Timing a localizer approach

[gsrcrsx68]

The guy I did my instrument with taught me I could descend to DH, then fly along until the timer expired...

I never bought it, but I think he still teaches it the same.

 

[FL420]

The guy I did my instrument with taught me I could descend to DH, then fly along until the timer expired....

I suggest you dig into the books and look up the definitions of Minimum Descent Altitude(MDA,) Decision Altitude(DA) and Decision Height(DH.)

I never bought it, but I think he still teaches it the same.

Why not? That's the way it's done. Again, I suggest a little IFR refresher is in order.

 

[gsrcrsx68]

Yeah one of us needs a refresher. I'll stand by what I said. DH and DA are both specific to an ILS. So I suggest you dust off one of your books and read it for yourself.

 

[FL420]

Yeah one of us needs a refresher. I'll stand by what I said. DH and DA are both specific to an ILS. So I suggest you dust off one of your books and read it for yourself.

Fair enough. I dusted off the books and this is what I came up with:

from FAA-H-8261-1 Instrument Procedures Handbook

from Appendix D
Decision Altitude (DA) –A specified altitude in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. The term “Decision Altitude (DA)” is referenced to mean sea level and the term “Decision Height (DH)” is referenced to the threshold elevation. Even though DH is charted as an altitude above MSL, the U.S. has adopted the term “DA” as a step toward harmonization of the United States and international terminology. At some point, DA will be published for all future instrument approach procedures with vertical guidance.

Decision Height (DH) – See Decision Altitude

from Chapter 5
MINIMUM DESCENT ALTITUDE, DECISION ALTITUDE, AND DECISION HEIGHT
DA is currently used on RNAV approach charts with vertical descent guidance. DA will replace DH for Category I precision IAPs. MDA and DA are referenced to MSL and measured with a barometric altimeter. CAT II and III approach DHs are referenced to AGL and measured with a radio altimeter.

So. Unless you and your aircraft are current and qualified to shoot CAT II or III approaches, you can eliminate the term Decision Height(DH) from your vocabulary. If you are shooting a Cat I precision approach(PAR, ILS, TLS and eventually LPV and LNAV/VNAV down to 200' HAT and ½ sm visibility) you will be using a Decision Altitude(DA) for your minimums. DHs are heights based on radar altimeters and DAs are altitudes based on barometric altimeters.

The original poster's question was regarding shooting a localizer approach using timing to determine the MAP. If a pilot knows he is going to shoot a non-precision Category I localizer approach and the IAP calls for timing to determine the MAP, that is what he must use. Furthermore, he cannot descend to the ILS DA and then drive to the MAP. He must go no lower than the published MDA then drive to the MAP(assuming he never sees the runway environment soon enough to make a safe and stabilized descent to land.)

If the pilot is shooting an ILS and loses a reliable GS indication after passing the FAF and the MAP for the localizer only approach can only be determined by timing, he should have started his timing passing the FAF because otherwise, he has no way of knowing where his MAP is. It can't be at the DA because he can't continue below the MDA without a valid glideslope signal.

Some commercial operators don't allow their pilots to revert to a localizer only approach after losing the GS signal inside the FAF. They are required to immediately execute a MA, rebrief for a localizer approach and come back and try it again, weather permitting. Some companies allow crews to revert to localizer only if they have specifically briefed both approaches as a contingency.

 

[gsrcrsx68]

<<The original poster's question was regarding shooting a localizer approach using timing to determine the MAP.>>
Well ok, but my comment didn't address his comment.

<<He must go no lower than the published MDA then drive to the MAP>>
That was my point. You can't go to DH or DA and drive along at all. The MAP for the ILS is on glide slope at DA(or DH, true, not good of me to interchange those).

So are you taking back you initial response to me?

 

[EA6BDriver]

Intersting and educational discussion.

I tend to agree with x68 on this. Why oh why would anybody teach to ignore that whole "precision" part of a precision approach by jumping right on down to DA and driving in straight and level for x distance?

Isn't "Decision Altitude", as its name implies, the time you need to decide to go missed? And if you're already at that altitude well prior to when you'd have been there had you followed that pesky glideslope needle, then when do you go missed?

Then again, I could be all wrong...

 

[FL420]

So are you taking back you initial response to me? [/FONT]

Okay. You obviously don't need any recurrent instrument training and probably never will.

 

[PURPLEHAZE21]

Guys,

This timing a Localizer with a clock or using GPS as backup is way out of control. Since the 30's guys shot NDB's with an ADF and Clock. They survived. It's a non-precision. The name implies it is not as accurate as a Precision. If you don't have the mental capacity to do or use both...then get out of the cockpit.

 

[maverick121]

Guys,

This timing a Localizer with a clock or using GPS as backup is way out of control. Since the 30's guys shot NDB's with an ADF and Clock. They survived. It's a non-precision. The name implies it is not as accurate as a Precision. If you don't have the mental capacity to do or use both...then get out of the cockpit.

Thank you, amen!!