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hvassk
26th Mar 2002, 11:00
Here I am trying to broaden my horizons and I can't figure out two items from European approach charts. Maybe someone out there can enlighten me. . .. .1. So far, all charts I've looked at have an . .OCA(H) listed in the profile view. Then there is a list of approach categories (A,B,C...) with associated MSL and AGL altitudes. Beneath the approach categories there is a GS out altitude and height. The GS out matches the LOC (GS out) MDA(H), however the other ones don't match anything else. It looks like this . .OCA(H) RWY 03 . .ILS A: 625(147) . .B: 635(157) . .C: 647(169) . .D: 661(183) . .GS out: 760(282) . .The legend is no help, it just tells me that it's the Obstruction Clearance Altitude (Height). Well, duh, even I figured that one out. . .So, what are those heights exactly and where are they measured from? There must be some point on the ground that I fly over in my C172 at 625 feet while I pass it at 661 feet in B747. These numbers are not listed on any American approach chart.. .. .2. On some charts there is listed Rwy Elev: 17 hPa. I am assuming that this is for QFE if a crew was interested in that. . .. .Anybody have any suggestions? Not even a phone call to Jeppesen has been helpful, because I can't speak to a live person.

Checkboard
27th Mar 2002, 09:48
The four minimas you have shown on the chart (The A, B, C, D things) are for four ICAO aircraft performance categories, determined by the approach speed of the aircraft.. .. .Category:. .A - Vat up to 90 knots. .B - Vat from 91 to 120. .C - Vat from 121 to 140. .D - Vat from 141 to 165. .E - Vat from 166 to 210. .H - helicopters.. .. .(Where Vat is the speed at the threshold based on 1.3 times the stall speed in the landing config. at max weight.). .. .So if you are flying a cat B aircraft, you fly the aproach to 635 feet AMSL (in your example), in a cat C aircraft you would fly the approach to 647 feet.. .. .In your example, the airfield elevation (at the aerodrome reference point) is 488 feet, and this is the difference between the altitudes and heights you have listed. (i.e. 625-147=488, 635-157=488 etc.)

N4641P
27th Mar 2002, 15:03
Herluf, . .I believe you are talking about Jeppesen approach charts. The main difference in approach minimums with state publications is that they tend to be slightly higher, on precision approaches. . .The Obstacle Clearance Altitude is a state defined minimum that (if I remeber correctly) does NOT include the altitude you lose at your decision altitude. What I mean is that at your DA you look up and say landing or go around. If you go around you will initially lose a little bit of altitude because of inertia, the heavier the aircraft the more you loose. Jeppesen takes this into account and adds a little margin on top of the state published OCA so that you dont go below the OCA when you arrive at your DA and look up.. .If you go below the OCA you would be "busting minimums" if you dip below your DA as a result of you looking up and deciding to go around, than this is somewhat accounted for.. . <img border="0" title="" alt="[Smile]" src="smile.gif" />

Tinstaafl
27th Mar 2002, 18:00
DA only applies to precision approaches. Precision approaches nearly always take you below the aerodrome OCA/H long before you get to the DA eg CAT 3 ILS etc.. .. .The DA is just a that - a decision altitude. The design of the precision approach allows for the dip below DA after the missed approach is commenced.. .. .In some ILS categories/aircraft it's even possible for the wheels to touch the runway during the missed approach. Obviously an OCA/H is nonsensical here.. .. .OCA/H applies during non-precision approaches. Different categories of a/c have different minimum clearance heights eg Cat A=300', Cat C=400' above the highest obstacle within the circling area (which in turn varies with a/c category eg 1.68nm for Cat A, 2.66nm for Cat B etc). .. .It is with non-precision approaches that the concept of a 'hard floor' to the minimum descent altitude is applied. The MDA is just that - the lowest altitude to which you may descend during the non-precision approach & missed approach. Unless certain defined criteria are met to enable further descent you are just not allowed to go any lower.

hvassk
28th Mar 2002, 09:40
Thanks for the responses, although, I would like it if I someone could confirm my answer. Here's an example:. .. .Heathrow ILS DME Rwy 09L 11-1. .I am a Category C aircraft with visibility above minimums, but the base of the fog layer is 285 feet MSL. I'm on the approach and as I approach 285 feet I see nothing...at 280 I see nothing, so I announce a go-around and commence it at the published DA of 279 feet MSL. I am a lazy pilot and the entire aproach was coupled and the aircraft naturally sinks slightly while transitioning from a descent to a climb. Am I safe (read legal) as long as I do not bust the OCA of 252 feet MSL which I read beside the profile view on the approach chart?. .. .Mere curiousity is killing me. I've asked a few check airmen buddies (US airlines) and they don't have an answer. Few of their approach charts have those numbers, and we don't have them over here.

BelowTenThousand
28th Mar 2002, 10:53
At the decision altitude you decide to land or go around. You don't have to be going round by the time you reach the DA. Only at DA do you decide to go round. As I see it you are legal in the situation you mention. As someone said earlier you could even touch down during the go round and be deemed legal.

hvassk
28th Mar 2002, 12:31
Then why the OCA? Does anyone really brief or use the OCA? It seems to me like a "nice to know" piece of information that is not applied in the real world.. . . . <small>[ 28 March 2002, 07:32: Message edited by: Herluf ]</small>

N4641P
28th Mar 2002, 16:03
It's been a while since I have seen these charts, is the OCA specified for CAT 1 approaches only, or also for cat 2 and 3? Because there it would be non essential indeed. . .From what I have seen on state charts (Netherlands) the story as previously laid out applies. Greetings. <img border="0" title="" alt="[Smile]" src="smile.gif" />

Tinstaafl
28th Mar 2002, 17:02
Don't have the chart so I'm only guessing, but does the chart also have a non-precision approach overlayed eg Localiser/DME or a circling approach after a descent on the ILS?. .. .That's very common in Oz so the same plate covers both the ILS precision approach and LOC/DME &/or LOC/Mkr Bcn &/or circling after a letdown on the ILS

bookworm
29th Mar 2002, 21:05
Herluf. .. .The OCH is the height of any obstacles plus an appropriate margin or minimum obstacle clearance. It doesn't take into account any limitations of the navigational aids used.. .. .The decision height that you are required to use (at least in the UK, in the absence of company issued minima) is the higher of the OCH and the system minimum. For a Cat 1 ILS presision approach the system minimum is 200 ft. . .. .Similarly the MDH for a non-precision approach is the higher of the OCH and the system minimum, which is 250 ft for a localizer approach, 300 ft for an NDB or VOR approach etc.. .. .For the UK, system minima are published in the AIP AD 1.1.2.. .. ....which begs the question "why put OCHs lower than the system minimum on the approach chart?" I suppose it could be argued that the information that the DH is limited by OCH rather than system minimum is useful to the pilot, but I suspect it's more of a historical anomaly.

dragchute
30th Mar 2002, 15:04
Herluf. .. .Altitudes are measured above mean sea level and read from the altimeter when set to the correct QNH. Therefore DA and OCA are referenced AMSL.. .. .However heights (DH or OCH) are measured from another datum such as the aerodrome reference point or the threshold of the applicable runway to which the chart relates. If the surface was dead flat then I suppose the Radar Altimeter may serve as the reference instrument.. .. .In that context DA or DH is the altitude above MSL or the height above the reference datum at which the pilot must decide to land or miss.. .. .OCA (reference MSL) or OCH (reference the applicable datum) is used to eatablish compliance with the appropriate obstacle clearance criteria.