On 3 November 1973 a Panam B707 freighter crashed near Boston USA following heavy smoke in the cockpit which the crew interpreted at the time as due to an electrical fire.
See :
ASN Aircraft accident Boeing 707-321C N458PA Boston-Logan International Airport, MA (BOS)
In those days the procedures for electrical fire or smoke required the crew to turn off various electrical systems in order to isolate the site of the fire or smoke. These procedures were essentially a trouble-shooting checklist and were often lengthy as a certain amount of time was called for between each selection to see if smoke eventually ceased or not.
In the case of the Panam 707, the flight engineer commenced trouble shooting action at cruise altitude. While he was doing this the captain maintained cruise. After some time and as the engineer continued to read through the checklist item by item trouble shooting, the smoke got worse until it was so dense the captain was forced to make an emergency descent from cruise altitude to get the aircraft on the ground as quickly as possible. The smoke was so thick that the first officer was unable to read radio frequencies. This all took time which in the event was fatal.
Control of the aircraft was lost at very low altitude, when the engineer continuing with his QRH trouble-shooting checklist switched off the battery switch causing the yaw dampers to be inoperative. Smoke was seen pouring from the copilot's window at the same time control was lost. The combination of spoilers, landing flap and no yaw damper made the aircraft uncontrollable. Investigation revealed it was not an electrical problem at all. Instead it was found that bottles of acid in the front cargo hold had broken releasing their contents on to wood chips surrounding the bottles to protect them from damage. The airflow carried the smoke from burning wood chips through cooling tubes to the circuit breaker panel on the flight deck. This led the crew to diagnose an electrical fire somewhere.
Boeing later changed the QRH for electrical fire or smoke and deleted reference to trouble shooting since smoke may not always have electrical origin. The checklist was reduced to bare essentials with the advice to land at the nearest suitable airport if the problem was not solved. The B737-300 QRH dated January 1999 for example had this checklist under the heading "Electrical Smoke of Fire"
Oxy Masks and Regulators (if required) ON, 100%
Smoke Goggles (if required) ON
If smoke source can be determined: Electrical Power....REMOVE.
If smoke source cannot be determined:
Bus Transfer...OFF
Galley Power....OFF
Land at nearest suitable airport.
............................................................ ........
Despite this revision to the QRH based upon the experience of the B707 accident and it's trouble shooting checklist, at least one European B737 operator elected to keep the trouble-shooting checklist for electrical fire or smoke. The reason given was the airline had over-water routes and the nearest suitable airport could be some distance away. The operator missed the whole point that smoke could quickly incapacitate the crew and that in the worst case an immediate ditching may be required before the crew succumbed to smoke fumes.
During the aftermath of the Swiss Air MD11 accident near Halifax where smoke and fire in the cockpit (electrical origin) caused the crew to become incapacitated, there was speculation that needless delays occurred as the crew went through a lengthy trouble shooting checklist and then circled to jettison fuel.
Since those days, more research by Boeing (737) resulted in further QRH changes and the words "electrical fire" were deleted. Possibly because it may be difficult to differentiate between electrical smoke and smoke from other causes, the Boeing 737 Non Normal checklist Section 6 Electrical, says nothing about Electrical smoke or fire but instead refers the reader to Section 8.8 under the heading of "Smoke, Fire or Fumes"
The reader is now confronted with no less than four pages of checklist items designed to trouble shoot the source of the problem and eventually directs the reader to a further page in Section 8 dealing with Smoke or Fumes Removal.
Yet only on page 3 of the "Smoke, Fire or Fumes" checklist do we see the first reference to consider an immediate landing if the smoke, fire or fumes situation becomes uncontrollable.
My point is this: After the Panam 707 accident referred to earlier, Boeing saw fit in the QRH to remove all reference to trouble shooting to isolate electrical system items in event of an electrical smoke or fire. In other words - forget trouble shooting. Turn off essentials and if that doesn't work, get the aircraft on the ground asap. Reason? Danger of incapacitation due smoke.
Later on, Boeing had another think and removed all reference in QRH Section 6 Electrical, to electrical smoke or fire. Instead transferred the scenario to Section 8 of the QRH under the heading Fire Protection - under the catch-all item "Smoke, Fire or Fumes". To adequately encompass that lot requires several QRH pages of trouble shooting.
Of course, there is always the possibility a crew can incorrectly diagnose the cause or origin of smoke. But it seemed to me when Boeing revised the old "Electrical Smoke or Fire" QRH item, and not only reduced it to bare essentials but left it under its rightful place in Section 8 Electrical, it was a good decision. The theory being if the revised shortened checklist did not fix the problem immediately, it was essential to land immediately. Incapacitation was recognised as the real danger.
Now we have gone the full circle and the lessons of the Panam 707 and Swiss Air MD11 accidents lost in time. Trouble shooting is back in vogue. Why the change? Is incapacitation now considered unlikely?
Comments welcomed.