737 Decision Height/radio altimeter questions

[Chris Catalano 0]

Can someone please tell me EXACTLY how and WHEN I am supposed to set the Decision height in the 737. I can't seem to figure out how to rotate the knob. Also, does is show up on the PFD below when I get it dialed in correctly. Also, WHEN do I use radio altimeter, and finally, is the value of the radio altimeter supposed to be the DH minus the TDZE height? Thanks a bunch! Chris Catalano

[SAS263 1]

Hi Chris, here

[Chris Catalano 0]

Thanks for all the help - only I am still having somewhat of a problem. I got the radio altimeter to set the way I wanted, but I cannot get the decision height to display. There is only a number that says -4 (like you described) in white. It looks like there are THREE settings on the EFIS display - "Baro", Radio" and "MINS" It only seems to let me select either baro or radio no matter how hard I try. If I theoretically could set the decision height, shouldn't it also appear as "DH" on my PFD? No matter WHAT I try, I can't seem to get that to display on the PFD, and I can't adjust it either. What am I doing wrong? Are you able to select all three settings with the dial (baro,radio, and mins) or can you just select 2 settings like me? Thanks for all the help.SincerelyChris Catalano

[michal 44]

>Just to clarify (hope I

[Guest tmetzinger]

>Let me correct above slightly. DH is used during precision>(CAT I) approaches, MDA is used during non precision>approaches. Both are based on baro altimeter. I repeat,>radar altimeter may not used as the primary altitude source>during such approaches, it may be used as a back-up>information and to help pilot perform a better flare during>landing. It is easy to see why RA should not be the main>source of altitude for DH (Cat I) and MDA - if you were flying>into say Aspen, Colorado, RA would fluctuate wildly during>approach because of highly uneven terrain around the airport.>As to CAT II/III - I don't know what role RA plays here.I'm gonna correct a bit more. Nowadays, the term for CATI approaches is getting standardized as DA (decision altitude) because it's an altitude and read from the altimeter and not a height read from the radio altimeter.So:DA - used for "normal" category I ILS approaches, set the minimums to BARO and dial in the altitude found on the chart for DA. When you hear "Minimums", you need to have the runway environment in sight or commence the missed approach procedure.DH - used for category II and III approaches, set the minimums to RADIO and dial in the height found on the chart. For CAT II,When you hear "Minimums", you need to have the runway environment in sight or commence the missed approach procedure. For CATIII, the airplane lands itself, and there are no "minimums" for height.MDA - Minimum Descent Altitude - the lowest you can go on a non-precision approach - set the mins to BARO and dial in the altitude shown on the chart. When you hear "Minimums", you can't go any lower, but you don't commence the missed approach until you reach the Missed Approach Point based on time, distance, or station passage.

[michal 44]

Good, I did learn something new today - that CAT II minimums are based on RA.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Chris Catalano 0]

Thanks Tim - I love reading your posts!Now - can you please help me learn how to set my decision height - no matter what I do, I can't get it to show up on my PFD. I can't seem to get the little dial to turn to MINS (only radio and BARO - everytime I try to set it to MINS it seems to skip right over the MINS setting on the dial). Is this a bug, or am I doing something so obviously wrong? I want my DH to show up in green on my PFD if it is possible. I still can't fugure out how to actually SET the DH, or the MDA. Thanks as always to you Tim, and to everyone!Sincerely,Chris Catalano

[Chris Catalano 0]

Actually, Tim I may have figured something out - MINS is not actually something you select, is it? It is just the category on that dial. Mins offers you 2 choices - either BARO or RADIO. Right? Now based on what you wrote above - here are my questions:"set the minimums to BARO and dial in the altitude found on the chart for DA"When you say to "dial in the altiude" - do I use the dial on the top right of the EFIS panel - the one that says BARO: Inches/Meters? If so, do I just dial in the number from the chart that is the DA? Is that how I "DIAL IN" the number?"Also, when you said:"DH - used for category II and III approaches, set the minimums to RADIO and dial in the HEIGHT found on the chart."Likewise, when I set MINS to "RADIO", do I just use the dial on the left this time and dial in the "Height" of the TDZE? What does "HEIGHT" refer to in this case - the height of WHAT? And, is this how I dial it in - by using the outer dial on the MINS selector?Finally, does the DH ever show up as a printed green number on my PFD like it used to on the 777?Thanks fo rall the help. I MAY be starting to get some of this after all these years!Sincerely,Chris Catalano

[michal 44]

You find the number for DH on the chart and set it as RADIO, or use the number DA and set as BARO. The chart will explicitely give you THOSE numbers. No, TDZE is not DH nor DA. DH is DH and DA is DA.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[SAS263 1]

Hi Chris, now that the experts have clarified all those abbreviations we

[Guest BlackDog]

I would like to thank everyone for this informative and well thought out thread.Keep up the good work and thanks again.

[Chris Catalano 0]

Thanks! I actually have Sim-plates. I purchased those last year & have printed out all my favorites. I can't wait to try this all out. Maybe the reason I don't see anything is because I didn't know that the screen is blank when the value is 0. Thanks for all the help.Sincerely,Chris Catalano

[Chris Catalano 0]

Still having trouble with one part of this - I did a landing tonight at Chicago's O'Hare (KORD) RWY 27 L. I switched the MINS dial to RADIO & dialed in the decision height from the chart of 852. It appeared in green on my PFD as "RADIO 852". It called out minimums at the correct time (200 feet AGL). Everything worked PERFECTLY!HOWEVER - whenever I tried to set the MINS dial to BARO (the altimeter was already correct with the shortcut key "B", and the altimeter was at 30.17)----- NOTHING HAPPENED!!! It never said "BARO" in green on my PFD, and rotating that same outer left knob that I used when setting the radio altimeter seemed to do NOTHING. I JUST don't get it - Is there a bug in my installation? Do I need to reinstall? I wanted to dial in the BARO altitude of 1060 from the chart, but, like I said, nothing seemed to happen when I switched over to BARO on the MINS dial. The only thing I noticed was that whenever I tried to set the outer left knob when working in BARO, there was a little green bug that seemed to move along the outer edge of my digital altimeter. I don't know what it did, if anything. I STILL don't get how to actually SET the BARO DA, or MDA, although I perfectly understand what you are all saying about what I am SUPPOSED to do (at least I THINK I understand!) Is there something I am STILL doing wrong? Is there a bug in my installation? For radio, the value I dial in appears in green on my PFD. Isn't the same thing supposed to happen when I switch to BARO - am I not supposed to see whatever value I dial in in green on my PFD? I am really losing heart with this whole topic. Can anyone help me figure this last part out?Radio seems to work fine. Now if I can just figure out how to dial in the BARO MINS, and what it LOOKS like when I dial it in correctly.Thanks so much for all your patience.Sincerely,Chris Catalano

[Mats_J 2]

Chris,Something else's fishy here as well. If you dial 852 on the RADIO ALTIMETER you should get the MINIMUMS call at 852 feet. Remember the RA measures actual height above the ground while the baro altimeter uses QNH to give you your altitude above ground. Cheers,

[Chris Catalano 0]

OK so that explaines why I got the MIMIMUMS call at 852. Was I supposed to dial in 200 and not 852? For BARO - what would the correct number have been - 408?Also, what does QNH stand for?Finally, any thought as to why I can't seem to get a numerical value to appear in green on my PFD when I switch MINS to BARO?ThanksChris Catalano