Plants & People

Mosquito manufacture mystery: Maths and materials

Why citing sources matters.
de Havilland DH-98 Mosquito (Expired Crown copyright)

I write about plants. They’re what interests me – that and their relationships to people. Somewhat unexpectedly therefore, I must make a foray into the subject of wartime aircraft design because of an issue I noted in Ash, a book by Edward Parker that I recently appraised. In that book, Parker stated that “several types of wood, primarily English ash (Fraxinus excelsior), Alaskan spruce and Canadian fir” (p. 151) were used in the construction of the World War II ‘fighter-bomber’ the De Havilland Mosquito.

Doubts emerge

On the face of it, this was a nice bit of information to have in a book devoted to ash and which extended that tree’s ancient association with weapons of warfare into the 20th century. It’s also a ‘fact’ that would be easy just to accept. However, having previously read Anna Lewington’s Birch, I recalled that she specifically stated that Canadian birch was used in the plane’s manufacture – along with Ecuadorean balsa (p. 111 in Birch). There is no mention of ash in Lewington’s book (nor reference to Canadian fir or Alaskan spruce…); Parker is silent on the use of balsa, although it may be included amongst the “several types of wood” he doesn’t specify.

Birch or ash..?

So, what wood was used in the construction of this celebrated aircraft known variously as the “Timber Terror, the Loping Lumberyard, the Wooden Wonder”? In their text regarding the Mosquito, both Parker and Lewington cite the same, single source, Stephan Wilkinson’s article on HistoryNet entitled ‘The miraculous mosquito’. Referring to the wooden nature of the plane’s construction, Wilkinson mentions by name “spruce, birch plywood and Ecuadorean balsa”; there is no reference to ash in his article, nor to ‘Canadian fir’. It would therefore appear that Lewington got it right regarding the use of birch (and balsa…), but Parker seems to have got it wrong regarding ash (and in respect of his omission of mention of birch – and we also should ask where he got his information about use of Canadian fir from, but we’ll focus on ash here…). [Although this blog item was inspired by information in Parker’s Ash, as an aside, it would appear that Lewington’s account in Birch was also a little light on the types of wood used in the Mosquito’s manufacture.]

Citing is everything

How could the same source be ‘interpreted’ so differently by Parker and Lewington? Well, in fairness to Parker, his in-text citation of Wilkinson’s article is placed before his statement regarding the construction materials used – which does leave it open to interpretation as to whether that single article is the source of all of his Mosquito manufacture facts.* Which begs the question: What is Parker’s source for this information? Has he inadvertently misinterpreted Wilkinson’s article, mistaking ash for birch? Is Parker’s statement on the employment of ash in the construction of the Mosquito just wishful thinking? Is there an uncited source Parker has used for this ‘fact’? Why have I spent so long on this point? Because it’s important and relates to an issue I’ve come across time-and-again in appraising factual plant-themed books, the necessity to provide sources to support one’s statements, and – as highlighted in Getting the numbers right below – related concerns over the accuracy of that information even when a source is cited.

Birch, and ash (apparently…)

Having identified the ash vs birch discrepancy I did some follow-up research for my loyal readers (but mainly to satisfy my own curiosity…). This is what I unearthed regarding the nature of the wood used in the construction of the Mosquito. Several woods are mentioned in this item: birch plywood, Douglas fir stringers, and Ecuadorian balsa wood. And, “Initial delays were caused by the unavailability of Canadian birchwood, and Australian coachwood had to be substituted”. “The wood consisted of three layers consisting of Ecuadorian balsa wood and two layers of three-ply birch wood … Also fir and spruce were used throughout the aircraft”. “The fuselage was a frameless shell made of balsa wood sandwiched between sheets of birch”. “Materials, Overall: Alaskan spruce, English ash, Canadian birch and fir, and Ecuadorian balsa”. “The Mosquito… was constructed primarily of spruce, birch plywood, and balsa wood”, and, “A variety of wood (Ash, Balsa, Birch, Douglas Fir, Spruce and Walnut) was used in Mosquito construction”. The greatest number of different woods – seven – I found was on Wikipedia: yellow birch, paper birch, Ecuadorean balsa, spruce, ash, walnut, and Douglas fir. Conclusion: There seems to be no consensus regarding the woods used in construction of the Mosquito, although birch, balsa (whether Ecuadorean or Ecuadorian), and fir appear to be mentioned most commonly. In the context of Parker’s book, there are at least three sources that specify use of ash, but none of those have been cited in Ash.

