My understanding... & using a B767. And of course re-emphasising the above post to read the introductory notes to your aircraft / Airline's DDG.
The DDG, is a "Dispatch Deviations Guide", and covers ALL issues with aircraft airworthiness, ie Minimum Equipments required, and deviations from normal configuration. In other words, the DDG is the overall document, and within that there are two 'sub' documents, specifically the MEL and CDL.
The MEL is used for EQUIPMENT failures (eg Pack inop, FMC, Rad Alt etc) and specifies the Minimum Equipments you can depart with.
The CDL is applicable to physical parts of the airframe - service panels, flap seals, static wicks etc - and will inform you how far you can 'deviate' from a complete airframe in order to depart. (forgive the over simplification.)
IF you cannot find any reference to your 'missing part' - eg Left Wing Detached - then that deviation is not allowed & dispatch not possible until rectification complete.
Ok - so when to use which, bearing in mind you ALWAYS refer to the 'DDG' (seeing as it is made up of both the MEL & CDL)
(1) On the walk-around you spot a pylon access panel missing. You open up the DDG, and go to the CDL within that & locate the info respective to said missing panel. Be sure to read all the introductory notes to the CDL, as >1 issue may be cumulative, ie have an ever-degrading effect on aircraft performance. (or eg a max of 3 related CDLs may only be allowed)
(2) An equipment (MEL) issue is not so straight forward, and divided into whether you're at:
- gate
- start (or moving under your own power)
- T/O thrust set
- less than 80 kts
- less than V1
- ie in line with the various EICAS Messages.
In essence:
(a) On the gate you note a STATUS MSG. These ARE relevant at this point in your 'flight', and WILL need reference to the MEL to ascertain whether you may depart or not.
After Engine start (or moving under your own power (dependant upon which definition your MEL uses) STATUS messages no longer require reference to the MEL and you may simply continue.
(b) Whilst taxing out you get an ADVISORY EICAS eg "L Pack Inop" - ie a higher level message than Status. Stop, and first carry out the appropriate (QRH) drill, and thence you MUST refer to the MEL. If it says "No Dispatch" - return to gate. If it allows dispatch but with Operational (O) considerations then you, as a pilot must apply those, eg limit your subsequent flight to FL350 and continue the taxi & take-off. (or return to gate for eg more fuel if that if the Flight level too limiting). If it allows dispatch but with Maintenance (M) considerations then you MUST return to stand for applicable engineering works to be performed (eg inhibiting valves.... whatever). No options here (M), as only a qualified engineer may do this work and subsequently sign off the aircraft as fit for flight.
(c) After T/O thrust is set you get ADVISORY (eg) PACK INOP. Do nothing, continue the take off, and only at a safe time, clear of terrain, high workload etc action the QRH. No need to consult the MEL as once T/O thrust was set you had 'dispatched' in terms of ADVISORY messages. Airmanship may suggest you 'educate' yourself by reference to the MEL when time is spare - but you certainly don't have to.
(d) Anytime below 80kts you get an EICAS CAUTION (eg FWD CARGO DOOR). Stop the aircraft (including reject the T/O) and then apply the appropriate QRH. With that out the way you then MUST open up the MEL to confirm whether the flight can continue or whether a return to stand is required. (it'll be obvious!)
(e) Anytime below V1, you get a WARNING EICAS. Stop the aircraft (inc reject if on the T/O roll), apply MEMORY / Reference items, and thence you MUST open up the MEL to determine whether dispatch is subsequently allowed. (it'll be obvious!)
Summarising...between..
Gate & Engine start / Taxy - all EICAS Status & above need MEL reference
Eng start / Taxy & Takeoff thrust set - all EICAS Advisory & above need MEL ref.
Takeoff thrust set & 80 kts - all EICAS Caution & above need MEL reference.
Above 80kts & <V1 - only EICAS Warning need MEL reference
>v1 the aircraft has dispatched in all events & nil reference to the MEL required.
So... a bit of a minefield? No, .... not if you apply your knowledge of EICAS message philosophy versus Boeings advices on continuing / rejecting flight. Hope this helps.
Last edited by Static in the Attic; 11th Jun 2012 at 14:09.