MDA/DH

[Dreamflight767 312]

Hi Folks:

Need some help here please.  Yesterday I was landing at an airport with an APP that had a DH of 1190'.  However I couldn't get the DH to go above 500'.

Secondly, how do you switch between DH and MDA?

Lastly, I have some keys on my keyboard and a button on my joystick set up to help me set decision heights.  However, when I attempt to use those shortcuts, the shortcut function I have programmed doesn't respond.  Which is interesting because other shortcuts (breaks, land gear, flaps, etc.) all work.

Thanks!

[Ephedrin 418]

1190 is very likely not a decision height but the minimum decision altitude.. DH is mainly used for Cat 2 and 3 approaches using autoland and is typically somewhere between 0 and 200, maybe 250 ft AGL. The MDA can be a lot higher and is barometric pressure (height= radio altitude AGL , altitude=baro alt MSL). Most of the time it‘s also settled around 200 ft above the rumway but if your airport is at 5670ft your MDA is somewhere at 5870 ft. And CAN also be at 6420 or so... 

to change it there is a switch somewhere.. or a button.. haven‘t had the time to fly the Maddog yet..

[Bob Scott 7,720]

Decision height (DH) is the altitude above the touchdown zone elevation used to determine the latest point on a Cat II or III ILS approach for the land/go-around decision, and is determined by reference to the radar altimeter.  The term is often conflated with decision altitude (DA), which is the altitude above mean sea level used to determine the latest point on a standard precision approach (Cat I ILS, PAR) for the land/go-around decision.  There are a number of high quality add-ons that have problems with setting large DA values like those legitimately found in mountain regions, for example Denver, Jackson Hole, etc.   A sim workaround, which is not legal in the real world, is to set the DH (normally depicted in parentheses after the DA on an approach plate).  It's not legal on non-Cat II/III approaches because the terrain on final approach may not be level, so a height value displayed on the radar altimeter 1/2 mile short of the threshold may be significantly different from the actual height above the TDZE due to the variation in terrain elevation.  On Cat II/III approaches, the TERPS eval used to draw the approach will take the topography leading to the precision missed approach point into account, so the published DH reflects the actual AGL height above the terrain under the missed approach point.  On some Cat II approaches, terrain irregularities on final prevent using a DH, so only a DA (MSL) value is published, and that is used instead.

A minimum descent altitude (MDA) is the minimum MSL altitude that you can descend to on a nonprecision approach prior to visually acquiring the runway environment sufficient to initiate a safe descent to the runway.  MDA and DA are both altitudes above sea level, and on most aircraft are set the same way where the panel allows it, where the DH value is set separately because it uses a different source (radar altitude).

Regards

 

[Meekg 86]

It is the normal behavior. There is no switching in the MD80... it is not a 737NG or a 747.

From the Maddog Operations Manual Vol 2 Chapter 2

DESCENT PREPARATION

Altitude Reference Bugs (If required)..................................................................................................SET 1/2
Set altimeter altitude reference bugs to MDA value for non precision approaches or DA value for precision approach.

Radio Altimeters ...................................................................................................................................SET 1/2
CM1 and CM2 set DH value for precision approach or 500 feet in case of non-precision or visual approach.

 

MDA/DA is the minimums in bold on the Jepp charts, DH is the respective value in brackets with reference to height over the terrain.

This procedure is an extract from the real operations manual of a specific operator.

[Omaniac 3]

You set your minima with the orange bug on the altimeter.

You set your DH with the knob you used. This is used low visibilty approaches only.

In your case you set the orange bug at 190 ft and you need to monitor and understand that your minima in this case is 1190 ft not 190 ft.

[Dreamflight767 312]

Thank you everyone for the clarification!