Getting the numbers right

The bulk of this blog item relates to the need to cite sources for statements made, which is important. However, it’s also important to ensure that statements attributed to a stated source are correct. This point is also highlighted in the Mosquito mix-up considered here. Wilkinson [and remember, this is the sole source cited by both Parker and Lewington for their Mosquito facts] states that “Exactly 7,781 Mosquitos were built”. Whilst 7,781 is the figure stated in Birch (although without Lewington explicitly stating its source as Wilkinson), Parker has gone for 7,787, citing Wilkinson’s article as the source.** But, since the Wilkinson source explicitly states 7,781 aircraft were made, how does Parker get his figure of 7,787?*** Has he misread Wilkinson? Has he used another – but undisclosed – source? Or, is what’s published a ‘typo’, and his notes will show 7,781 but which has been inaccurately published as 7,787 [7 and 1 can be confused – see here , and here]..? Regardless of the answers, the rather sad conclusion from this numerical excursion is that you can’t necessarily accept as fact a ‘fact’ even if it’s supported by a source. Which, again, underlines why it’s important to state one’s sources – even it means they can be checked and one’s own shortcomings identified. It’s all about academic rigour (and maybe stricter proof-reading?).

And back to ash again

In Ash‘s Timeline for 1940-50, Parker writes “7,787 De Havilland Mosquito planes were made, crafted primarily [my emphasis] from English ash” (p. 193). Which is internally consistent in terms of stated numbers of units made on p. 151. However, with no mention here of Alaskan spruce or Canadian fir, it significantly alters the wood composition mix of the Mosquito in favour of English ash as the primary component – which is rather at odds with the sources I managed to track down regarding this matter…


Although my particular interest in penning this piece is to promote the need for better-evidence-based plant-writing, this article should be taken to apply to non-fiction writing generally. I will finish – for now, I feel sure this is a topic that I’ll need to return to… – in the form of a plea: Please provide sources for all of the statements of fact you make – and quote them correctly. Doing that can only bolster any claim to intellectual rigour and scholarship by the writer. It rightly credits those whose work was used to provide the facts incorporated in the author’s work. And it is also a respectful service to your readers, especially those who may be critical or sceptical and rightly don’t just accept unsupported statements as fact. Or, putting this into a three word mantra that appears to be all the rage these days: Show your sources! Your reviewer – and critical readers – will thank you for it.

Post-script: I did notice the issue considered above as I was writing my review of Edward Parker’s Ash, but chose not to include it there to avoid making my appraisal overly-long. However, a general need for the author to provide more sources for statements made was included. And, I recognise that – with the benefit of hindsight having now read Ash(!) – my appraisal of Lewington’s Birch needs some amendment…

* For balance, it should be stated that Lewington’s placement of the Wilkinson reference is at the end of her block of text dealing with the Mosquito – which rather ambiguous citation-siting will be interpreted as Wilkinson’s article being the source for all of her Mosquito manufacture information. And, as a piece of advice to those who practise ‘scientific writing’, such a citation placement is poor practice. As is placing a source at the beginning of a paragraph that contains several statements of fact. The best advice is that each statement needs its source to be stated – in the interests of: removing doubt over assigning sources to statements, giving credit to the original author, and avoiding a charge of plagiarism.

** For completeness, and since I have the data to hand, of the sources cited above concerning the types of wood used in the Mosquito’s construction, most use the figure of 7,781 aircraft built, except for the Flying Heritage site which rather unhelpfully states “over 7,700”; the Bomber Command Museum of Canada is silent on the matter of numbers made.

*** Interestingly, but further muddying the already-murky Mosquito mathematics’ waters, I tracked down one source – authored by “mathscinotes” – which gives a Mosquito production figure of 7,778. Somewhat helpfully, that site also states: “As is often the case with WW2 data, different sources have different totals. The source used in this post shows 7778 units while other sources list 7781, a discrepancy of 3 units, which I view as minor”. Numerically minor, but in the context of this blog item quite major. Unfortunately, I didn’t manage to discover mathscinotes’ source for its quoted figure of 7,778. But, is it too much of a coincidence, that 7778 is a transposed version of the 7787 as used by Parker..?

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