Radio Altimeter

[AC560]

Can you legally use a radio altimeter to determine MDA on a CATI ILS or non-precision approach? I think the answer is no but just wanted some clarification.

Thanks!

 

[xjcaptain]

Short answer: no. Not on a CAT 1. Assuming you're using Jepps, RA is only published on cat II and cat III approaches. You have no way to determine what would be shown on the radar altimiter when reaching decision altitude on your altimiter.

 

[AC560]

Short answer: no. Not on a CAT 1.

Thanks and I think I know the long answer just needed somebody to reassure me.

 

[Amish RakeFight]

Ive seen RA's used before in 135 baron pax runs which were always CAT. I

I havent yet really been exposed to those helpful devices, but is one not allowed to really activate one for say 200'?

Aren't they installed in many aircraft that dont fly CAT II & III approaches. Is it that they can only be used as a secondary source, even though you have it set to soemthing close to the DA/MDA according to altimeter?

 

[MauleSkinner]

Ive seen RA's used before in 135 baron pax runs which were always CAT. I

I havent yet really been exposed to those helpful devices, but is one not allowed to really activate one for say 200'?

Aren't they installed in many aircraft that dont fly CAT II & III approaches. Is it that they can only be used as a secondary source, even though you have it set to soemthing close to the DA/MDA according to altimeter?

Tell ya what...fly the approach into Telluride (TEX), and let me know what you see when your radar altimeter gets to the appropriate HAT. That would make it self-explanatory

The reason it's not valid except as an aid to situational awareness on CAT I approaches is that the HAT isn't necessarily the same as the radar altitude when you get to minimums...the ground just isn't that flat.

Fly safe!

David

 

[FL420]

Can you legally use a radio altimeter to determine MDA on a CATI ILS or non-precision approach? I think the answer is no but just wanted some clarification.

Thanks!

No. Think about this.

Lots of runways have terrain on approach sloping down toward the runway(e.g.-KSAN Rwy 27.) Lots of runways near the water have cliffs of various heights just off the end of the runway(e.g.-KSAN Rwy 09.) Sometimes ships cross or are tied up off the approach end of the runways. Although it's not an IFR airport, Catalina Island has huge cliffs off both ends of the runway.

When I was an instrument instructor in TA-4F/J's driving around the GCA pattern at NKX with a student in the back under the bag they would have the bug set at 200' on the RadAlt to get a warning tone and red light. Even though PAR's and ASR's use baro. alt. for minimums we used the RadAlt around the boat for backup. It was incredibly boring for the IP after the first couple of trips around the pattern. Occasionally, our downwind vector north of NKX for Rwy 24R would take us over Black Mountain(4051' MSL) at 4200' MSL. The student couldn't see it coming of course but I would start getting all antsy to see how the student would react when the RadAlt warning would go off and the needle would unwind to something less than 200' AGL as we passed over the top of the mountain.

When the warning went off some guys would do a little dance with the stick followed by a WTFO! Other guys just ignored it and assumed the RadAlt was attacked by queer electrons or something. It was a real eye-opener for them when I took control and rolled the airplane while I told them to pop the bag and look back at the mountain.

We had to get our chuckles when and where we could. It was a good graphic example to the students of the importance of a correct altimeter setting and precise altitude control in IMC. Sometimes the GCA controllers would forget about you for awhile on downwind(I'm sure they were bored too.) No big deal in day VMC but at night or in IMC, BIG DEAL because you were flying into rising terrain in the form of the foothills of the Laguna Mountains. In the days before GPWS and EGPWS, sometimes the RadAlt would give you the only warning you were gonna get if your radio or your controller quit on you and you didn't recognize it in time.

 

[Amish RakeFight]

Tell ya what...fly the approach into Telluride (TEX), and let me know what you see when your radar altimeter gets to the appropriate HAT. That would make it self-explanatory

The reason it's not valid except as an aid to situational awareness on CAT I approaches is that the HAT isn't necessarily the same as the radar altitude when you get to minimums...the ground just isn't that flat.

Fly safe!

David

So for those who regularly use an RA for other than CAT II & III approaches, how can you effectively and safely use it as a back up.

 

[FL420]

So for those who regularly use an RA for other than CAT II & III approaches, how can you effectively and safely use it as a back up.

Here are a couple of accounts of a very interesting British Mediterranean Airways A320 near CFIT at Addis Ababa where the RadAlt saved the day when the VOR and EGPWS were no help:

http://www.aviationtoday.com/sia/20040201.htm

http://www.caa.co.uk/docs/33/FOD200316web.pdf

 

[MauleSkinner]

So for those who regularly use an RA for other than CAT II & III approaches, how can you effectively and safely use it as a back up.

Well, if your radar altimeter says you're 500 feet above terrain in flat country, and your baro altimeter says you've got 1200 feet to go to minimums, it's a good bet that you need to check your altimeter settings to avoid becoming a CFIT statistic...basically, a little knowlege of the area will go a long way toward making a radar altimeter a useful safety feature.

Fly safe!

David

 

[sharpeye]

hahaha, I would go a step further and say if you see your RA at 500' and your baro reading 1200' to go for mins in flat country territory, you better go missed and climb like hell.

As far as this post goes, I agree with what has already been said, NO, NO, and NO, you can not use your RA for mins on any approach whatsoever unless the Jepp is based on RAs, and that is usually for CAT3 approaches.