MDA/DH Question

[Airline2Sim 489]

Guys, a question from our Facebook page about setting up decision altitudes that you may find interesting, answered by Josh...

 

David:

Hi Ben, During you Base training Josh sets up an MDA altitude for an ILS approach. I was under the impression that MDA was for non-precision approaches like a VOR approach and that DH should be used for an ILS precision approach.

 

Josh:

David, you're quite correct in your understanding of MDA for non precision approaches (NPAs) and a DH for precision, it's just a the way the Q400 interprets it. The setting 'knob' on the Q4 side panel is marked DH/MDA, it really should say something like 'Rad Alt' and 'Baro Alt' as when you set an 'DH' you're actually setting a bug based on a radio altitude and, when the aeroplane reaches that rad alt it'll generate a 'Minimums' call.

 

When you set an 'DH' you're setting a bug based on barometric altitude which will show on the right hand side of the PFD but won't generate any automated calls.

 

The key point here is that on a CAT I ILS approach we still use a DA however that DA is based on barometric altitude, not radio altitude, therefore to set it we have to use the side of the rotary switch labelled MDA which is rather misleading. Later on you'll see us shooting some CAT II approaches which also have a DA, however that DA is radio altimeter based (at 100ft above ground level rather than the 200ft above threshold level that Cat I uses) rather than barometric altitude based so we'll set it using the side of the switch labelled DA.

 

I hope that makes sense, cheers, Josh

[Tamadeez 7]

CAT2 is a more precise approach so your allowed to descend lower? And Radio Alt is used instead of Barometric because a laser measurment is more accurate then air pressure which is important at lower altitudes? Is this correct?

[Airline2Sim 489]

Essentially yes that's pretty much it! With you only becoming visual around 100 feet above the ground the extra accuracy of the radio altimeter is required.

[Maelstrom Aviation 0]

Hi Tamadeez,

 

A small correction to your summary. A Radar altimeter does not receive its height information by the means of lasers but by the means of radar waves. A different, and higher, frequency wave than for example the VHF range used for communication. Radar altimeters normally only measure from 2500ft AGL. I am not sure however if the radar altimeter on the Q400 accounts for the gear height. Maybe Airline2Sim can clarify?

 

Cheers,

 

Jeroen

[Airline2Sim 489]

Jeroen, the rad alt reads zero on the ground, unlike some aircraft that display a minus figure after touchdown to account for the body angle in the flare, showing zero essentially at the point of main gear touchdown so I don't believe it does. 

[AZWildk4t 0]

Can someone explain how you dial in the DH/MDA? I know how to switch between DH and MDA via the knob. However, I can't figure out how to change DH to say '400'. What dial is used to adjust this value?

[Airline2Sim 489]

Can someone explain how you dial in the DH/MDA? I know how to switch between DH and MDA via the knob. However, I can't figure out how to change DH to say '400'. What dial is used to adjust this value?

 

The knob is split into two parts. The front part of the knob selects either DH or MDA. The rear part of the knob rotates to set the desired DH or MDA. Move your mouse back a little and you can change it with the mouse scroll wheel.