Large Transport; Low Wing; 2 Turbojet Eng
Situational Awareness; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors; Procedure
On descent into ATL; we were given holding instructions; to reduce speed; and hold (on either the LGC or HONIE intersection) radial; as published at FL180. Due to a low altimeter setting; less than 29.92; I queried my First Officer (FO) to ask ATC to reaffirm our holding altitude. After doing so; and with numerous aircraft receiving clearances; the ATC controller; seeming somewhat agitated; restated that we were instructed to hold at FL180. I instructed my FO that this was an unacceptable FL due to the altimeter setting and to advise ATC. Unfortunately; due to numerous arrivals and simultaneous ILS PRM approaches in use; we were not able to advise ATC of the unacceptable clearance. At this time; we were descending on the arrival; and entering the hold. Slowing; and turning while entering the hold; alpha protection started climbing on our PFDs. Noticing this; my FO called out 'airspeed'. I responded to his callout; looked at my altimeter; believing I was passing through 18;700 and called for 'flaps one'. He checked his altimeter and positioned the flap handle in flaps one position. At this time; knowing the max flaps extension altitude being 20;000 feet; we both believed and confirmed that we were within aircraft limitations.Although aircraft saturation and weather were a significant factor; what appeared to be an ATC controller unaware that FL180 is unusable with a local altimeter setting of less than 29.92; added to the confusion; not only in communications; saturated approach control frequency; but the workload in the cockpit of transitioning from the descent phase to the approach phase. ATC must limit speed restrictions in the flight levels to nothing less than 250kts IAS.
A321 Captain reports being given holding instructions at FL180 with a low altimeter setting making FL180 unusable. The distraction of attempting to clarify with ATC results in low airspeed and the flaps being extended above 20;000 feet.