Aerospace and defense: how the sub-sector actually works

Two different businesses sitting under one coverage banner

The single most common mistake candidates make in an aerospace and defense conversation is treating A&D as one thing. It is two businesses that happen to share a coverage team, a supplier base, and a lot of the same machine shops.

Commercial aerospace is driven by airline fleet decisions. Airlines order aircraft based on where they want capacity in five to ten years, how old their existing fleet is, what the fuel burn differential looks like against a new-generation aircraft, and whether their lessors and financing partners will fund the order. Passenger traffic growth matters because it eventually shows up in fleet plans, and retirements matter because an aging fleet has to be replaced whether or not the airline is having a good year. What actually converts into revenue for an original equipment manufacturer is the announced production rate on a given aircraft program, because that is what pulls parts through the supply chain.

Defense is driven by government budgets and procurement priorities. A defense prime's revenue is a function of which programs get authorized and funded, at what quantity, and on what schedule. Demand does not respond to consumer confidence, credit conditions, or industrial production. It responds to appropriations and to shifting procurement priorities between platform types.

Same coverage team, genuinely different demand drivers, genuinely different risk profiles. An interviewer who hears you describe "A&D demand" as a single variable will assume you have not looked closely. If you are still orienting yourself on where A&D sits relative to the rest of the group, the industrials sub-sectors map lays out the coverage landscape.

The value chain, and why position determines everything

Commercial aerospace has a clear hierarchy, and where a company sits in it determines its margin, its negotiating power, and how it gets valued.

At the top are the airframe OEMs, which design the aircraft, integrate everything, and own the customer relationship with airlines and lessors. Below them sit tier-one structural and systems suppliers, which deliver major assemblies (wings, fuselage sections, landing gear, avionics suites, interiors) and increasingly carry design responsibility and development risk alongside the OEM. Engine manufacturers occupy a category of their own, because a jet engine is effectively a program within a program, sold and supported on its own economics.

Below tier one sit components and sub-systems suppliers, then the fastener, forging, casting, and machined-parts businesses that make the physical hardware. Running parallel to all of it is MRO, maintenance, repair and overhaul, the business of servicing aircraft and engines over a service life that can run decades.

The rule of thumb worth having in your head: negotiating power comes from being hard to replace on a qualified program. Parts on a certified aircraft go through a qualification process, and swapping a supplier mid-program means re-qualification, cost, and schedule risk. A supplier holding a sole-source position on a flying program has real pricing leverage even if it is small. A supplier making a commoditized part with three qualified alternates has almost none, and gets squeezed every time the OEM runs a cost-reduction program. This is why two companies with identical revenue can trade at very different multiples.

The core concept: original equipment is the loss leader, aftermarket is the business

If you learn one thing about A&D before an interview, learn this.

An engine or an airframe is frequently sold into a new program at a thin margin, and on engines specifically, the original equipment unit can be sold at a loss. The manufacturer is not being irrational. It is buying a position on a platform that will fly for decades, and every one of those units becomes a permanent annuity: spare parts, overhauls, service agreements, and upgrades, sold at far higher margin, for as long as the aircraft stays in service. This is the razor-and-blade model, and in aerospace it operates on a timeline no consumer business can match.

That structure has three consequences you should be able to state without hesitation.

First, original equipment versus aftermarket mix is the most important disclosure line in the entire sub-sector. Two companies can report identical revenue and identical growth and be fundamentally different businesses if one is 70 percent original equipment and the other is 70 percent aftermarket. Any A&D model you build should split revenue on this line before anything else.

Second, aftermarket revenue is stickier and structurally higher margin. It depends on the installed base flying, not on new orders. When production rates fall, original equipment revenue falls with them, but installed aircraft keep flying, keep needing overhauls, and keep pulling spare parts. Aftermarket does eventually respond to how intensively the fleet is being used, but it decouples from the new-build cycle in a way that materially dampens earnings volatility.

Third, and this is what interviewers actually reward, aftermarket mix earns a premium multiple. Higher margin, more recurring, less cyclical, and backed by an installed base that is already committed. A hypothetical illustrates it: two suppliers each doing 500 million of revenue, one at 15 percent EBITDA margin on mostly original equipment work, the other at 28 percent on a heavy aftermarket mix. The second is not just more profitable, it is a better business, and a buyer will pay a higher multiple on the higher EBITDA number. That compounding effect on enterprise value is exactly why aftermarket-weighted assets get fought over in processes. The mechanics of how mix flows into a multiple are covered in how industrials companies are valued.

The trap is assuming aftermarket is automatic. It is not. On many parts, independent providers and licensed alternatives compete for the service and spares business, airlines can choose where to send an overhaul, and used serviceable material can substitute for new spares. Aftermarket capture is a competitive outcome, not a guaranteed one, and the strongest answer acknowledges that.

Program accounting, learning curves, and long-cycle economics

A&D runs on programs, not quarters. A commercial aircraft program involves years of development spend before a single delivery generates revenue, and a defense program can run development, low-rate initial production, and full-rate production across a decade or more.

Two long-cycle features matter for modeling. The first is the learning curve: unit cost falls as cumulative production volume rises, because labor hours per unit come down, tooling gets refined, and the supply chain matures. Early units on a new program are often produced at negative gross margin, and profitability arrives only once the program moves down the curve. If you model a new program at steady-state margin from unit one, you have misunderstood the business.

