Quick MDA/RVR Question

coa787

Unknown Member
On the ILS RWY 28 plate for CLE, it says "1018/24 227(300-1/2)" for the straight-in ILS 28 arrival for all aircraft catagories.

I know that "1018" is the MDA and "24" is the RVR (2400'), but what is meant by "227(300-1/2)"?
 

BobDDuck

Island Bus Driver
227 is what your AGL will be at minimums and if I recall the (300-1/2) is the DOD mins. But it's been a while since I've looked at a NOS plate.
 

JLF

Well-Known Member
That 300 1/2 is a DOD thing.

Ceilings are reported in 100 foot increments. I don't have any NOS charts either, but I bet they correspond to the "practical" minimums a lot of the time.

So the wx mins for the approach are 227' ceilings and 1/2 sm vis, or rvr reported to at least 2400. You'll never see 227' reported in a METAR which effectively makes the ceiling mins 300'. (300-1/2)

Not sure about part 91 flying, but the ceilings are usually only a factor for planning purposes. Visiblity is the big one when it comes to shooting an approach.
 

nosehair

Well-Known Member
I bet they correspond to the "practical" minimums.

So the wx mins for the approach are 227' ceilings and 1/2 sm vis, or rvr reported to at least 2400. You'll never see 227' reported in a METAR which effectively makes the ceiling mins 300'. (300-1/2)

Not sure about part 91 flying, but
..but now you can be sure.

You, Sir, are correct.

~Old Army Guy
 

mjg407

Well-Known Member
The (300-1/2) is the for weather mins for takeoff/alt, etc, which depends on the pilots qualifications, aircraft equip etc. The 227 is your HAT at the DH/MDA for straight in or the HAA for circling....
 

fish314

Well-Known Member
That 300 1/2 is a DOD thing.

Ceilings are reported in 100 foot increments. I don't have any NOS charts either, but I bet they correspond to the "practical" minimums a lot of the time.

So the wx mins for the approach are 227' ceilings and 1/2 sm vis, or rvr reported to at least 2400. You'll never see 227' reported in a METAR which effectively makes the ceiling mins 300'. (300-1/2)

Not sure about part 91 flying, but the ceilings are usually only a factor for planning purposes. Visiblity is the big one when it comes to shooting an approach.

This is CLOSE, but not quite, the way it works. In the Air Force, there are several rules regarding when you use the ceiling and visibility when you use just the visibility.

1st. If you are going to fly a circle to land, you must use ceiling and visibility rather than just the visibility. This means that you cannot start the approach if the ceiling or the vis is below that posted on the plate before you start the descent for the approach, and if you plan to file to a field where you must circle, the weather must be forecast to be above both the ceiling and the visibility (ETA +/- 1 hour) for you to file there. In addition, for a circling approach the prevailing visibility is the prefered visibility information rather than the RVR.

2nd. For flying an approach straight in, you are allowed to use just the visibility, and ignore the ceilings, however there is a penalty that you pay in terms of the amount of fuel you are required to carry. If you plan to file to a field using "vis only" rules, you need to plan for enough fuel to get to the intended destination, fly the approach and the missed approach at the destination, and then fly to the alternate for an approah at the alternate, plus required fuel reserves. For straight in type approaches, the preferred visibility information is the RVR, rather than the prevailing vis.

3rd. If you file to a field using "ceiling and vis" rules, then you only need to plan to have enough gas to fly to the intended destination, then to the intended alternate, then the approach at the alternate, and required reserves. You don't need to plan to have fuel to fly an approach and the missed approach at the intended destination. So you save yourself some gas.


BTW, the USAF definition of "required reserves" is 10% of the total flight time (including the alternate fuel) or 20 minutes whichever is higher, up to a maximum of 45 minutes of fuel, computed at maximum endurance airspeed at 10,000'.

227' isn't the "mins" for the approach. As mjg posted, it's the HAT (or HAA if we're talking about a circle).

HAT means "height above touchdown" and is computed by subtracting the DH or the MDA from the touchdown zone elevation (TDZE). The touchdown zone elevation is the highest elevation in the "touchdown zone", which is the first 3000' of the runway. So in practical terms, for the example given when you are at 1018' MSL you will be 227' above the highest point in the first 3000' of the runway.

For HAA, it's "height above aerodrome". It's computed the same way, but instead of using the TDZE, you use the highest point on any landing surface of the aerodrome.... which is the same as the "field elevation." The TDZE and the field elevation could be the same, but the don't have to be and frequently they aren't. If this line were refering to the circling minimums, then 227 would be referencing an HAA and at 1018' MSL you would be 227' above the field elevation.
 

JulietBravo

On Call, On Demand
227 is what your AGL will be at minimums and if I recall the (300-1/2) is the DOD mins. But it's been a while since I've looked at a NOS plate.
:yeahthat:... simple as that. 227 is your agl at 1018msl and the (300-1/2) is the DOD mins for the military... usually for civilian you just ignore the parentheses.
 
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