Air carrier relief pilot reported a missed altitude restriction while on the GIBBZ2 arrival to iad. ...

Attributes
ACN	1590288
Time
Date	201809
Local Time Of Day	1801-2400
Place
Locale Reference	IAD.Airport
State Reference	DC
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 129
Flight Phase	Descent
Route In Use	STAR GIBBZ2
Flight Plan	IFR
Person 1
Function	First Officer Pilot Not Flying Relief Pilot
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 34 Flight Crew Total 5946 Flight Crew Type 15
Events
Anomaly	Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Undershoot Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

Narrative:

Aircraft was first cleared to FL270; subsequently the clearance descend via the GIBBZ2 was given and read back by the captain. Both pilot flying and pilot monitoring did not understand that meant they could descend below FL270 via the arrival. Additionally it was briefed the gibbz arrival is quite steep and drag may be needed; I queried the first observer if he agreed we were cellar via the arrival; he replied I was not paying attention. He then queried the pilot monitoring who said he thought it was FL270. I explained to him the decent via clearance at which point ATC was queried. They advised we were to comply with the altitude requirements on the arrival. They then issued a new altitude clearance and a heading. By the time I was able to clearly communicate to the crew we had already missed the FL250 crossing restriction at jimve. A contributing factor to this was the lack of intercom and boom mic for the second observer on the aircraft as I had to stand up to talk to the pilot monitoring clearly; additionally there is no port page annotation about the descend via clearance or the known steep arrival iad combined with typically high tailwinds on this arrival.

Original NASA ASRS Text

Title: Air carrier Relief Pilot reported a missed altitude restriction while on the GIBBZ2 arrival to IAD. The crew misunderstood the 'descend via' phraseology.

Narrative: Aircraft was first cleared to FL270; subsequently the clearance descend via the GIBBZ2 was given and read back by the Captain. Both Pilot Flying and Pilot Monitoring did not understand that meant they could descend below FL270 via the arrival. Additionally it was briefed the GIBBZ Arrival is quite steep and drag may be needed; I queried the first observer if he agreed we were cellar via the arrival; he replied I was not paying attention. He then queried the Pilot Monitoring who said he thought it was FL270. I explained to him the decent via clearance at which point ATC was queried. They advised we were to comply with the altitude requirements on the arrival. They then issued a new altitude clearance and a heading. By the time I was able to clearly communicate to the crew we had already missed the FL250 crossing restriction at JIMVE. A contributing factor to this was the lack of intercom and boom mic for the second observer on the aircraft as I had to stand up to talk to the Pilot Monitoring clearly; additionally there is no port page annotation about the descend via clearance or the known steep arrival IAD combined with typically high tailwinds on this arrival.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Air carrier relief pilot reported a missed altitude restriction while on the GIBBZ2 arrival to iad. The crew misunderstood the 'descend via' phraseology.