There could be a reason for the difference; read #1219. I know that there is a set of Spitfire II notes on Scribd; these show that there have been no amendments and it is possible that the operational limits for the Merlin XII were originally set at +9 lbs boost, later pushed to +12 1/2 lbs. (Paragraph 4 of the attached document does state that the Merlin XII was cleared for this boost.) It is possible that on operational units an amendment slip specifying +12 1/2 lbs was issued with the notes.

There is a small possibility of printer error, for example on p3 it describes the Spitfire II as being "powered by a Merlin III..."

Correction: This particular set of notes does incorporate some amendments, on top of page 6 and 19 "Amended by A.L.No.6" - interesting; this indicates they could have been republished later than July 1940.

Yes that's my conclusion from the available data.

Definitely it was physically available and the amended pages confirm it. However we don't know since when the pilots where authorized to use the emergency boost with a Merlin XII. The pages from the Pilot's Notes Spitfire II from the 1940 time-frame that I've seen so far do not authorize the use.

I would be glad if someone could provide a proof since when +12 boost was authorized for emergency conditions and not only take-off.
In addition we don't know what happened if the pilot used the +12 take-off boost for example during combat (engine damage, no boost increase, nothing ... we don't know).

Compare it with the Hurricane II with Merlin XX which was introduces at about the same time (August-September 1490) and had +12 take-off boost authorized since introduction and emergency boost was approved shortly later in November 1940.

Now that's a interesting find. The page without amendments only give +12 boost for take-off, but the 1939 documents states +12.5 boost for take-off.

However it's still clear from the June 1940 "List of content" that the page that contains the +12 emergency boost was not contained at that date and was added later and at that date only the page without +12 emergency boost was contained.

IMHO the easiest would be to get the combat reports of the "units concerned" (pun intended ;) ) to find one that proofs the use of +12 in a Spitfire II or search in the National Archives for a similar doc like the one that clears the use of +12 emergency boost for Merlin XX.

On Spitfire Is the pilot did have to push the throttle lever through a thin wire, which was replaced by the ground crew. If for some reason the pilot failed to notify the ground crew that +12 Lbs boost had been used - a note had to be made in the engine's log book - the broken wire was probably a good way of showing the engine had been subjected to overboost.

With regard to the pilot notes, RAF Air Publication (AP) amendments are well known for being behind the times as far as up-to-date common practise at the front line is concerned. It may be many months and over a year or so before an AP is fully updated to reflect what is actually taking place on the ground and in the air.

A fighting force does not wait for the AP to be updated before taking actions that are operationally required to provide an advantage in combat (AP amendments are not a priority when fighting a war). In the mean time, RAF personnel may be informed and corporate knowledge developed by other means of authorised advanced information contained in a variety of communication methods, such as signals, memos, letters, advanced information leaflets (destroyed once formal amendment leaflet is incorporated into the AP), briefings and local training.

The date an Amendment Leaflet is issued is not an indication of when the subject practise was first authorised or carried out. Moreover, the vast number of aircraft AP copies in existence would not have all been amended with updated changes on the same date; different copies of the same AP held across the RAF at squadron and flight level will have different dates recorded on the amendment leaflet record for the incorporation of the amendment. Also, it is not unusual for amendment leaflets to go missing in transit and for a unit to receive an amendment leaflet out of sequence, or for the AP to have a number of missing amendment leaflets; no system is perfect.

As for the pilot in the air in a life and death situation over his own territory, given a situation where extra boost is available, I do not believe that he is going to wait for authorisation to use extra boost if it enables him to win a fight, survive another day or save the life of another pilot or people on the ground. After all, pilots were known to ram enemy aircraft and such authorisation will not be found in AP pilot notes (tongue in cheek, LOL).

Lane has posted the proof that a Spitfire II used emergency boost on 21 August 1940:
http://forum.1cpublishing.eu/showpos...postcount=1441

That emergency boost is +9 lbs I believe. Its pity the documents do not specify.

And thats just it, thats just your 'belief'.

You know how the mechanism works though, don't you.....

Its a tad bite more than that. The BoB era Spitfire II manual clearly states that the maximum boost allowed in flight is +9 lbs.

Just like the Spitfire I manual states +6.25 as normal maximum boost (better known as "All out" boost by RAF terminology).

The "cut-out control ... allows the normal maximum boost to be exeeded for special purposes ("emergencies")."

http://forum.1cpublishing.eu/attachm...1&d=1335714873

Pilot's Notes General, 1st Edition June 1941.