If your decision of going around or landing is made at the DA, you WILL go below DA. If you check the practical test standards for the instrument rating in the US it says you can maintain your MDA plus +50/-0 ZERO. That´s ´cause you´re leveling off at that altitude.
When mentioning DA margins it specifies a -50 feet margin. This is ´cause they know you´ll go slightly underneath it, even in a 172.
For example. Let´s say a 747 approaching at 150 knots groundspeed, flying a 3º glideslpe the rate of descent will be close to 750 feet per minute, maybe a bit more (I do Groundspeed times 5) If that plane reaches DA
(H)and doesn´t see anything, it must go missed. The maneuver is preety sudden, I´ve done it, but there´s no way the aircraft will initiate an immediate climb without losing 50 feet or so. TERPS knows this and ILSs are flight checked with that taken into account.
Glideslopes are flight inspected all the way to 100´(CAT 1), I´ll get back to that.
I would like to say that unless you´re flying a CAT II or III approach, heights in parenthesis ARE NOT radio altimeter heights. They are the difference between the altitude at that position and the Touchdown zone elevation of that runway.
Unless you are flying a CAT II or III you´re required to use your altimeter to determine when it´s time for you to decide whether to continue approaching or go around.
In the States, the required RVR for an ILS CAT I is 2400 ft (1800 ft if the airport has touchdown zone lights)
When the airplane is at the DH, it is usually at some 4,000 feet distance from what´s called the ground point of intercept, where the glidepath meets the ground, and it is at 3,000 feet from the landing THRESHOLD, roughly.
If that day RVR is 2400 feet the airplane will not see the runway at the DH, if he doesn´t see the runway, obiously he won´t see the the PAPI or VASI, located at another 1000 feet down the rwy.
However, if he sees the Approach light system, he is allowed to continue descending to 100 feet above the touchdown zone elevation.
The portion of flight from the DH to 100 feet above tdze is what´s called COMPOSITE FLIGHT, a combination of instrument and visual.
The pilot can use visual references to determine runway alignment, and bank information, but he still uses the glideslope and the ADI to monitor his profile and pitch. At 100 feet above tdz the aircraft will be at 1,000 feet or so from the landing threshold, and at another 1,000 feet from the touchdown point. He still needs an extra 400 feet to see the far VASI light so there you have 1000+1000+400 equals 2400, so that´s why ILSs require RVR 2400 feet. At that time, the pilot sees the lights, the runway, and the VASI. He can now continue 100 percent visual...Plus at 100 feet above tdz you are supposed to stop using the electronic glideslopes, since they´re not flight inspected below 100 feet.
Hope it helps.
Good luck to you all !!!