MD11 flight crew experiences dual ADC failures at FL360. Stall warnings are activated and altitude is lost as the First Officer recovers using standby instruments. Many of the problems clear up at low altitude and flight continues to destination after declaring an emergency.
Synopsis
MD11 flight crew experiences dual ADC failures at FL360. Stall warnings are activated and altitude is lost as the First Officer recovers using standby instruments. Many of the problems clear up at low altitude and flight continues to destination after declaring an emergency.
Narrative
Stick shaker occurred; but both primary flight displays initially showed normal airspeed. During the next several minutes; various alerts/warnings and conflicting data continued. Following the stick shaker onset; and during the time needed to process and interpret the instruments; I witnesses what I perceived as a stall buffet. I immediately added maximum power then lowered pitch. Stall indications continued during descent with considerable altitude loss. Captain could neither verify/interpret the instruments initially; and was concerned about my altitude deviation as radio congestion and primary efforts of 'aviating' prevented the prompt declaration of our emergency or setting of the appropriate transponder code. The multiple distractions of alerts and conflicting instruments led to considerable confusion; and I found it extremely difficult to follow the standby instruments to recovery efficiently. With this scenario and the Captain's desire to return to altitude; I allowed a secondary stick shaker/stall buffet. The other crewmembers were summoned forward to assist and to complete appropriate checklists; and the Captain assumed flying pilot duties at some point after stall recovery was complete and while I was attempting stabilized flight via reference to the standby instruments. We felt strongly that it was not a simple failure of one Air Data Computer; and that all the symptoms did not result from an ADC failure as we had experienced in training. The Captain and I both feel it is imperative to require crew training in high speed stall situations by use of only standby instruments; to include distractions attendant with erroneous presentations and the various alerts both aural and displayed. As the Captain is the check airman; he has already received assurances that such training is to be placed in the company program.
Second reporter narrative
We were at FL360 in clear weather approximately 11 hours into the flight. We had been level at FL360 for several hours. We got a stick shaker followed by aircraft buffet. Both primary airspeed indicators showed Mach .82 or approximately 280 KTS. The standby airspeed indicator however indicated 220 KTS. We advanced power however we could not recover the airspeed without giving up altitude. We tried to contact radar but aircraft control required our full attention. We were getting multiple alerts and warnings at this point. We accomplished a lost/suspect/erratic airspeed checklist and used the standby instruments. Once confident of aircraft control and the situation we declared an emergency and got vectors to land at destination airport. At low altitudes many of the problems cleared up. We obviously had a dual ADC problem. One of the biggest issues in this situation was how insidious the onset of this problem was. A single ADC problem and comparators will alert you prior to the issue. In this case they were adding to the confusion.
NASA callback
Reporter stated that about an hour prior to the Stall Warning the Airspeed Comparator light illuminated intermittently. The pilots ran the checklist; looked at the airspeed indicators and believed that #2 was the most correct and so selected ADC BOTH ON 2. The crew did not remember this aircraft having a record of erratic airspeed indications. When the actual failure event happened; the cockpit was dark except for instrument illumination. Suddenly the aural alerts sounded at the same time as the stall buffet so the reporter reverted to 'seat of the pants' flying and began a descent. He did not expect that when maximum power was added at FL360 the aircraft would not respond as it did in the simulator at 10;000 feet. In fact he could not hear the engines accelerate; did not see an airspeed increase and realized quickly that descending was the only way of increasing airspeed and flying out of the stall. They did not call ATC to declare an emergency until several minutes and thousands of feet later because of the confusing information and attention to simply flying the aircraft. In retrospect; the first thing this pilot would have done is turn the cockpit lights to bright so that all of the instruments could be seen. He had difficulty seeing the standby airspeed indicator and so did not include that in his scan. The PFD and other instruments immediately in front of him became the primary scan. According to the reporter the major focus was responding to the stall warning because his training told him that reliably and unequivocally the stall warning is indicating true information. The Captain; who was a check airman; ran the checklist and when they did not get resolution called the other crew to the cockpit. The crew did not determine during the flight what was supplying accurate data and even on final approach a 5-10 knot airspeed difference existed. The reporter would suggest that pilots maintain their manual flying skills and learn to rely less on automation so that in extreme circumstances such as this they are able to respond correctly by instinct. Maintenance replaced #1 ADC but the log book response did not indicate that an ADC failure was found prior to it being replaced. Reporter stated they examined their actions and could not remember if the ADC selector remained BOTH ON 2 throughout the entire event. If so; [reporter questioned] could the fact that #2 was selected for all user system inputs have prevented the Airspeed Comparator from detecting and warning about a difference if it malfunctioned because BOTH #1 and #2 ADCs were not selected?
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.