B1 vs B2: Comparing the Part-66 Mechanical and Avionics Licences

B1 vs B2 is the first big decision for most people heading towards a Part-66 licence. The two categories share half their exams and most of their training, but lead to different jobs: B1 engineers certify the structure, engines and mechanical systems, and B2 engineers the avionics and electrical systems. This guide compares the privileges, the modules and paper sizes, the experience and the career fit, and explains how to add the other category later.

Part-66 Prep is an independent study aid, not EASA, the UK CAA or a Part-147 organisation. Privileges are set in 66.A.20, modules in Appendix I and paper sizes in Appendix II of Part-66: see the EASA Easy Access Rules for Continuing Airworthiness for the EU and UK Part-66 on legislation.gov.uk for the UK. For every category, including A, B2L and B3, see Part-66 licence categories.

B1 vs B2 licence privileges side by side

	B1	B2
Main scope	Aircraft structure, powerplant, mechanical and electrical systems	Avionic and electrical systems
Avionics work	Replacement of avionic line replaceable units needing only simple tests to prove serviceability	Full avionic work, including fault-finding
Engine and mechanical systems	Full scope	Electrical and avionic tasks within them that need only simple tests
Category A privileges	Included (B1.1 includes A1, B1.3 includes A3)	Not included as a subcategory, but B2 may certify minor scheduled line maintenance and simple defect rectification within its ratings
Base maintenance	May act as B1 support staff	May act as B2 support staff
Subcategories	B1.1 turbine aeroplanes, B1.2 piston aeroplanes, B1.3 turbine helicopters, B1.4 piston helicopters	None: B2 covers all aircraft (B2L is a separate, limited category)

The practical meaning: a B1 engineer can change an engine, rig a flight control or repair structure and release it, and can swap a radio box if a simple test proves it works, but cannot troubleshoot the radio system. A B2 engineer can troubleshoot the radio, autopilot or electrical generation system and release that work, but cannot certify a structural repair or an engine change. On a complex aircraft, the two work side by side.

Key fact: "Simple test" is the dividing line. If proving the work needs fault-finding or more than a simple go/no-go check, it belongs to the B2 for avionics and to the B1 for mechanical systems.

B1 vs B2 modules and paper sizes

Both categories sit Modules 1 to 10, but not all at the same size. This table compares B1.1 (the most common B1 subcategory) with B2, under both systems. Figures are multiple-choice questions per paper.

Module	B1.1 EASA	B1.1 UK	B2 EASA	B2 UK
1 Mathematics	32	32	32	32
2 Physics	52	52	52	52
3 Electrical fundamentals	52	52	52	52
4 Electronic fundamentals	20	20	40	40
5 Digital techniques / EIS	40	40	72	72
6 Materials and hardware	80	72	60	60
7 Maintenance practices	80 + 2 essays	80 + 2 essays	60 + 2 essays	60 + 2 essays
8 Basic aerodynamics	24	20	24	20
9 Human factors	28	20 + 1 essay	28	20 + 1 essay
10 Aviation legislation	44	40 + 1 essay	44	40 + 1 essay
11A Turbine aeroplane	140	140	—	—
13 Aircraft systems (B2)	—	—	188	180
14 Propulsion	—	—	32	24
15 Gas turbine engine	92	92	—	—
17A Propeller	32	32	—	—
Total questions	716	692	684	652

So the totals are close. B1.1 sits 13 papers and B2 sits 12. The differences are in where the weight falls: B2 has nearly twice the electronics in Modules 4 and 5 and one huge aircraft systems paper, while B1.1 has heavier materials and practices papers and two separate aircraft and engine papers. The other B1 subcategories swap the aircraft and engine modules: B1.2 sits 11B, 16 and 17A, and B1.3 sits 12 and 15. The Part-66 B1 licence guide has their sizes.

Same module, different depth

Even where the paper size is the same, the knowledge levels can differ. In Module 6, B1 sits most materials topics at level 2 and corrosion types at level 3, while B2 sits many at level 1. In Module 5, B2 is examined at level 2 on microprocessors, integrated circuits and multiplexing, which B1 does not sit at all. The Module 5 questions page shows the full split.

The aircraft modules

The biggest single difference is the aircraft systems paper:

• B1.1 Module 11A covers turbine aeroplane aerodynamics, structures and systems from a mechanical point of view: 140 questions. See the Module 11 questions page.
• B2 Module 13 covers aircraft aerodynamics, structures and systems from an avionics point of view, including autoflight, communication, navigation, surveillance, instruments and integrated modular avionics: 188 questions under EASA rules, 180 under UK rules. It is the longest basic paper in Part-66. See the Module 13 questions page.

B1.1 then sits engines in depth (Module 15, 92 questions) and propellers (Module 17A), while B2 sits a shorter propulsion paper (Module 14, 32 or 24 questions) focused on engine indicating, starting and ignition.

Exam tip: Under EASA rules both Module 11A and Module 13 may be split into partial exams because they are over 90 minutes; Module 13 at 235 minutes may be split into three. Ask your exam centre how it runs them before you plan your revision.

Which is harder, B1 or B2?

