747-400 Encounter With A Volcanic Ash Cloud

 

In December 1989, a KLM Boeing 747-400 descending for landing in Anchorage, encountered the volcanic ash cloud from Mt. Redoubt, a volcano located about 120 miles southwest of Anchorage. The airplane was about 70 miles north of Anchorage, when the Captain reported entering what appeared to be light clouds at FL260. Then suddenly things turned dark black and he noted that lighted particles were visible, like fireflies in the dark. The Captain ordered a climb to get out of the clouds and climb power was applied. Engines reached better than 100% N1 and the airplane pitched up, but then suddenly all four engines dropped to approximately 45% N1 with no response to the thrust levers. The stall warning and stick shaker sounded. It was an all-engine flame out.

With the loss of all engine power, all electrical generators ceased operating and the airplane was on battery power only. The right-hand pilot, who had been the one flying the airplane at the time, lost his primary flight display (PFD) and navigation display (ND). The controls were therefore transferred to the left-hand pilot. [There were three crew on the flight deck at the time. The Captain, was sitting in the right-hand observers seat while the other two pilots were at the controls.] He reported that intense smoke entered the flight deck and that the oxygen masks were donned. The Captain then announced to the passengers that they had encountered a volcanic ash cloud. Then the fire bell sounded. It was “CARGO FIRE FWD”, most likely from the smoke and not an actual fire. The rate of descent at this

time was 1500 ft/min, but the PFD had no airspeed indication, so the Captain ordered to maintain that rate of descent while he commenced engine restart procedures.

The Captain reported that every time he placed the fuel control switches to cut-off, the left hand PFD and ND displays went dark. Then the displays came back when he placed the fuel control switches to ON. During the restart procedures, one or more of the engines had high EGT and the procedure had to be repeated. He says that he may have had to repeat the procedure 7 or 8 times. After reaching 14,000 feet engines 1 and 2 restarted and they were able to maintain 13,000 feet altitude. Shortly after that they were able to restart engines 3 and 4. With all four engines now running, the Captain traded positions with the left-hand pilot and executed the approach and landing.

An examination of the aircraft found that the front windshields were sandblasted and distorted, the airspeed indicating system was inoperative due to plugged probes, and the entire interior was contaminated with ash. The plugged pitot probes would explain the loss of airspeed indication and the stall warning/stick shaker warnings. But what would account for the apparent cycling off and on of the left hand PFD and ND displays? These are cathode-ray-tube display devices that require electrical power for operation. The two left hand displays are powered from standby power under this condition. The electrical buses that remain powered upon loss of all main generators along with some of their connected services are:

1. Main hot battery bus (always powered with a battery installed), 24 volts DC.

  • a) Engine fire extinguishing
  • b) Engine fuel shutoff valves (1-4)
  • c) IRS backup
  • d) 115V AC standby inverter

2. Main battery bus, 24 volts DC.

  • a) Flight deck dome lights
  • b) Flight deck Capt. Indicator lights
  • c) Engine fuel control valves
  • d) Oxygen valve and indication
  • e) Standby instruments altimeter vibrator
  • f) Standby attitude instrument
  • g) Aural warning L
  • h) Stick shaker L

3. APU hot battery bus (always powered with a battery installed), 24 volts DC.

  • a) IRS backup
  • b) 115V AC APU standby inverter

4. APU battery bus, 24 volts DC.

  • a) Engine start air control
  • b) Engine speed sense
  • c) VHF-L
  • d) Interphones
  • e) Passenger address

5. 115V AC standby bus, powered from an inverter supplied from the main battery.

  • a) Electronic flight instrument system, L (EFIS)
  • b) Standby ignition all four engines
  • c) Upper EICAS
  • d) EIU-L
  • e) Air data computer-L
  • f) Standby instrument lighting
  • g) FMCS CDU-L

6. 115V AC APU standby bus, powered from an inverter supplied from the APU battery.

  • a) PFD -L
  • b) ND-L
  • c) Flight management computer system FMC-L
When the airplane was later tested on the ground, the standby power transfer was confirmed to be operating properly. The loss of the right side PFD and ND are explained by the loss of all main generators. They were not designed to be operated from standby power, However, since the L-PFD and L-ND are designed to be operable from standby, the question here is what would explain the apparent loss of these displays with operation of the fuel control switches?

The answer was found from the digital flight data recorder (DFDR). Of the several restart attempts, one or more of the engines had high EGT and the attempt was aborted by putting all four fuel switches to cut-off. Sometimes one or

more of the engines had high EGT and the attempt was aborted by putting all four fuel switches to cut-off. Sometimes one or more engines did begin to start. Since main generator power is restored each time N2 reaches or exceeds 57%, there were several power transfers between main generator power and static inverter power. In between each power transfer is a momentary power interruption. These interruptions could range from 1.0 milliseconds to 100 milliseconds. By specification, the interruption can be as long as 400 milliseconds. Such power interruptions normally cause this equipment to blank for 0.5 to 8 seconds, depending on the length of the interruption. With the confusion of the situation, the blanking of the displays could very well seem to be associated with the operation of the fuel control switches.

APU AC Standby Power (Figure 1)

With normal generator power, the APU Stby XFR Relay is energized and the L-FMC, L-PFD and L-ND are powered from the Capt XFR Bus. With the loss of all generators, the Capt XFR Bus becomes unpowered and the APU Stby Relay drops out. This causes the APU Inv XFR Relay to pick up, energizing the APU Static inverter. The initial power interruption was found to be 45ms before the APU static inverter begins outputting AC power. It remains on for 15ms and then for some reason shuts down for 25ms before coming on again and staying on. So, there are actually two power interruptions for the generators going off. When one or more generators come back on, the Capt XFR Bus is initially connected to main AC Bus-1 and after a 1ms interruption the displays receive power from AC Bus-1.

However, after approximately one second, the Capt XFR Bus transfers to AC Bus-3 with yet another power interruption of 15ms. Thus there are another two, but lessor, power interruptions to the displays upon return of a generator. One can imagine that a succession of such interruptions could cause blanking of the displays.

Two other 747 aircraft have been known to inadvertently fly into a volcanic ash cloud. Like in the KLM 747-400 incident, all four engines were lost and subsequently restarted. In each of these cases, standby power operated as expected.

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