RNAV (no TAA) - Cleared for the approach. When to descend?

[John Collins]

Having flown a lot of TAAs, not sure why the author was unfamiliar with the sectorized presentation of the safe altitudes in each sector. Since often fly in fairly rugged semi-mountainous terrain, on a new approach will check the sector altitude to insure Center hasn't given me too low an altitude - i.e., "they've descended us to 5100', were are cleared for the approach, and were are 24 miles away in a sector where the minimum safe altitude is 4500'." All good. No need to actually descend to the minimum until on a feeder "T" then hold that altitude until GS intercept. Unless you like to reconfigure and drop down to a lower altitude.

Unlike an MSA which may only be used in an emergency, a TAA sector is considered as part of the approach depicted route. It is in effect an RNAV feeder route to the "T" IAF points so that once inside the TAA sector and having been cleared for the approach, you are cleared to the descend to the sector altitude. ATC is not allowed to clear you for an approach without assigning you an altitude to maintain until you are on a charted segment of the approach and at or above the charted segment minimum altitude. So if you are outside the TAA and cleared for the approach, once you enter the TAA segment you can descend to the charted TAA altitude. If you are inside the TAA and cleared for the approach, you can descend to the TAA segment altitude without further clearance. If one was cleared direct to one of the IAF fixes and cleared for the approach at an altitude below the TAA segment, you would have to immediately climb to the TAA segment altitude. Assume RBASE was the right hand T base leg IAF with the TAA altitude as 3500 feet and the next segment connecting to the center IAF fix had a charted minimum of 3000 feet and ATC had you on vectors inside the TAA at 3000 feet and at or above the MVA for the area, ATC could clear you "direct RBASE, Maintain 3000 until crossing RBASE, cleared RNAV 12 approach". That is acceptable because your approach clearance only applies after crossing RBASE and before that ATC is providing vectors at or above the MVA and has assigned you an altitude to maintain. Once your approach clearance is in effect, ATC may no longer assign altitudes below a charted procedure altitude. Here is an example of a case when ATC gave an ambiguous clearance to a Beech 1900 crew at PADL, resulting in a fatal crash into mountains. https://aviation-safety.net/database/record.php?id=20130308-0

 

[Steamflyer]

If you are inside the TAA and cleared for the approach, you can descend to the TAA segment altitude without further clearance.

Agree. Though it may not be necessary right away. Often I'll get the descent to a little above the TAA altitude for that sector and then when cleared for the approach, told to cross the IAF at an altitude not below the TAA altitude.

 

[John Collins]

Agree. Though it may not be necessary right away. Often I'll get the descent to a little above the TAA altitude for that sector and then when cleared for the approach, told to cross the IAF at an altitude not below the TAA altitude.

That should not be needed because once cleared for the approach, you are supposed to follow the charted altitudes, unless ATC restricts you. If all they are doing is restricting you to follow the chart, that is redundant, but might happen anyway.