Jump to content
Sign in to follow this  
mgh

MDA and DH

Recommended Posts

Guest Boeing747-430

could someone please explain what these two terms mean? i know that they stand for minumum descent altitude and decision height, respectively, but not much after that. what do they mean and how do they differ? thanksadam

Share this post


Link to post
Share on other sites

"Minimum Descent Altitude" is the lowest altitude you can descend to without seeing the ground during a non-precision approach, ie the airport has a NDB and no ILS."Decision Height" is the lowest (Radar) altitude that you can descend to without seeing the runway during a precision approach, ie ILS approach CAT I, II, III.

Share this post


Link to post
Share on other sites
Guest UltraRunner

You can descend lower than either of those without seeing the runway or ground, if you have the approach lights you can go down to 100' above the touchdown zone.

Share this post


Link to post
Share on other sites
Guest Adverse Yawn

All correct as above. The other difference is the way you fly the approach. There is no 3 deg glideslope, just recommendation which maybe 6 deg. Basically you descnd to MDA as quick as it is safe do so within normal operational limits because this gives the best chance of becoming vis with the terrain permitting you to continue the descent.That is all generally speaking and specific procedures may and often do apply. The plates have the specific details.

Share this post


Link to post
Share on other sites

In the UK the Decision Height is defined as the height in a precision approach at which a missed approach procedure must be initiated if the required visual reference to contine the approach has not been established. This more rigorous than the previous definitions.Also in the UK, unless the pilot is an Instrument Rating Holder in Current Practice it's recommended that the DH/MDH is increased to at least 500 ft for a precision approach and 600 ft for a non-precision approach for IMC Rating Holders in Current Practice. Pilots not in Current Practice are advised not to make instrument approaches in bad weather and, if they have no choice, are recommended to add at least 100 ft and more depending on his experience and knowledge.

Share this post


Link to post
Share on other sites

The word "Decision Heigt" (This is height above ground) and "Decision Altitude" The altitude for you to decide to land or go around for precision approach (ILS)


Manny

Beta tester for SIMStarter 

Share this post


Link to post
Share on other sites

>"Decision Height" is the lowest (Radar) altitude that you can>descend to without seeing the runway during a precision>approach, ie ILS approach CAT I, II, III. DH is defined based on radar altitude only for Cat II and III ILS approaches. For other precision approaches, i.e. ILS Cat I, MLS, PAR...it is a barometric altitude.Also, the DH is the lowest altitude on a precision approach at which the crew may make the decision to land or execute the missed approach. If the decision is to go missed approach, the aircraft will descend below the DH as the missed-approach is initiated, and that is factored into the TERPS criteria used to build the approach. The MDA, on the other hand, is like a glass floor...unlike the DH, the aircraft may not descend below it until the airport environment is in sight. The crew must plan the level off so as to not break the glass floor. As an old flight examiner once told me..."Break the glass, I'll bust your ..."RegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile


Bob Scott | President and CEO, AVSIM Inc
ATP Gulfstream II-III-IV-V

System1 (P3Dv5/v4): i9-13900KS @ 6.0GHz, water 2x360mm, ASUS Z790 Hero, 32GB GSkill 7800MHz CAS36, ASUS RTX4090
Samsung 55" JS8500 4K TV@30Hz,
3x 2TB WD SN850X 1x 4TB Crucial P3 M.2 NVME SSD, EVGA 1600T2 PSU, 1.2Gbps internet
Fiber link to Yamaha RX-V467 Home Theater Receiver, Polk/Klipsch 6" bookshelf speakers, Polk 12" subwoofer, 12.9" iPad Pro
PFC yoke/throttle quad/pedals with custom Hall sensor retrofit, Thermaltake View 71 case, Stream Deck XL button box

Sys2 (MSFS/XPlane): i9-10900K @ 5.1GHz, 32GB 3600/15, nVidia RTX4090FE, Alienware AW3821DW 38" 21:9 GSync, EVGA 1000P2
Thrustmaster TCA Boeing Yoke, TCA Airbus Sidestick, 2x TCA Airbus Throttle quads, PFC Cirrus Pedals, Coolermaster HAF932 case

