Question 1 of 7: Updating a 50-Year-Old Aircraft — Team, Objectives, Concept Selection and Specification Flow-Down

Question 1: Updating a 50-Year-Old Aircraft — Team, Objectives, Concept Selection and Specification Flow-Down (40 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Design direction adopted (and held consistently through A–E). The subject aircraft is a 180-seat single-aisle transport of a 1970s type certificate: a 79 000 kg maximum take-off mass, a 3 000 km design mission, an aluminium semi-monocoque airframe, first-generation low-bypass turbofans, mechanical flight controls and analogue instrumentation. The commercial decision taken at the outset, and defended in part B, is a derivative programme under an amended type certificate rather than a clean-sheet aircraft: it preserves the existing certification basis, the production tooling and — decisively for an airline customer — the pilot type rating. Everything that follows is judged against that direction.

Part A — The four anchor skill sets

A derivative aircraft programme fails for organisational reasons far more often than for technical ones, so the four anchors are chosen to close the four ways this particular programme can die: it can be uncertifiable, it can be unbuildable, it can be unsellable, and it can be ungovernable. Naming a discipline is not enough; each anchor is stated below as a skill set with the decision authority that goes with it.

1. Certification and airworthiness engineering. The anchor must be able to write and defend a certification basis — which paragraphs of Airworthiness Manual Chapter 525 apply, which changed-product-rule elements are triggered by each proposed change, and where a Transport Canada Civil Aviation finding of compliance will be by analysis, by similarity, or by test. On a fifty-year-old type certificate this is the single most valuable skill on the team, because the difference between an amended type certificate and a new one is measured in hundreds of millions of dollars and several years, and that boundary is decided by the detail of what changes, not by its magnitude. This person holds a veto over configuration changes.
2. Systems engineering and requirements management. The second anchor owns the requirements architecture: the flow-down from the three high-level objectives of part B to allocated, verifiable, budgeted specifications, and the interface control between the changed and unchanged parts of the aircraft. On a derivative, most of the aircraft is not being redesigned, so the interfaces between new and legacy are where the programme risk concentrates. The skill wanted is model-based systems engineering discipline — one authoritative requirements and interface model, not a stack of documents that disagree.
3. Propulsion and aerodynamic integration. The third anchor is a domain specialist in installed propulsion performance: nacelle and pylon aerodynamics, engine and airframe thermodynamic matching, and the book-keeping that decides whether an advertised engine specific-fuel-consumption improvement survives installation. Part C shows why this skill is the pivotal one for the objective selected: an uninstalled 15 % fuel improvement becomes roughly 12.8 % once nacelle drag and engine mass are charged against it, and a team without this skill will write the wrong number into the specification.
4. Manufacturing and supply-chain engineering (design for production and for retrofit). The fourth anchor keeps the design producible on the existing line and, where the change is retrofittable, installable in a customer maintenance visit. Aerospace derivative programmes routinely erase their own business case in the industrialisation phase, because a change that is elegant on the drawing is a new jig, a new qualified supplier and an eighteen-month lead-time part. This anchor also owns obsolescence: fifty-year-old avionics and actuation components are frequently no longer procurable at all, which is by itself a reason the update is happening.

Two skills deliberately not made anchors are stress analysis and detail design. They are indispensable and will be heavily staffed, but they are mature, well-tooled and readily contracted; they do not decide whether the programme is viable. The anchor test used here is the one Dieter and Schmidt apply to team composition — anchor the roles whose judgement cannot be recovered later by more effort.

Part B — Three high-level design objectives

The three objectives below are chosen because each is a condition an airline imposes before it will place an order, and because each is measurable, which is what allows part E to turn them into specifications.

