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jordanal

Help Setting MDA & DH Bugs

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Looking up the subject bug settings in the PMDG 747 manuals, I still to this day find it somewhat confusing. I'm looking for further RW instructions to set these realisticly during my initial descent.MDA: This is the "Baro" setting? Is this the same as MSA (Minimum Safe Altitude ?) found in approach charts? I also happen to use Radar Contact (RC) and see that when setting up the controller page, it fairly accurately lists the MSA at the planned arrival. Would setting the MDA on the PFD to this RC listed MSA altitude be realistic at all? Example, arrival in KATL lists a MSA of 2800ft. Should I set the MDA "Baro" to 2800ft?DH: This is the "Radio" setting? As I understand it, this is the land/no-land altitude whereby the pilot has to decide if the landing will take place. What is a fairly quick way to determine this alitude for a given arrival? Is it specific for each Rwy or for the entire airport?I'm always willing and trying to improve my semi-detailed understanding of the majestic Queen. Thanks in advance for the enlightenment...

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>Looking up the subject bug settings in the PMDG 747 manuals,>I still to this day find it somewhat confusing. I'm looking>for further RW instructions to set these realisticly during my>initial descent.>>MDA: This is the "Baro" setting? Is this the same as MSA>(Minimum Safe Altitude ?) found in approach charts? I also>happen to use Radar Contact (RC) and see that when setting up>the controller page, it fairly accurately lists the MSA at the>planned arrival. Would setting the MDA on the PFD to this RC>listed MSA altitude be realistic at all? Example, arrival in>KATL lists a MSA of 2800ft. Should I set the MDA "Baro" to>2800ft?>>DH: This is the "Radio" setting? As I understand it, this is>the land/no-land altitude whereby the pilot has to decide if>the landing will take place. What is a fairly quick way to>determine this alitude for a given arrival? Is it specific>for each Rwy or for the entire airport?>>I'm always willing and trying to improve my semi-detailed>understanding of the majestic Queen. Thanks in advance for>the enlightenment...>>Hi Al,First off, MDA is not the same as MSA. I think this is the information you're looking for:There are three terms used in instrument approaches:MDA=Minimum Descent AltitudeDA=Decision AltitudeDH=Decision HeightMDA is the minimum altitude you can descend to on a non-precision (VOR/ADF/GPS/RNAV) instrument approach until you can make a normal landing visually. On the NG, you set this using BARO MINS, and that will give you the "minimums" call out when you pass this altitude.DA is the altitude on a precision approach (ILS) where you have to see the runway or immediately execute the missed approach. It's an ALTITUDE, read on the altimeter, and is used on CAT I ILS approaches. On the NG, you set this using BARO MINS.DH is the HEIGHT (radio altimeter) above ground on a CAT II or CAT III ILS approach. At this height, you either see the runway or immediately execute the missed approach. It's set using the RDR mins, and is usually 100 for CAT II and IIIa, down to 50 for CATIIIb.Ken.

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Ken, thanks for replying. So, if I almost always perform a cat II or III automated landing (just cuase I like to watch the FMC and MCP do its thing) then I should only need to set the DH Radio altimeter, which in the case of the 747, defaults to 100ft so I don't really even need to change the DH. If this is correct, then there's nothing realy for me to do as I should also ignore the default 200ft setting of "baro", correct?Thanks again.

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Ken has covered this thoroughly, but one additional thing comes to mind. Some airlines set the decision height bug (I think the Barometric one) to 400ft AGL before DEPARTURE to signify the minimum emergency action altitude after takeoff. If you suffer a failure or emergency after V1 and thus continue with the takeoff, you should not carry out any emergency actions below 400 feet AGL, other than cancel any fire warnings. This - as I understand it - is to allow the crew to focus on the most important task of getting the aircraft transitioned from the takeoff roll to the initial climb before their attention is in any way compromised by the emergency at hand.On paper, it seems like an unneccesary delay in dealing with the problem, but next time you're taking off, check the altitude as you call for gear up, already you're well on your way towards 400 feet anyway.I'm pretty sure I've got all this correct, but as always corrections to the above are welcomed!

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I know on the Boeing 767-300 800ft is the minimum acceleration height from V2+10 on a single engine so I set the orange altimeter bug to 800ft.That way if I have a failure/fire/emergency I get the airplane up to 800ft then deal with it.I will try Mark's tip on my next [self imposed] currency check.

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Bear in mind that the minimum emergency action altitude and the acceleration height are two distinct values. Passing 400 feet the PNF can commence the memory items and then run through the QRH, meantime the PF monitors while flying the aircraft, (normally) handling ATC, and commencing the acceleration shortly afterwards passing (minimum) 800 feet.A lot of airlines have their own higher acceleration height minimums, often 1,500 feet but it can vary.

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If you look on the CAT 2 chart it will list the setting that needs to set on the Radio alt ( DH). This will vary with the type of approach (Cat 2 or Cat 3 a or :( and the surveyed terrain prior to the runway. Remember the heights are above the Touch down zone so if the land slopes up or down then this will affect the setting and this is reflected on the DH on the chart. The Baro is set to the CAT 1 minma (DA) and you will find this on the standard ILS chart. Depending on some failures or unsericebilities you might be able to cntinue down to CAT 1 and therefore you dont have to reset bugs.CheersSteve

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