Decision Height etc 737

[Guest markyp]

Hi all,The other question.A friend of mine and myself have been trying to work out when etc and what decision heights and the aircraft calling out "minimums" they use in the real world?We saw a table like this, please read on below------------------------->> Just trying the HK approaches and noticed some are Cat 2 and others Cat 3. >> Anybody knows what the difference is between ILS Cat 2 and Cat 3, and are >> there any others such as Cat 1 or maybe Cat 4?>>The differences between Cat I through CAT III are visibility/ceiling>minimums. I stole this chart off the web:>>Type Ceiling Visibility >========================================>Cat I - 200 feet - 2400 feet >Cat II - 100 feet - 1200 feet >Cat IIIa- >100 feet - 700 feet >Cat IIIb- >50 feet - > 700 feet but not > 150 feet >Cat IIIc- No DH - No RVR limitation ------------------------Is the minimum alt nothing to do with the CAT side of things?? Or weather or equipment on the aircraft? ? I am really confused by the above could someone please explain this to me? ThanksMark

[Guest tmetzinger]

There are three terms used in instrument approaches:MDA=Minimum Descent AltitudeDA=Decision AltitudeDH=Decision HeightMDA is the minimum altitude you can descend to on a non-precision (VOR/ADF/GPS/RNAV) instrument approach until you can make a normal landing visually. On the NG, you set this using BARO MINS, and that will give you the "minimums" call out when you pass this altitude.DA is the altitude on a precision approach (ILS) where you have to see the runway or immediately execute the missed approach. It's an ALTITUDE, read on the altimeter, and is used on CAT I ILS approaches. On the NG, you set this using BARO MINS.DH is the HEIGHT (radio altimeter) above ground on a CAT II or CAT III ILS approach. At this height, you either see the runway or immediately execute the missed approach. It's set using the RDR mins, and is usually 100 for CAT II and IIIa, down to 50 for CATIIIb.Hope this clears it up. When you adjust the MINS on the EFIS panel, that alters the altitude at which you'll get the "minimums" call out.Best Wishes,

[Guest markyp]

Hi there,Thanks for the reply.So when would you use each of these approaches is the thing I am struggling to understand?Or I guess when to use what type of CAT Landing and use one of the above?? I don't quite understand that bit?!Would you be told which to use on the charts then use that or??ThanksMark

[Guest markyp]

Following on from the above from the question back to you Tim!Why do you set Baro for all decision heights and not Radio? I thought you used Radio?! I take it i am wrong!How on earth do you set RDR Mins in the 737NG?? I can find Baro mins but nothing else?ThanksMark

[Guest tmetzinger]

>Following on from the above from the question back to you>Tim!>>Why do you set Baro for all decision heights and not Radio? I>thought you used Radio?! I take it i am wrong!>>How on earth do you set RDR Mins in the 737NG?? I can find>Baro mins but nothing else?>>Thanks>>MarkOnly certain runways are "good enough" for CATII or CATIII approaches. They have a surface area that is flat for a minimum distance beyond the runway ends. Because it's flat, it's safe to use the radio altimeter for that approach.The way you can tell what sort of approaches that you can fly to at a specific airport is to get the instrument approach plates for that airport. You can download US airports at http://www.myairplane.com.In general, ILS approaches allow you to land when the clouds are low and the visibility is lousy. the other non-precision approaches generally require that you be able to see the airport from farther away and a higher altitude.

[Guest wesl]

I think some of the confusion is why is altitude used for the missed approach point when visibility is the criteria for making the approach. If the ceiling is 100 ft (below minimums), and the vsby is 10 miles, a pilot can make a CAT 1 ILS (part 121 or part 135). If however the ceiling is 500 ft and the vsbly 1/4 mile (RVR 1200) the same pilots can not make the approach because the vsby is the less than required. With a precision approach, when established on the glideslope, the DH will be lower as visbility requirements decrease. Look at KBOS RY4 ILS. Cat I mins are 200 and RVR 1800, CATII are as low as 100 and RVR 1200. These rules do not apply to Part 91 (general aviation) but are still good to use. That being said, there are exceptions to every rule. Wes

[Guest tmetzinger]