1. Objective 1 — Reduce block fuel per seat-kilometre by at least 12 % at the design mission. Fuel is the largest single operating cost of a single-aisle aircraft on a 3 000 km stage, and it is the cost the customer keeps paying for twenty-five years after purchase. It is also now a regulatory quantity rather than merely a commercial one: aeroplane carbon-dioxide emission standards apply to new and derivative type designs, so an un-updated fifty-year-old configuration is progressively excluded from new production regardless of what any airline thinks of it. This is the objective carried forward into parts C, D and E.
2. Objective 2 — Bring the flight deck and aircraft systems to a current, supportable and obsolescence-free standard. Two forces act together. Commercially, required navigation performance, controller-pilot data link, automatic dependent surveillance and future air-traffic-management mandates make an analogue flight deck progressively unable to access the most valuable airspace and approaches. Practically, fifty-year-old avionics, electromechanical actuation and wiring are no longer procurable; the supply chain has ended the product whether or not the operator has. An update that does not address obsolescence merely postpones the same programme.
3. Objective 3 — Reduce maintenance cost per flight hour and lengthen the maintenance interval, while raising cabin appeal at constant seat count. Maintenance is the second-largest controllable operating cost, and it is where fifty years of materials, sensing and structural-health knowledge pay directly — corrosion-resistant alloys and composite secondary structure, condition-based rather than calendar-based tasks, and on-board data that turns unscheduled removals into planned ones. The cabin is included here rather than as a fourth objective because it is the only part of the aircraft the paying passenger ever evaluates, and on a derivative it is the cheapest visible differentiator: a new interior, larger bins and modern lighting change the product the airline sells without touching the certification basis of the aircraft itself.

Objectives 1 and 3 compete with one another for mass and for capital, and objective 2 competes with both for electrical power, cooling and panel space. Stating that tension now is deliberate: part E has to arbitrate it, and an objective set that does not conflict has not been set ambitiously enough.

Part C — Three competing approaches to Objective 1

Given. The design mission and datum aircraft are as follows.

Find. The cruise fuel burned per sector by the datum aircraft and by each of three competing approaches to Objective 1, so that the three can be compared on a single quantitative footing in part D.

Approach. Each candidate changes exactly one term of the Breguet range relation — specific fuel consumption, lift-to-drag ratio, or mass — so inverting that relation for fuel burned at fixed range isolates the contribution of each and makes the three genuinely comparable.

1. State the governing relation and invert it for fuel. For cruise at constant Mach number and lift coefficient, the Breguet range relation gives $$R=\frac{V}{c}\,\frac{L}{D}\,\ln\!\left(\frac{W_1}{W_2}\right)$$ where $V$ is true airspeed, $c$ the installed thrust specific fuel consumption expressed as fuel weight flow per unit thrust, $L/D$ the cruise lift-to-drag ratio, and $W_1$, $W_2$ the start- and end-of-cruise masses. Solving for the fuel burned, $W_f=W_1-W_2$, gives the working form used throughout this question, $$\boxed{\,W_f=W_1\left[1-\exp\!\left(-\frac{R\,c}{V\,(L/D)}\right)\right]}$$
2. Evaluate the datum. Substituting the datum values, $R c/(V\,L/D)=(3.0\times10^{6})(1.60\times10^{-4})/(230.1\times17.0)=0.12271$, so $$W_{f,0}=79\,000\left(1-e^{-0.12271}\right)=9\,122.8\ \text{kg per sector.}$$ Every candidate below is measured against this figure.
3. Option 1 — re-engine with a modern high-bypass geared turbofan. A current-generation engine of this thrust class offers roughly 15 % lower cruise specific fuel consumption, so $c$ falls to $1.36\times10^{-4}\ \text{s}^{-1}$ and the exponent becomes 0.10430: $$W_{f,1}=79\,000\left(1-e^{-0.10430}\right)=7\,824.7\ \text{kg},$$ a saving of 1 298.1 kg per sector, or 14.23 %. This is by a wide margin the largest single lever, which is exactly why re-engining dominates real derivative programmes.
4. Option 2 — aerodynamic refinement. Blended winglets or raked tips, a re-profiled wing-body fairing, revised flap-track fairings and a natural-laminar-flow nacelle raise cruise $L/D$ from 17.0 to about 17.8, a 4.7 % improvement. Then $$W_{f,2}=79\,000\left(1-e^{-0.11719}\right)=8\,736.4\ \text{kg},$$ a saving of 386.4 kg per sector, or 4.24 %.
5. Option 3 — structural and interior mass reduction. Carbon-fibre secondary structure (fairings, control surfaces, floor beams), a composite interior and lighter seats remove about 1 800 kg of operating empty mass. At fixed range the fuel scales with the start-of-cruise mass, so $$W_{f,3}=77\,200\left(1-e^{-0.12271}\right)=8\,915.0\ \text{kg},$$ a saving of 207.9 kg, or 2.28 %. Note the structural result: because $W_f$ is proportional to $W_1$ at fixed range, the percentage fuel saving equals the percentage mass saving exactly, $1\,800/79\,000=2.28\ \%$. That identity is worth carrying into part D, because it means mass reduction can never beat a good engine on this objective unless the mass saved is enormous.

