The Often Fatal Consequences Of Improperly Handled Aircraft Switches
In aviation, pilots are faced with a myriad of switches. If in the wrong position, these switches can kill. Let’s examine a few recent and notable examples, and the important safety lessons they convey.
In last week’s issue, ASW reported a leading Brazilian aviation journalist’s theory, based on input known only to her, that an inadvertent switch actuation may have caused the Sept. 29, 2006 GOL 737/Legacy collision over the Amazon.
Eliane Cantanhede, a leading journalist from the newspaper Folha, got access to 290 pages of transcripts of the cockpit and ATC conversations. She’d deduced, from the pilots’ references to consulting a DVD, that the Legacy crew had placed their electronic flight bag (EFB) laptop on the center console for viewing by both pilots attempting to resolve an ongoing communications outage.
In her scenario, the opening lid of the EFB had contacted the pushbutton on the right of the Primus display that puts the transponder to standby, and established that the screen-lid was at about the right height to do so. The laptop’s screen would have then also hid the status light. TCAS and transponder would have then been disabled by that one inadvertent actuation, compounding ATC’s Flight Level allocation error and leading directly to the deadly collision.
Even worse, the indications on the panels after an accidental transponder (and TCAS) switch-off are not very prominent and can’t be clearly seen by the pilots. Consider the following examples:
- TCAS OFF is written in small white letters on the Primary Flight Display (PFD) — and not in red warning colors.
- A similar white message is put on the Navigation display (in very small lettering).
- The TCAS needle, displaying vertical escape movements to the crew when a TCAS Resolution Advisory is triggered, remains green despite the fact that the equipment is turned off and not functioning (in aviation green means on and functioning).
- An amber “TCAS fail” light is only activated when there is a technical malfunction of the transponder.
A saner option would be to have the Master Caution system trip whenever the transponder goes to Standby in flight. Loose articles in cockpits? Never a good idea, because of turbulence and inadvertent actuation of both switches and CBs. But see the April 2, 2007 ASW coverage of the FAA’s endorsement of class 2 EFBs for “own position” disclosure via GPS while taxiing. These EFBs are now to become a de rigeur floating flight safety tool in cockpits.
Another stark example of the potent demonology of switchology is the Flash airlines accident at Sharm-el-Sheikh. That 737’s autopilot had a pushbutton ON selection rather than the electronically “held” toggle switch typical of earlier designs.
The complexity there is that engagement of the autopilot could be unnoticeably rejected if the push-button attempt was made while rolling into a turn at a significant yoke aileron deflection or with a significant pressure being input to the control surfaces. Any pilot used to regularly getting his autopilot selection in an established coordinated turn might not notice that it hadn’t “taken”.
The pilot could become distracted enough by other tasks to allow an unusual (and irrecoverable) attitude to develop sight unseen. With a pushbutton autopilot selection, its non-engagement isn’t evident. However, DISengagement of an engaged autopilot is. That will set off clackers and visual alerts.
Then there’s the Helios 737 accident in Greece. That airplane flew all the way on autopilot after its human pilots succumbed to hypoxia on climb-out. The Outflow Valve’s switch had been left in MAN (manual) instead of AUTO by mechanics hurriedly completing a ground pressurization check. Most switches have a non attention-getting green advisory caption associated with them if they are selected to a position.
If a system function doesn’t accord to a switch position (i.e., system malfunctions), that’s a different matter. You can then expect an amber CAUTION light at the very least (or an alerting caption flashing on a PFD/MFD display).
In a May 2000 accident, a Super KingAir 200 (VH-SKC) flew almost 2000nms from Perth Western Australia with a dead crew and passengers before crashing in Queensland. Bright sunlight upon a non-flashing pressurization caption had probably prevented the pilot seeing it on climb. However, in other similar cases, a night flying light’s rheostat had been left cracked, dimming all cockpit captions into relative invisibility for day-time viewing.
