Hey all,

During my multi add on checkride I was briefing the approach to KGON ILS 5 circle 23 using the jepp charts and the MSA is located on the right side of the briefing strip so I came to that last.

http://204.108.4.16/d-tpp/0805/05049IL5.PDF
(this is the nos chart)

I told him the MSA altitudes and why the one is higher than the other (there is a 1511ft tower over there) and he then said.....well you said the MSA is 2100 where we are? (we were inbound from Hampton VOR about to intercept the ILS) I said yes...and he said...well then why can we go to 1900 once established? Thats below the sector alt.....I said well its becuase were going to be on the ILS and that gives us ground clearence....so he said well were going to be circling, what about then. I said well as long as were within the airport enviroment (gliding distance) were fine. I know I was wrong there but I couldnt think of the right answer until later and I think its based on airspeed and that gives you a radius of like 1.3 nm around the airport thats safe? Anyhow, does anyone know for sure what the answers are? Its been bothering me. When the MSA says 2100 within 25nm of GON why can we come down to 1900 when established and why can we on the feeder routes as well? What about circling what protection do you get then? Sorry its so long I hope its understandable. Thanks!

 

ok here is my insight. Somewhere inside that imaginary 25nm circle there is an obstacle (maybe an antenna) that will not be of concern as long as you are at your MSA. Evidentially, that “antenna” is not located in the ILS or feeder route course/path (otherwise it would be impossible to fly the approach- If its published, it has been tested and flown by the FAA) and therefore descending to 1900 will not be a problem. Same goes for the circling altitude…lets say for a CAT A aircraft, the FAA knows that no obstacles exists within a 1.3nm radius from the runway threshold (not sure about that) and circling at 560ft will clear you of hitting anything.

 

Got this info from another thread (PAR/ASR Differences)

c. Minimum Safe/Sector Altitudes (MSA) are published for emergency use on IAP charts. For conventional navigation systems, the MSA is normally based on the primary omnidirectional facility on which the IAP is predicated. The MSA depiction on the approach chart contains the facility identifier of the NAVAID used to determine the MSA altitudes. For RNAV approaches, the MSA is based on the runway waypoint (RWY WP) for straight-in approaches, or the airport waypoint (APT WP) for circling approaches. For GPS approaches, the MSA center will be the missed approach waypoint (MAWP). MSAs are expressed in feet above mean sea level and normally have a 25 NM radius; however, this radius may be expanded to 30 NM if necessary to encompass the airport landing surfaces. Ideally, a single sector altitude is established and depicted on the plan view of approach charts; however, when necessary to obtain relief from obstructions, the area may be further sectored and as many as four MSAs established. When established, sectors may be no less than 90° in spread. MSAs provide 1,000 feet clearance over all obstructions but do not necessarily assure acceptable navigation signal coverage.

USMCFLYR

 

Once you are established on any section of the approach and are cleared for the approach, you should descend according to that approach. This is merely following your clearance. In any case, the MSAs are usually used when you aren't in contact with ATC (like a practice approach). If you're talking to a controller they can give you a lower altitude.

 

Don't forget, either, that in order to circle you must remain in visual contact with the runway. Once the circling portion of the approach begins, you can see-and-avoid any relevant obstacles.

 

thanks everyone! So basically the MSA is for emergencies, anytime your not on a airway or approach, or off route not talking or seen by atc?