How is an MDA calculated?

[ROB-x38]

Simple question... how do they calculate the lowest altitude you can go on a non-precision approach?

I'm sure it's related to a certain amount of obstacle clearance, a certain area etc...

What are the specifics and any links to published documents?

 

[A Squared]

Simple question... how do they calculate the lowest altitude you can go on a non-precision approach?

I'm sure it's related to a certain amount of obstacle clearance, a certain area etc...

What are the specifics and any links to published documents?

It's all in the TERPS. That is THE refrence for approach design.

Real briefly, there is an obstacle clearance zone for the final approach. the size and shape varies with the particulars of the approach. The MDA is seledted so that it gives a minimum clearence over any obstacle in that zone. THe clearence may be as low as 250 feet. SOme approaches (NDB with no FAF for example) require as much as 350 feet clearence above an obstacle in the zone.

 

[Mercury]

Just curious, (no terps infront of me) what does that fact that there is no FAF(such as an NDB located on the field) have to do with the clearance in the obstacle zone??? The final approach segment and descent to MDA still has to begin at some point in space and meet the obstacle clearance requirements. I guess i just dont see the correlation.

 

[A Squared]

Just curious, (no terps infront of me) what does that fact that there is no FAF(such as an NDB located on the field) have to do with the clearance in the obstacle zone??? The final approach segment and descent to MDA still has to begin at some point in space and meet the obstacle clearance requirements. I guess i just dont see the correlation.

Well, I don't have an explanation. I've thought about it before, but haven't come up with a logical explanation. All I can say is that the existence of an MDA makes a 50 foot difference in the obstacle clearence. For a VOR with a FAF it's 250' vs 300' without ,and an NDB with a FAF it's 300' vs 350' without.

I can see having a wider obstacle clearence area for the no-FAF approaches (and they do have a wider area), but like you I'm scratching my head about the increased obstacle clearence.

 

[ROB-x38]

Thanks for the replies. I have another question re: approach design if anyone's willing...

Regarding the calculation of circiling visibility in Australian approaches http://www.casa.gov.au/manuals/regulate/mos/026r0201.pdf (see page2-13).

I believe I understand the philosophy but do not understand the significance of 0.9 = minimum downwind spacing to achieve alignment on final approach. Can anyone explain this term?

Thanks in advance

 

[A Squared]

Thanks for the replies. I have another question re: approach design if anyone's willing...

Regarding the calculation of circiling visibility in Australian approaches http://www.casa.gov.au/manuals/regulate/mos/026r0201.pdf (see page2-13).

I believe I understand the philosophy but do not understand the significance of 0.9 = minimum downwind spacing to achieve alignment on final approach. Can anyone explain this term?

Thanks in advance

SOrry, can't explain that. as far as I can see, the way it's inserted into the equation for computing minimum vis, it's a factor that reduces the minimum visibility for a circling approach to 90% of the diameter of a 180 degree turn for a given category, which would imply that if you were on a perfect downwind spacing during your circle manuver at min vis, the airport would be just out of sight ......seems not to make sense to me.