Despite being a repeat of an incident on Dec. 12, 1999, involving VH-OYA, CASA (the Australian regulator) opted not to make an aural warning of non-pressurization a mandatory retrofit item.
To cite their reasoning: “CASA has been unable to find any support for the proposal to mandate the fitment of aural warnings amongst foreign regulators responsible for certification of affected aircraft, nor from the manufacturers of these aircraft. However, it is noted that some manufacturers are fitting aural warning devices in newer aircraft types such as the Raytheon King Air B300.” [tinyurl.com/yqcgp4 ].
The upshot: miss a switch and fail to see a dimly lit caption, and it’s a potential death sentence for all onboard!
But what about a flight-crew that can check switches that are off and yet repeatedly see them as being ON? Believable? You’d better believe it. It’s all about viewing angle and insufficient switch “throw”. See image at tinyurl.com/26ch3c
VH-QOD, a Bombardier DHC-8-402 was climbing out of Brisbane Queensland on June 29, 2006 when, passing through FL220, the cabin altitude warning light illuminated, accompanied by the associated aural warning. Visual checks by the crew indicated that the cabin differential pressure and cabin air flow appeared to be normal.
The aircraft’s bleed air switches also appeared to the crew to be correctly selected to the ON position. Luckily, the pilot was a “hands on” type of person so he manually cycled the switches and found that the bleed air switches had been unnoticeably on OFF.
His selection of the switches to the ON position extinguished the cabin altitude warning light and the associated indications, and the aircraft’s pressurisation system commenced normal operation.
The operator was concerned sufficiently by the deceptive “visual only” switch position check that it amended the aircraft checklist to include:
a. Revised responses to the pressurisation-related checklist items;
b. An additional Pressurisation checklist requirement to be conducted at Transition Altitude; and
c. The addition of a requirement for a tactile confirmation of some checklist responses.
The potential for a Helios repeat? Quite probably. Time of useful consciousness was 3 to 5 minutes and the pilots didn’t don masks.
How had the bleed switches come to be OFF? On the flight immediately prior to the Mackay flight, the copilot, transitioning to the Q400, had selected the aircraft’s bleed flow rotary switch to the Minimum (MIN) position, and the engine bleed air switches to OFF for takeoff.
That was the procedure in the earlier models of the Dash-8. During the pre take-off checks for that prior flight, the captain had spotted the mis-selection, confirmed that the bleed flow rotary switch was selected to MIN, and re-set the bleed air switches to ON for the takeoff as required for the Dash 8-400. During the taxi for takeoff to Mackay on the incident flight, the bleed air switches were again incorrectly selected to OFF by the First Officer.
This time, the incorrect switch position remained unnoticed by the captain. As the aircraft climbed through 10,000 ft above mean sea level (AMSL), the cabin pressure differential was in the normally seen range of about 4 to 5 psi.
This apparently normal cabin pressure differential at the time of the cabin altitude alert was a combination of the ram effect due to the aircraft’s high speed, and the integrity of the brand-new aircraft’s tight pressure hull. However, the captain expressed surprise that the flight attendants had not commented on the quality of the cabin environment or temperature.
The Q400’s rotary flow and bleed air switches are located on the right overhead panel. There is only 3 mm switch movement between OFF and ON selections.
The bleed air switches don’t have associated lighting for night operations, instead relying on residual ambient lighting from the electronic flight instrumentation system screens to illuminate the switches and their position. At night, it is difficult to visually confirm the position of the bleed air switches from either seat, but particularly from the captain’s.
The illumination of the cabin altitude warning light presented the captain with seemingly contradictory information. The cabin pressure differential appeared to be in the right range and a visual check of the bleed air switches led the captain to believe that they were correctly selected ON. It was only fortunate that he then decided to cycle the bleed air switches, thus rectifying their mis-selection.
A more serious incident was avoided by mere whim. However, the failure of the pilots to immediately don oxy-masks as a first priority response is just another indicator of how a mis-positioned switch can crank up a lethal scenario on the other side of those locked cockpit doors.