The three approaches are genuinely competing rather than three flavours of the same idea: one changes the propulsion system, one changes the external aerodynamic shape, and one changes the structure and furnishing. They also differ completely in certification path, in whether they can be retrofitted to aircraft already in service, and in capital cost — which is what makes part D a real decision rather than an arithmetic exercise.

Part D — Selection strategy, and its application to the three options

Given. The three candidates of part C, their computed fuel savings, and the capital cost of each per aircraft: CAD 4.60 M for the re-engine (shipset price premium plus amortised non-recurring and certification cost), CAD 1.15 M for the aerodynamic package, and CAD 1.90 M for the structural mass reduction.

Find. A defensible selection, produced by a strategy that is stated before the candidates are scored and that survives a sensitivity test.

Approach. A three-stage strategy is used, deliberately ordered so that cheap filters run first: (i) Pugh screening against the do-nothing datum to eliminate anything that is not at least neutral overall; (ii) a weighted objective matrix with the weights fixed and signed off before any rating is entered; (iii) two hard economic and regulatory gates, followed by a sensitivity sweep on the dominant weight. Stage (iii) exists because a weighted matrix produces a number, and a number that is within its own noise is not a decision.

1. Stage 1 — Pugh screening against the datum. Each concept is scored $+$, $0$ or $-$ against the unchanged aircraft on eight criteria. The purpose is not to rank but to eliminate, and to expose concepts whose negatives cluster in one place.

   Criterion	Datum	Option 1 re-engine	Option 2 aerodynamics	Option 3 mass
   Block fuel per seat-km	0	+	+	+
   Emission-standard compliance	0	+	+	+
   Certification effort	0	−	−	−
   Capital cost per aircraft	0	−	−	−
   Maintenance cost per flight hour	0	+	0	−
   Payload-range capability	0	+	+	+
   Community noise	0	+	0	0
   Retrofit to in-service fleet	0	−	+	−
   Net (plus minus minus)	0	+2	+2	0

   All three survive screening, which is the expected outcome when the concepts have been generated systematically rather than opportunistically. The screening is still worth the ten minutes it costs, because it shows that option 3 is only break-even against doing nothing and that option 1's negatives are concentrated in certification and capital — precisely the two criteria the next stage weights most heavily after fuel.
2. Stage 2 — fix the weights before scoring. Six criteria are weighted to sum to unity. The weights are set by the programme steering group from the objective set of part B and recorded before any candidate is rated, which is the single most important procedural safeguard in weighted scoring: weights chosen after ratings are seen are simply a rationalisation of a decision already taken. $$\begin{aligned} &\boxed{\,S_j=\sum_{i=1}^{6} w_i\,r_{ij}\,} \\ \sum_i w_i &= 1 \\ &r_{ij}\in\{1,\dots,5\} \end{aligned}$$
3. Stage 2 (continued) — rate and total. Ratings are on a 1–5 scale, 5 best, anchored to the part C computations wherever a computation exists.

