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Is there a procedure for substituting a discontinued part which grounds the aircraft?

The TB21 electric fuel pump, made by Weldon, used on the old (12V) aircraft is finished.

The TB20/21 tachometer, made by NORIS in Germany, likewise.

What are the options?

For the tacho, there are some instruments “certified as primary” (I believe EI do some) so maybe that is one way.

For the fuel pump, you are grounded unless you can get one from a crashed or parted-out plane.

Is there any concession for dealing with these situations?

It’s a bit like this thread

Obviously transferring to N-reg is not an option when you get this because you cannot transfer an unairworthy aircraft to N, can you?

Would substituting another similar fuel pump be a Minor alteration, on the (FAA) grounds that it is not a “basic change to the fuel system”?

Shoreham EGKA, United Kingdom

If it’s a change to the parts shown in the IPC, then it’s a modification. If we take FAA, then for it to be a Minor alteration (log book entry) you need acceptable data. If it’s a Major alteration (337), you need approved data. If there is a PMA replacement and the aircraft is in the PMA list, then it’s straightforward – just install and sign-off.

Just for clarity. An FAA Major alteration is effectively the same as an EASA Minor change. An FAA and EASA Major Change are the same and both require an STC.

Avionics geek.
Fairoaks. EGTF

That’s wrong, surely…

Shoreham EGKA, United Kingdom

FAA Part43 App A does refers to Major Alterations and Major Repairs…..AFAIK Major Change is not an FAA term…

Correction: Part21D does in fact define minor and major changes….

21.93 Classification of changes in type design.
(a) In addition to changes in type design specified in paragraph (b) of this section, changes in type design are classified as minor and major. A “minor change” is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are “major changes” (except as provided in paragraph (b) of this section).

Last Edited by AnthonyQ at 02 Mar 18:24
EGPD / OMDW / YPJT, United Kingdom

Wigglyamp’s 2nd para is one big typo. Especially the bit about the STC… there are multiple routes to a Major Alteration e.g. field approval, DER 8110, STC… I am sure he didn’t mean to write that.

EASA stuff I am not sure about. AIUI they don’t have a viable Minor route (meaning: there is no straightforward way for an engineer to decide a mod is Minor and back the decision up by reference to a readable regulation) so while an EASA 21 company has some discretion to make the decision, most situations need EASA approval, so in effect a Minor mod is still a load of hassle but costs a lot less money.

Shoreham EGKA, United Kingdom

A ‘major alteration’ and a ‘Major change to type design’ are not the same thing.

A major alteration is one that does not alter the type certificate but still meets the requirements for significant effects to weight, balance etc.

If the aircraft manufacturer wants to make a significant change to the TC, this would be a Major Change to Type Design resulting in a revised issue of the TC. If the same was done by a third party (such as a replacement type of engine), the same Major Change to Type Design would be certified under an STC.

Avionics geek.
Fairoaks. EGTF

OK… in that case, how would one approach a different model of an electric fuel pump?

IMHO, FAR 43 Appendix A, this is a Minor alteration (not a “basic change to…”) unless there is some other reg which explicitly states otherwise.

I don’t believe the fuel pump is an engine component, otherwise every TIO-540-XX would be grounded. I believe it is an airframe component.

The above is for N-reg of course.

I also don’t see how the fuel pump would invalidate the TC, because it isn’t listed on the TCDS. The engine is listed…

Shoreham EGKA, United Kingdom

Well Peter, as you are aware, my area of design certification is specifically avionics. However, the principles are the same.
I would agree that changing an airframe electric fuel pump will be either a Major or Minor alteration, so where to start?
First, do you have a possible replacement that is form/fit and function identical to the faulty unit? If not and you need to make structural or pipe work changes, then you get into the potential need for approved data. This may immediately make it a Major Alteration if you need an DER and an 8110-3 to address the changed airframe interface.
Does the replacement pump meet the same safety requirements as the old unit – bypass conditions in the event of an internal failure etc.
Conduct a safety assessment as required to meet FAR23-1309 using the appropriate guidance:
Finally, take advice from your IA or an appropriately qualified systems DER.

Avionics geek.
Fairoaks. EGTF

I think my original Q was a general one: whether there are any special concessions when a significant number of airframes risk being grounded. I guess the answer is No, but more interesting may be why these things don’t become the big disasters which they might be expected to be. Perhaps a way forward is found.

On the fuel pump, why is a DER necessary if the new pump is not an exact fit (and assume it is a suitable pump otherwise e.g. is a standard aviation model)?

AIUI, normally a DER is needed (non pressurised context here) when the job cannot be supported by the stuff in AC43-13. That is how work seems to be done in the USA.

Where is the reference for the identical form/fit/function aspect?

Shoreham EGKA, United Kingdom

I wasn’t suggesting you HAD to use a DER, only that it MIGHT be necessary if you don’t have acceptable data to perform a Minor Alteration and hence need to produce approved data (with an 8110-3) for a Major alteration. Of course you can actually perform a Minor alteration using acceptable OR approved data. Since it it down to the certifying A&P or repair station to determine if the data is ‘acceptable’, it’s something of a grey area – just Google it and see the many articles on the subject!

The form/fit/function discussion isn’t from regulations but has been used in training seminars I’ve attended (presented by the AEA) as one part of assessing whether an alteration can be classified as Major or Minor. It immediately dispenses with the need for any physical change to the aircraft which could then possibly be construed as outside the scope of using acceptable data. If the part is form/fit/function, and where necessary meets the same TSO approval, then it’s pretty much a no-brainier decision to be a Minor.
So in the case of the fictitious fuel pump, if you could find one from another aircraft type with the same engine (hence the same fuel delivery requirements) and it would directly fit in place of the defective one, then again you’re half way there. You just need some acceptable data (perhaps spec sheets for both pumps, plus also confirming any fail-safe modes) to justify the decision.

Last Edited by wigglyamp at 02 Mar 23:44
Avionics geek.
Fairoaks. EGTF
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