Welcome Guest. Sign in or Signup

2 Answers

Minimums in CAT II & III chart

Asked by: 6405 views Instrument Rating

Have a look at this chart: SEA, ILS RWY 16R (CAT II & III) (http://155.178.201.160/d-tpp/1701/00582I16RC2_3.PDF)

While reading this chart, I'm kind of lost on the minimums for the CAT II approach. Why the RA (which I guess refers to the radio altimeter), which is 139 AGL, is different from DH, which is 100 AGL? Does the difference come from the difference in ways of measuring altitudes between the radio altimeter and barometric altimeter?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

2 Answers

  1. Best Answer


    Russ Roslewski on Jan 31, 2017

    Local terrain is the reason.

    DH is the height above the Touchdown Zone of the runway when the airplane is at that minimum altitude. So, in this case DH is 100 feet above the touchdown zone. Easy enough.

    RA, however, is the reading on the radar altimeter when the airplane is at that point. Since the earth isn’t flat, it often varies, sometimes significantly, from the DH. At SEA, for example, the ground drops off quite steeply at the end of the runway. In fact, there is an tall retaining wall built between the runway and a local street.

    Take a look at the Google Street View:
    https://goo.gl/maps/w9pTzPWPcZt

    You can see the approach light “bridge” structure sticking out, that’s where the runway centerline is, so it’s pretty clear in this case that there’s a large difference between how high the airplane is above the ground directly below it, and how high it is above the runway.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  2. Lemontree on Jan 31, 2017

    Thank you Russ. Very clear now.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.