24 opposite E1.

From the Philippines AIP (RPLL AD 2) the position of
MIA is 14 30 28.7 N 121 01 18.2460 E,
IMA DME 14 30 39.6330 N 121 01 34.2590 E,
ZULU 14 35 31.60 N 121 10 35.44 E

From MIA to ZULU the GPS distance is 62,672 ft, when at a true altitude of 3,500 ft, the calculated slant (DME) distance is 10.33 nm. ZULU is actually 10 GPS from IMA DME, I calculated it to be 10.01 IMA DME.

Be careful with the profile views, 3.01 vs 2.81 degree approach paths.

Thanks for the explanation. A couple of observations;

Originally Posted by swh

Be careful with the profile views, 3.01 vs 2.81 degree approach paths.

That isn't the source of the discrepancy here. The difference between 2.81 degree glidepath and a 3.06 degree glide path only makes about a 3 ft difference in horizontal distance at 2 nm (the MAP on the VOR approach) even out at ZULU, the difference is about 13 ft. For that matter the difference between the Slant Range and the and horizontal ground distance out at ZULU is less than 100 ft, which isn't meaningful in the context of navigation and obviously not the source of the difference here.


As far as your distances based on the geographic positions, that all checks. However. you're still going to be above the glideslope. Not a lot, and not too hard to push it over and catch it, but you aren't approaching it from below. The GS altitude at ZULU is, like I said earlier, going to be 3316, and as you approach ZULU, you're going to be at D10.3 MIA, still within the 3500 MVA sector, and still not established on a published portion of the approach (until passing ZULU). As AtoBsafely noted, higher than standard temperatures which are the norm there will place you even further above the GS. So, you're either going to have to capture the GS from above, or after crossing ZULU, descend rapidly until below the GS, then capture it from below. I'm not sure that the latter is really "better" than the former in any meaningful way.

As for my earlier claim that MIA and IMA distances were coincident, I agree that seems to not be the case. Apparently the distances on the VOR Z Rwy 24 profile which I used to are simply in error. Just looking at the Taxi Diagram, the location of the VOR appears to be more than 0.2 nm (1200 ft) from the Rwy 24 threshold. Measuring it in Google Earth, shows the VOR is about 0.5 nm from the runway threshold, so there's the missing 0.3 nm.

[swh]

Originally Posted by A Squared

That isn't the source of the discrepancy here. The difference between 2.81 degree glidepath and a 3.06 degree glide path only makes about a 3 ft difference in horizontal distance at 2 nm (the MAP on the VOR approach) even out at ZULU, the difference is about 13 ft. For that matter the difference between the Slant Range and the and horizontal ground distance out at ZULU is less than 100 ft, which isn't meaningful in the context of navigation and obviously not the source of the difference here.

The orthometric height change was not my point. The distance to the runway threshold is not published in the Philippines AIP, they are numbers calculated by Jepp. The published distance in the AIP for the LOC MAP is 0.92 DME IMA, Jepp published 0.9. In the AIP the MIA DVOR/DME is 193.66M FM RWY06/24 CL and 117.83M FM extended RWY13/31 CL.DME24 is 125M right of RWY24 & 410M FM RWY24 THR.

In the profile view you need to be aware of the rounding and the source of the data.

Originally Posted by A Squared

As far as your distances based on the geographic positions, that all checks. However. you're still going to be above the glideslope. Not a lot, and not too hard to push it over and catch it, but you aren't approaching it from below. The GS altitude at ZULU is, like I said earlier, going to be 3316, and as you approach ZULU, you're going to be at D10.3 MIA, still within the 3500 MVA sector, and still not established on a published portion of the approach (until passing ZULU). As AtoBsafely noted, higher than standard temperatures which are the norm there will place you even further above the GS. So, you're either going to have to capture the GS from above, or after crossing ZULU, descend rapidly until below the GS, then capture it from below. I'm not sure that the latter is really "better" than the former in any meaningful way.
.

The numbers I did above were just quick and dirty calculations, accurate calculations. MIA to ZULU 10.3 nm (19132.9166 m).

