If you’re Climbing Via a SID or Descending Via a STAR and
• ATC vectors you off track, or
• You are deviating for weather
Be sure to get an assigned altitude from ATC.
Recently, an aircraft was cleared to “descend via” an RNAV STAR, but needed to deviate away from the STAR’s lateral track due to weather. ATC approved the deviation, but did not assign an altitude to maintain. ATC’s expectation was that the aircraft would still comply with the published altitude restrictions.
If the cross-track deviation from the lateral track becomes great enough, VNAV guidance may no longer be available resulting in the automation reverting to another vertical mode.
Depending on the setting of the altitude pre-selector, the aircraft may no longer honor the altitude restrictions published on the procedure.
In addition, the published altitude restrictions on a SID or STAR provide obstacle and terrain clearance only when the aircraft is established on the procedure’s track. If the aircraft deviates from the procedure, an ATC-assigned altitude will ensure obstacle clearance.
The FAA is addressing this issue in a future update to the pilot and controller guidance. In the interim, pilots should request an altitude to maintain if a “climb via” or “descend via” clearance has been issued and the aircraft must deviate away from lateral track of the procedure.
Remember, the assignment of an altitude-to-maintain is required regardless of whether the pilot request a deviation or ATC issues a clearance that removes the aircraft from the procedure’s lateral track.