Portable Sys3 (P3Dv4/FSX/DCS): i9-9900K @ 5.0 Ghz, Noctua NH-D15, 32GB 3200/16, EVGA RTX3090, Dell S2417DG 24" GSync
Corsair RM850x PSU, TM TCA Officer Pack, Saitek combat pedals, TM Warthog HOTAS, Coolermaster HAF XB case

Share this post


Link to post
Share on other sites

>If the decision is to go missed>approach, the aircraft will descend below the DH as the>missed-approach is initiated, and that is factored into the>TERPS criteria used to build the approach. >>The MDA, on the other hand, is like a glass floor...unlike the>DH, the aircraft may not descend below it until the airport>environment is in sight. The crew must plan the level off so>as to not break the glass floor. As an old flight examiner>once told me..."Break the glass, I'll bust your ...">>Regards>>Bob Scott>ATP IMEL Gulfstream II-III-IV-V L-300>Santiago de Chile>Interesting... I didn't know about that you can descend below the DA.can you do that for the IFR Checkride?


Manny

Beta tester for SIMStarter 

Share this post


Link to post
Share on other sites

"... I didn't know about that you can descend below the DA. can you do that for the IFR Checkride? "No! :)Bruce.

Share this post


Link to post
Share on other sites

A lot of good info in this thread.As some have already indicated, a DH occurs on a precision approach, and is an ALTITUDE where the approach must be aborted if the "runway environment" is not visible. This includes the threshold, touch-down zone or runway, and identified as the actual object, the markings on the object, or the lights on the object. Other items that qualify are VASI and PAPI. And, of course, the required visibility must be present for the approach, and you must continously be in a position to land.Just as the DH is the missed approach point for a precision approach and is an altitude, the missed approach point for a NON-precision appraoch is a PLACE, and is displaced or coincident with a fix or waypoint. An example is the missed approach for my home airport (KBJC) for the VOR 29R approach, the missed approach point is 1.9 DME from the VOR BJC. This place requires an altitude in 3 dimensional flight, which is the MDA. You must be at or above the MDA, then have the same criteria as above for continuing lower or further without executing a missed procedure. The requirement to be continously in a position to land is more restrictive here than in the precision approach. Sometimes the missed approach point is coincident with the runway threshold, and if you're still at the MDA at that point, you may not be in a position to make a "normal" landing.Bruce.

Share this post


Link to post
Share on other sites

>"... I didn't know about that you can descend below the DA.>can you do that for the IFR Checkride? ">>No! :)>>Bruce.>>>>DOH!ROFLMAO... :)


Manny

Beta tester for SIMStarter 

Share this post


Link to post
Share on other sites

> Sometimes the missed approach>point is coincident with the runway threshold, and if you're>still at the MDA at that point, you may not be in a position>to make a "normal" landing.>>>Bruce.>>You know Bruce..I believe it was in this forum someone had said.. in most cases for GA pilots the threshold is more than sufficiant to start a normal descent.They usually have a reasonable length of runway where you have ILS approaches and you are flying a single engine prop or a twin prop.Manny


Manny

Beta tester for SIMStarter 

Share this post


Link to post
Share on other sites
Guest Adverse Yawn

You mustn't fly below the DH without viz, but if you choose to go around at DH (reaosnable) in a heavy then you will almost certainly sink below it. In fact it has been known for an aircraft's mains to kiss the runway during a go-around.

Share this post


Link to post
Share on other sites

>You mustn't fly below the DH without viz, but if you choose>to go around at DH (reaosnable) in a heavy then you will>almost certainly sink below it. In fact it has been known for>an aircraft's mains to kiss the runway during a go-around.But the fact still remains, I can't use that explanation on my IFR Checkride while flying a Seneca I.LOL :)


Manny

Beta tester for SIMStarter 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...