Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Aircraft; Human Factors
We were level at FL300 and were cleared to descend to FL280. I (pilot flying) set the altitude preselect to FL280 and stated; 'FL280 set.' My partner confirmed and stated; 'FL280 seen.' As I began to brief my partner on the anticipated descent; approach; and landing I noted the airplane was descending near FL280. When I scanned back I noticed that the airplane had descended through FL278. I disconnected the autopilot and returned the aircraft to FL280; from an altitude of FL277. During the recovery I noticed that the altitude preselect was now set to FL275. After being level at FL280 for about 30 seconds; the ATC controller asked us to confirm that we were level at FL280. To which we replied that we were. No further action or conversation was had between the crew and ATC regarding a possible deviation.It is widely known that the altitude selector in the GIV is extremely sensitive; as well as not having specific detents to feel that the knob has moved additionally. Also; my personal technique is to rest my finger on the knob after setting the desired altitude and leaving it there until my partner has responded that the new altitude is 'seen.' it is possible due to the operation of the knob that as I removed my finger from it; it moved additionally to the next lowest setting; although I cannot affirm that this occurred. The accurate altitude was both 'set' and 'seen;' so there had to have been additional movement of the knob. So perhaps a mechanism that I had created to ensure that both crewmembers had acknowledged the altitude change; created a new issue of not noticing immediately after confirmation of a change. Going forward; I personally; will employ the SOP of setting and confirming the change as well as confirming again on my PFD that the desired altitude is in fact set. Also; confirming that the correct altitude is still set when making the 1;000 foot call.
Although their cleared descent altitude of FL280 was both set and confirmed in the altitude alert window the pilot flying a G-IV noted the aircraft descending through FL278; stopped the descent and returned to FL280.  He noted that the altitude alert window had changed to 27;500.  He attributed the change excessive sensitivity of the set knob.