There is no official answer and no published pass-rate data. What can be said:

• B2 is harder if electronics is new to you. Module 4 at 40 questions, Module 5 at 72 and the avionics systems in Module 13 assume you are comfortable with circuits, logic and data.
• B1 is harder if you dislike detail on materials and procedures. Module 6 at 80 questions (EASA) and Module 7 at 80 plus essays are long, identification-heavy papers.
• Module 13 is the single longest paper either category sits, so B2 candidates face one very big exam rather than two large ones.

Try both before you commit. The free B1.1 diagnostic and B2 diagnostic each give 20 questions across the category's modules, and comparing your scores is a quick, honest test of where your background helps.

Experience: B1 vs B2

The basic experience requirements in 66.A.30 are identical for B1.1, B1.3 and B2. B1.2 and B1.4 are shorter.

Background	B1.1, B1.3, B2	B1.2, B1.4
No relevant technical training	5 years	3 years
Skilled worker in a relevant technical trade	3 years	2 years
Part-147 approved basic course	2 years	1 year

At least one year must be recent experience on the category you are applying for, and the experience must be practical and representative of that category: a B2 applicant needs avionics and electrical work in their logbook, not just mechanical tasks. The UK CAA currently recognises no course as qualifying for the skilled-worker reduction. The experience requirements guide explains what counts.

Getting both: adding B2 to B1 or B1 to B2

Plenty of engineers hold B1 and B2 together, which makes them very flexible on the line. There is no conversion course that turns one into the other; you add a category in the same way you got the first.

1. Pass the missing modules. From B1.1 to B2: Modules 13 and 14, plus the B2-size Modules 4 and 5. From B2 to B1.1: Modules 11A, 15 and 17A, plus the B1-size Modules 6 and 7.
2. Check credits for the shared modules. Whether an existing pass at one category's size counts for the other depends on your authority and on syllabus differences, so ask before you book anything. Modules 1, 2, 3, 8, 9 and 10 are the same size and level for both, which helps.
3. Gain the experience. You need experience relevant to the new category. For an addition to an existing licence, 66.A.30(d) allows the recent experience to be less than one year, but at least three months.
4. Apply to your authority for the extension. As of October 2026 the UK CAA charges £455 per added category (CAA scheme of charges); see the CAA engineer licensing pages.

Who releases what: worked examples

The split is easier to see with real defects. These examples assume a complex turbine aeroplane in line maintenance and engineers who hold the relevant type rating and authorisation.

Defect	Who typically certifies	Why
Hydraulic leak at a brake unit, unit replaced	B1	Mechanical system; the leak check is part of the mechanical task
Main wheel change after tyre wear	B1 (or a category A mechanic with the task on their authorisation, for their own work)	Minor scheduled task
Weather radar shows no returns; fault traced to the transceiver and wiring	B2	Avionic fault-finding
A display unit swapped after the crew report a blank screen, and a simple built-in test passes	B1 or B2	LRU replacement proved by a simple test is within B1 privileges
Generator fails to come online; fault traced to a generator control unit	B1 or B2	Both categories cover electrical systems, which is why the shared Module 3 is the same size for both
Autopilot disconnects in flight; fault traced to a flight control computer input	B2	Avionic system fault-finding; a B1 could only replace a unit if a simple test proves the fix

In practice, organisations write their own procedures for which certifier signs each part of a job, and some tasks fall clearly on one side while others need both signatures.

Common misunderstandings

• "B2 engineers never touch engines." They do: B2 covers electrical and avionic tasks within powerplant and mechanical systems that need only simple tests, and the B2 syllabus includes a propulsion module.
• "B1 engineers cannot work on electrics." B1 covers electrical systems; the limit is on avionic troubleshooting.
• "You have to pick one forever." You can add the other category later, as explained above.

Career fit: how to choose

Some questions to ask yourself:

• What do you enjoy? Hands-on mechanical work, structures and engines point to B1. Wiring diagrams, test sets and fault-finding point to B2.
• What does your employer need? Line stations and hangars need both, but the mix varies. Ask the maintenance organisations you want to work for what they are short of.
• Which aircraft? For helicopters and piston aeroplanes, B1.3, B1.2 or B1.4 are the common routes. B2 covers all aircraft, which suits engineers who move between types.
• How do you want to progress? Both lead to type ratings on complex aircraft and to category C (base maintenance release), which can be reached from either B1 or B2.

On pay, there are no official figures. It varies by employer, location, aircraft type, shift pattern, overtime and the ratings you hold, so compare current job adverts for both roles rather than relying on averages quoted online. For the full B1 route, read how to become a B1 engineer; for avionics, see the B2 licence guide.

Your study plan

1. Take the free B1.1 diagnostic and the free B2 diagnostic and compare your scores.
2. Read the licence categories study note for the privileges in full.
3. Start with the shared modules, 1, 2, 3, 8, 9 and 10, which count towards either category; Module 1 quick practice is a good first step.
4. Before committing to B2, try the free B2 Module 5 paper 1; before committing to B1, try B1.1 Module 6 paper 1.
5. Talk to employers about which category they need, then plan your modules and experience around it.