   Criterion	Weight wi	Opt 1	Opt 2	Opt 3
   Block-fuel reduction (computed, part C)	0.30	5	3	2
   Certification risk and schedule	0.20	2	4	3
   Capital cost per aircraft	0.20	2	4	3
   Payload-range and fleet commonality	0.15	5	3	4
   Maintenance cost and reliability	0.10	4	3	2
   Retrofit to in-service fleet	0.05	2	5	2
   Weighted total Sj	1.00	3.55	3.50	2.70

   Option 1 leads, but by 0.05 on a five-point scale — about 1.4 % of the range. Reporting that as a result would be dishonest, and stage 3 exists to deal with it.
4. Stage 3a — sensitivity sweep on the dominant weight. Hold the ratings and let the fuel weight $w$ vary, rescaling the other five weights proportionally by $(1-w)/0.70$. Options 1 and 2 exchange rank where $$5w+\frac{2.05(1-w)}{0.70}=3w+\frac{2.60(1-w)}{0.70} \;\Longrightarrow\; 2w=\frac{0.55}{0.70}(1-w) \;\Longrightarrow\; \boxed{w^{*}=0.282}$$ The chosen weight is 0.300. The decision is therefore fragile: an 0.018 shift in one weight inverts it. Figure 1.2 shows the crossing.

1. Stage 3b — break the tie with two hard gates the matrix cannot see. Because the matrix cannot separate options 1 and 2, the decision is referred to two criteria that are absolute rather than weighted. The first is the cost of saved fuel, $g = I/(\Delta W_f\,N)$, the capital spent per kilogram of annual fuel avoided; the programme gate is CAD 6.00 per annual kilogram. The second is simple payback, $t_p = I/(\Delta W_f\,N\,p)$, against a five-year gate set by the airline's own fleet-planning horizon. $$\begin{aligned} g_1&=\frac{4.60\times10^{6}}{1\,298.1\times750}=4.72 \\ g_2&=\frac{1.15\times10^{6}}{386.4\times750}=3.97 \\ g_3&=\frac{1.90\times10^{6}}{207.9\times750}=12.19 \end{aligned}$$ in CAD per annual kilogram, and $$\begin{aligned} t_{p,1}&=\frac{4.60\times10^{6}}{1\,298.1\times750\times1.057}=4.47\ \text{yr} \\ t_{p,2}&=3.75\ \text{yr} \\ t_{p,3}&=11.53\ \text{yr} \end{aligned}$$
2. Read the gates and decide. Option 3 fails both gates decisively — CAD 12.19 per annual kilogram against a CAD 6.00 ceiling, and an 11.5-year payback — and is eliminated outright, despite having survived Pugh screening and scored 2.70 in the matrix. Options 1 and 2 both pass, and option 2 is in fact the more efficient use of capital on both gates. What separates them is that option 2 is capacity-limited: 4.24 % is the whole of what aerodynamic refinement can deliver, and it does not reach the 12 % objective set in part B, whereas option 1 does. The decision taken is option 1, the re-engine, with the option 2 aerodynamic package retained as a funded second work-package — it clears both gates on its own merits, it is the only one of the three that retrofits to aircraft already in service, and its benefit is very nearly additive to the engine's because the two act on different terms of the range relation. Option 3 is deferred to a later cabin refresh, where the mass reduction can be bought incidentally rather than paid for directly.

Part E — Converting the chosen solution into specifications, documenting it and communicating it

Given. The selected solution is the re-engine of option 1, whose uninstalled benefit is a 14.23 % cruise fuel reduction. The installation charges against it a nacelle and pylon drag penalty of up to 1.1 % of aircraft cruise drag and an engine-plus-pylon mass increase of up to 480 kg. The specification written in part B demands 12.0 %.

Find. Whether the installed configuration still meets the specification, with what margin, and how that single number is decomposed into allocated, owned and verifiable requirements for the rest of the design team.

Approach. Specification flow-down on this kind of programme is a budget: the top-level number is decomposed into signed allocations that must roll back up with positive margin. The budget is computed first, then documented as a requirements tree, then communicated through interface control and configuration management.