The second is that development spending is committed long before revenue arrives, which makes near-term free cash flow a poor read on the health of a company in the middle of a development-heavy phase. Free cash flow conversion is still a fair thing for an interviewer to ask about, but the honest answer includes where the company sits in its program cycle.

This is also why backlog carries more analytical weight in A&D than almost anywhere else in industrials. A multi-year order book on programs with decade-long lives gives real visibility, and backlog, book-to-bill and cyclicality covers how to read those disclosures across the broader group.

Two A&D-specific distinctions sit on top of that.

Total backlog versus funded backlog. On defense programs, total backlog can include work under contracts that have not yet been fully appropriated. Funded backlog is the portion for which money has actually been allocated and can be spent. Quoting a total backlog number as if it were contractually certain revenue is one of the fastest ways to lose credibility in a technical conversation. The follow-up question is always some version of "how much of that is funded, and how much is cancellable."

Contract type. Cost-plus contracts reimburse allowable costs plus a fee, which pushes overrun risk onto the government customer and, in exchange, carries a lower margin. Fixed-price contracts pay a set amount regardless of what the work costs, which means higher margin potential and real overrun exposure sitting with the contractor. Fixed-price development work is the riskiest combination in the sub-sector, because you are committing to a price on work that has never been done before. When a defense prime takes a large charge, a fixed-price development program is very often the reason.

Revenue types side by side

Deal dynamics

A&D M&A has a few recurring shapes worth being able to name.

Supplier consolidation is the steady drumbeat. OEMs prefer fewer, larger, more capable suppliers who can take design responsibility, and tier-ones respond by buying scale and content. A supplier that adds proprietary content on a growing platform is buying future aftermarket, which is the real prize.

Sponsor interest concentrates on aftermarket, components, and MRO assets, for exactly the reasons above: recurring revenue, high margins, and cash flows that hold up better across a cycle. Those characteristics also make the assets more financeable, which matters when a sponsor sizes leverage against a business with genuine cyclicality in part of its mix (see leveraged finance terms for the vocabulary).

Defense M&A carries a regulatory and national-security review layer that does not exist elsewhere in industrials. On top of standard antitrust, transactions touching classified work, sensitive technology, or foreign acquirers can face national-security review, foreign ownership mitigation requirements, and customer objections to vertical combinations that would put a supplier inside a competitor. That layer lengthens timelines, adds conditionality, and sometimes kills deals that would clear on competition grounds alone. If your process knowledge is thin, the M&A guide covers the generic mechanics; the A&D-specific point is that the review layer is a live deal risk you should name.

Portfolio separations are the fourth pattern. Conglomerates with mixed commercial and defense exposure separate them, because the two halves attract different investors, support different capital structures, and get valued off different multiples. Sitting together, the higher-quality half is often obscured. The structuring logic behind those separations is in industrials M&A and spin-offs.

What interviewers ask, and the traps waiting for you

The traps are consistent enough that you can prepare for them directly.

Quoting backlog without qualifying it. If you cite a backlog figure, be ready to distinguish funded from total and note whether orders are cancellable. Confidence about an unqualified number reads as unfamiliarity.

Calling defense non-cyclical. Defense is cycle-driven, just on a different cycle. It does not follow GDP or industrial production, it follows budget cycles and shifting procurement priorities. Individual programs get restructured, stretched, or cancelled, and a company concentrated on one platform can face a sharp revenue decline in an otherwise stable budget environment. Say "differently cyclical," not "non-cyclical."

Assuming aftermarket falls into your lap. Installed base creates the opportunity. Capturing it is a competitive fight against independent service providers, licensed alternatives, and used serviceable material.

Confusing airline profitability with OEM demand. Airlines order aircraft against multi-year fleet plans, replacement schedules, and financing availability. A profitable airline year does not automatically produce orders, and a weak one does not automatically stop deliveries on an existing order book. Fleet decisions drive orders. Profits influence the timing at the margin.

Treating a supplier as a supplier. Ask what content it holds, on which platforms, whether it is sole-source, and what the original equipment versus aftermarket split looks like. That is the analysis. Revenue alone tells you very little.

If you are shaping a fit answer around this sub-sector, how to answer why industrials covers the structure, and specificity about A&D economics is what makes that answer land.

Practice question

Why do aerospace suppliers with high aftermarket exposure trade at a premium, and how would that change how you value one?

Aerospace runs on an installed-base model. Original equipment, particularly on engines, is often sold at thin or even negative margin to win a position on a program, because the manufacturer is buying decades of spare parts and service revenue on every unit that flies. So aftermarket is where the profit actually sits, and it behaves differently from original equipment. It is higher margin, it is recurring, and it depends on the existing fleet flying rather than on new orders, which means it holds up when production rates come down.

That combination, higher margin plus more recurring plus less sensitivity to the new-build cycle, is what earns the premium multiple. So the first thing I would do in valuing a supplier is split revenue between original equipment and aftermarket, and then look at margin by segment, because two companies with identical revenue can be completely different businesses on that split. From there I would look at what content the company actually holds: which platforms, whether it is sole-source, and how long those platforms are likely to fly, because that determines how durable the aftermarket stream is. I would also check whether the company genuinely captures its aftermarket, since independent service providers and used serviceable material compete for that work. It is an advantage, not a guarantee.

What the interviewer is listening for: Whether you understand that the money in aerospace is made after the sale, not at it, and can tie mix directly to margin, recurrence, and multiple rather than just asserting that aftermarket is good. The extra credit is acknowledging that aftermarket capture is competitive, which separates candidates who have read about the model from candidates who have thought about it.