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Reading the clearance: "Climb via SID except maintain FL180."

What should the pilot do about the FL?

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Assuming you are in the , this is correct phraseology. It means that you are to comply with all constraints published in the assigned SID—lateral, vertical, and speed constraints—but once you reach the specified altitude or flight level (FL180 in this case) you are to stay there until receiving another climb or descent instruction from ATC. Even after you level off, you are to continue complying with all other constraints from the SID; from that point on you disregard the vertical information only.

From the AIM 5–2–9h:

4. (Using the Example 3 flight plan, ATC determines the top altitude must be changed to FL180). The clearance will read:
“Cleared to Johnston Airport, Scott One departure, Jonez transition, Q-One Forty-five, Climb via SID except maintain flight level one eight zero.”
NOTE- In Example 4, the aircraft must comply with the Scott One departure lateral path and any published speed and altitude restrictions while climbing to FL180. The aircraft must stop climb at FL180 until issued further clearance by ATC.

See ATC's guidance for this, the 7110.65 4–3–2e4(b) for "full" departure clearances and 4–3–3e2 for "abbreviated" departure clearances:

Instruct the aircraft to “Climb via SID except maintain (altitude)” when a top altitude is not published or when it is necessary to issue an interim altitude.
EXAMPLE-
“Cleared to Johnston Airport, Scott One departure, Jonez transition, Q One Forty-five, Climb Via SID except maintain flight level one eight zero, expect flight level three five zero one zero minutes after departure.”

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It means the controller wants you to climb, according to all the instructions in the SID, except maintain the altitude that they gave you, as opposed to the maximum listed on the SID.

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If it is not a typo then pilot should maintain FL180. A SID may allows flight to a point higher than FL180 but ATC wants the plane no further than 180.

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    $\begingroup$ Typos are rarely preceded by the word "except"... $\endgroup$ Jan 9 at 16:00
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Edit…..

Apparently it’s not a typo. As a non-FAA pilot, I think it is a ambiguous and requires clarification.

What should the pilot do? First they need to understand the clearance and then read it back correctly.

This is how I would respond: “Climb via the SID, maintain FL180. Confirm that all altitude restrictions are cancelled and that we should climb directly to FL180 and maintain FL180”

The controller will respond affirmative, or give further clarification to this POORLY worded clearance.

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    $\begingroup$ The clearance was not "cleared via SID, maintain FL180." The clearance was "cleared via SID and also climb via SID, except maintain FL180." This is neither a typo nor a lazy controller; it is acceptable phraseology in the USA. $\endgroup$
    – randomhead
    Jan 9 at 14:23
  • $\begingroup$ It may be acceptable phraseology in the USA, but I think it is ambiguous and requires clarification. $\endgroup$ Jan 9 at 14:27
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    $\begingroup$ That is why the FAA publishes the AIM, to provide clarification. If you are unfamiliar with the AIM then by all means please ask rather than making an assumption, but the correct answer to OP (assuming they are not currently sitting in an aircraft copying their IFR clearance!) is not "ask" but rather "check the AIM." $\endgroup$
    – randomhead
    Jan 9 at 14:28
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    $\begingroup$ Does the FL180 limitation supercede my EFC+10 altitude? If I have a comms failure, do I proceed to FL180, then proceed to FPA or Expect +10 alt 10 minutes from departure? Or would I have to complete the trip at FL180? $\endgroup$
    – John K
    Jan 9 at 17:57
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    $\begingroup$ @John that would better be a separate question, but: The "except maintain" is part of your initial altitude instruction. "Climb via SID except maintain FL180" is all one chunk. The "expect" altitude will be issued separately, whether by voice or as information printed on the SID, and it is to be complied with separately. So in the case of lost comms you would still evaluate your "expect" altitude as part of the decision. $\endgroup$
    – randomhead
    Jan 9 at 19:24

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