1. Compute the installed benefit rather than quoting the engine brochure. Re-evaluating the range relation with all three installation effects applied simultaneously — $c=1.36\times10^{-4}\ \text{s}^{-1}$, $L/D$ reduced by the factor $1/1.011$, and $W_1$ raised to 79 480 kg — gives $$W_{f,\text{installed}}=79\,480\left[1-\exp\!\left(-\frac{(3.0\times10^{6})(1.36\times10^{-4})}{230.1\times(17.0/1.011)}\right)\right]=7\,954.4\ \text{kg}$$ so the installed reduction against the 9 122.8 kg datum is $$\boxed{\,\frac{9\,122.8-7\,954.4}{9\,122.8}=12.81\ \%\,}$$ against a 12.00 % specification: a margin of 0.81 percentage points. Read as a budget, the 14.23 points delivered by the engine are reduced by 0.89 points of nacelle and pylon drag and a further 0.53 points of installed mass, $-14.23+0.89+0.53=-12.81$, so 1.42 of the 14.23 points — a tenth of the headline benefit — is consumed by installation before any sub-team has made a single detail-design decision. This is the whole reason the propulsion-integration anchor of part A exists, and the reason the specification is written against installed, flight-tested performance rather than against the engine supplier's uninstalled deck.
2. Turn the margin into allocated requirements with named owners. The 12.81 % is decomposed into four allocations, each with a numeric limit, an owner and a verification method. Written in the form used on the requirements tree below, they are: propulsion, $\Delta c/c \le -15.0\ \%$ at the cruise rating; installation, $\Delta C_D \le +1.1\ \%$ of aircraft cruise drag; and mass properties, $\Delta m \le +480\ \text{kg}$ for engine, nacelle, pylon and mounts combined. The 0.81 point residue is held as programme margin by the chief engineer and is not visible to the sub-teams — margin that is published is margin that is spent. Bleed and electrical offtake re-allocation, which a modern engine also permits, is deliberately not claimed in the budget: it is tracked as an unallocated opportunity, so that if the drag or mass allocation is exceeded there is something left to trade.

1. Document the design in four controlled artefacts, not in a report. (i) A system requirements document holding every allocation above with its rationale, its verification method and its trace to the part B objective — requirements without rationale are re-litigated at every review. (ii) Interface control documents for each boundary between the changed and the unchanged aircraft: pylon-to-wing structural interface, engine-to-aircraft electrical and bleed interfaces, engine-indication data to the flight deck, and the nacelle-to-airframe aerodynamic interface. On a derivative these ICDs are the most valuable documents on the programme, because most of the aircraft is not being redesigned and every defect will be found at an interface. (iii) A certification plan agreed with Transport Canada Civil Aviation stating the certification basis, the changed-product-rule assessment, and each means of compliance. (iv) A mass and performance budget under formal configuration control, so that every proposed change is scored against the 0.81-point margin before it is approved.
2. Communicate through a single authoritative model and a cadence of reviews. The four artefacts live in one configuration-managed baseline — a model-based systems engineering environment where the requirements, the interfaces and the digital mock-up are linked, so that a change to an allocation propagates visibly to everyone it affects. Around that baseline the team runs a fixed cadence: weekly integration meetings at the interfaces, a formal change-control board that is the only body permitted to move an allocation or spend margin, and the conventional gate reviews — preliminary design review to confirm the allocations are achievable, critical design review to confirm the detailed design meets them, and a test-readiness review before certification flight test. Every change request that arrives after the baseline is frozen must state its effect on the budget in the same units as the budget itself; a change that cannot say what it costs in percentage points of block fuel is not evaluable and is rejected on that ground alone.
3. Close the loop with verification. Each allocation names how it will be proved: the engine deck by supplier rig test and by installed flight test at four cruise ratings; the drag increment by computational fluid dynamics, confirmed in the transonic wind tunnel and finally by flight-test drag polars; the mass by component weigh-off at first article and by aircraft weigh-off before delivery; the offtake credit by systems load analysis. The programme is not finished when the drawings are released — it is finished when the roll-up of verified evidence reproduces the 12.81 % with the margin intact.