OCA and OCH on Approach Charts

[VHOJT 166]

Hi allBasically - what are they?Also, for any ILS approach anywhere, is the decision height standard for that category ILS approach?Thanks for any helpRudy

[Guest FlyMD11.nl]

Hello Rudy!OCA and OCH are the Obstacle Clearance Altitude and Obstacle Clearance Height. The difference between them is obviously the Altitude and Height. The OCA is based on the altitude above sealevel and the OCH is based on the Height above threshold or aerodrome elevation.OCH and OCA are derived as follows:If at any height an aircraft would make a go-around a certain theoratical height loss will occur before the aircraft starts to climb again. There's a table for these losses in different aircraft categories. i.e. a cat D aircraft could lose around 160 feet (Altimeter) or 85' (Radialt, more accurate so lower value). This will be added to the critical obstacle in the approach area.The result is the OCH. To get the OCA just add the elevation of the threshold or aerodrome. You now have a save height/altitude which can be used as the lowest descent limit for the approach.The descent limit for the approach is the higher of:* Company Minimum* State Minimum* OCA/H(State/System) Minima for the different categories of ILS are:CATI : 200'DH / 550m RVRCATII : <200'DH but >100'DH / 200m RVRCATIIIA: <100'DH / 200m RVRCATIIIB: <50'DH or no DH / <200m RVR but not less than 75mSo if the derived OCA/H is higher than the standard minima as prescribed above, the published descent limit for that approach will be higher... If not than the published descent limit will be standard for that CAT approach.i.e. Schiphol (EHAM) lies 10' below sealevel. The terrain in the approach segment is almost flat. If we derive the OCH CATI (adding 160' to the highest terrain (0')) than the OCH CATI will be 160'. The System Decent Limit for CATI is 200' so the published descent limit is 200'DH/190'DA.For CAT II approaches the math is the same only that we base everything on the radioaltimeter. So we add 85' to the highest terrain to derive the OCH. We compare OCH to System Descent limit (between 200' and 100') and see that for EHAM 100' as a DH height would be sufficient.With these questions you actually helped me refresh everything!!! Pfff I had to go into the books on some of the items!!!Happy flying, and don't forget to ask me if things are not completely clear!

[VHOJT 166]

Hi thereThanks so much for the helpful and detailed reply - it has really made things much much clearer to me! Everything's been answered.Thanks again for your help!Rudy