First Station : IML DME
----------------
LAT = 14 30 39.63300 North
LON = 121 1 34.25900 East
EHT = 15.2400 Meters

Second Station : ZULU
----------------
LAT = 14 35 31.60000 North
LON = 121 10 35.44000 East
EHT = 1066.8000 Meters

Forward azimuth FAZ = 60 59 59.4440 From North
Back azimuth BAZ = 241 2 15.4164 From North
Ellipsoidal distance S = 18520.1913 m
Delta height dh = 1051.5600 m
Mark-to-mark distance D = 18551.5845 m

Now by using the inverse of that projecting out where the intercept will be on an azimuth of 60 59 59.4440 from the IML DME at 3500 ft (1066.8 m), 10 DME (18520.01 m ) I get the glidepath intercept to be at N 14 35 31.10177 E 121 10 34.51561, that is 31 meters east of ZULU. Most aircraft will capture a GP within 150 ft.

First Station : INTERCEPT
----------------
LAT = 14 35 31.10177 North
LON = 121 10 34.51561 East
EHT = 1066.8000 Meters

Second Station : ZULU
----------------
LAT = 14 35 31.60000 North
LON = 121 10 35.44000 East
EHT = 1066.8000 Meters

Forward azimuth FAZ = 61 2 14.8453 From North
Back azimuth BAZ = 241 2 15.0782 From North
Ellipsoidal distance S = 31.6228 m
Delta height dh = 0.0000 m
Mark-to-mark distance D = 31.6281 m

[InSoMnIaC]

An intercept from above scenario is the least of your worries in Manila. The real danger begins once safely on the ground

[aterpster]

Originally Posted by InSoMnIaC

An intercept from above scenario is the least of your worries in Manila. The real danger begins once safely on the ground

I guess it depends where you are going in town.

[flite idol]

Originally Posted by Tee Emm

Back in the 1980's I was flying a 737-200 radar vectored for Manila ILS 24. If I recall we were initially coming from Guam (PGUM) and over a VOR called Jomalig. The radar vector kept us at 4000 ft until intercepting the Localiser. We were above low cloud. We did not get established on the localiser until around 8 miles and were fully configured at VREF+5 in anticipation of having to descend quite steeply to intercept the glide slope from above.

At 8 DME ILS we should have been around 2400 ft but were at around 1000 feet higher due poor radar vectoring. Yet the ILS glide slope indication showed half a dot high instead of full deflection too high.
The DME versus height simply wasn't working out. We broke visual about 4 miles out and obviously very high yet the glide slope showed on slope. The TVASIS was all over the place and displayed obviously erroneous light signals. We landed safely and I reported to ATC I thought there was a false but flyable glide slope at about five degrees. I also reported the TVASIS lights were useless for guidance.

A few weeks earlier a Air Manila (?) Boeing 707 had crashed short of the runway at a high rate of descent. I think everyone survived but the crew swore they were on glide slope all the way to impact. I wondered if they had followed a false but flyable glide slope to impact. On my return through Manila from Hong Kong a couple of weeks later my inquiries revealed that a calibration flight test on the 24 ILS did indeed find a flyable glide slope at 5.5 degrees. Also the reason for the TVASIS being useless was that there had been earth tremors which displaced the TVASIS installation. I asked in that case why was the TVASIS left switched on when it was obvious erroneous indications were likely and why no NOTAM? The answer came back that it was policy to have the TVASIS on for all jet aircraft landings. Words failed me.

interesting report. Obviously still vivid in your memory. False G/S capture from above us a very real and present danger. Cross check of fix altitudes is vital once established to mitigate.

[wiedehopf]

Originally Posted by flite idol

interesting report. Obviously still vivid in your memory. False G/S capture from above us a very real and present danger. Cross check of fix altitudes is vital once established to mitigate.

Except that this was bad equipment. There should be no flyable glideslope at 5.5 degrees. A reversed glideslope is preset at 6 degrees normally but that is not flyable because it will show low when you are actually above that false glide slope.

The 9 degree false glideslope is flyable again as down and up will indicate "correctly".

The need to cross check altitude with DME remains of course no matter what the problem with the ILS.
Just saying that this is was not what a typical false G/S capture from above looks like.

[foelic]

So, assuming you are VMC, visual with terrain, but still within that 10-15D 3500 MVA, can you ‘legally’ request a visual descent to 2500?

[extricate]

Originally Posted by foelic

So, assuming you are VMC, visual with terrain, but still within that 10-15D 3500 MVA, can you ‘legally’ request a visual descent to 2500?

i would have requested to descend on glide.

[aterpster]

Originally Posted by extricate

i would have requested to descend on glide.

Not saying they don't do it, but ATC is not supposed to do that. Neither is the pilot if outside the ILS PFAF.