I'm not sure that's true... I'll went and read 121 and 135 again, but minimums are NOT just visibility. You cannot even begin an approach under part 121 or 135 unless the reported weather at the field is above both the ceiling and visibility minimums. When flying an ILS, assuming one expects the weather to be above minimums, one flies the airplane down to the DA (for CAT I) or DH (CATII). If the pilot flying cannot make out the necessary visual references (runway, or the approach lighting system), then a missed approach is initiated.As an example, let's assume you're performing a CAT I ILS to Sacramento Runway 16L. You can't fly a CATII or III to that runway, it's only certified for CAT I.As you approach the airport, you check the weather. It needs to be at least a 200 foot ceiling, and 1/2 mile visibility if the approach lights are working. Let's say the ATIS calls for 200 overcast and 3/4 mile in rain and fog. Good enough, so you commence the approach, and set your DA to 228 feet. Down you go, properly configured. At 250 feet the view out the window lightens a bit, but you can't make out anything yet. At 228 feet the airplane (and the FO) calls out "minimums":If you can see the approach lights or the runway environment you can continue the approach down another 100 feet, and if you don't have the runway environment by then you must miss the approach.If you cannot see the approach lights or runway environment, you initiate the missed right there at 228 feet.The reason the decision point on a CAT I ILS is an ALTITUDE an not a HEIGHT is due to the fact that many aircraft don't have radio altimeters or other qualifications for CAT II, and because the runway environment may not be adequate for radio altimetry.An example. I was waiting to fly from IAD to DTW one morning on Northwest. Other flights using CRJs were departing on time, but our DC9 stayed at IAD for many hours because it wasn't equipped/certified for anything beyond CAT I minimums. Weather at DTW was 1 mile visibility but ceiling was around 150.

[Guest andyman]

If NWA ever gets out of bankruptcy they should sell their DC9's off and replace them with A320/A319s. If NWA was really listening to me; they'd buy 737's(get the sims and train the pilots too).... WHY? Because NG's are cool.

[Guest markyp]

ok!So Tim you always set whatever it is MDA, DH, DH in Baro, I think in the Airbus it actually tells you which one you are using, I am taking it that you just set the Baro to whatever it is in the Boeing 737....It does not display what mode I am in?! That is fine just want to confirm with you! If it does show how does it do that?!ThanksMark

[Guest markyp]

Looking at the Chart for Sacremento 16L I cannot see anywhere on it that is is only authorised for a CAT I ILS Landing??!!Mark

[Guest UKPilot]

Hi all, In the real world its fairly simple how this is set.For Departure:-BARO is set to the MFRA (minimum flap retraction altitude)this is in case of an engine failure on take off, so for example at Manchester it is airport elevation so 257ft + 800ft = 1057ft BAROFor Landing:-BARO is set for a CAT1 approach to Runway Elevation plus 200ft, so for example again at Manchester on 24R this is 249ft + 200ft= 449ft BARORA (Radio Altitude) is set if the vis falls below that stated in the first post and this is always 50ft RA, at the same time on BARO this will be sent to 1057ft (eg at Manchester 24R again)just in case of an engine failure on the Go Around.Hope this helpsRgdsUKP

[Guest szp]

cat 2/3 approaches use a separate approach plate (u.s. anyway). if it's an ils approach and doesn't have cat 2/3 in the title, it's a cat 1 approach.

[Guest joediamond]

>>I'm not sure that's true... I'll went and read 121 and 135>again, but minimums are NOT just visibility. You cannot even>begin an approach under part 121 or 135 unless the reported>weather at the field is above both the ceiling and visibility>minimums. This is incorrect. For part 121/135 operations only the visibility minimum is required. The only time a published ceiling minimum is required is if one is listed as a takeoff minimum.For example if the reported weather at the airport is 100' ceiling visibility 1/2 mile it would be legal to conduct a CAT1 approach with a minimum visibility of 1/2 mile and a DH of 200'C McCarthy

[Guest tmetzinger]

That's half correct:121.655 Applicability of reported weather minimums.In conducting operations under

[jazer 0]

Hi, can anyone tell me when you hear approaching minimums and approaching decision height. And whats the difference?THanks,Jacir