When Airlines Use Domestic Law to Erode Montreal Convention Rights
Swiss Airlines delayed a passenger's baggage for a month, then invoked Swiss unjust enrichment doctrine to challenge reimbursement of necessary purchases. The Montreal Convention has legal supremacy, but enforcing that supremacy in full requires escalation few passengers can practically pursue. The case was settled through court-mediated negotiation with most expenses awarded.
The situation
A claim was submitted against Swiss Airlines following the delayed return of checked baggage on a flight from Barcelona to Zurich. The bag contained the passenger's belongings from a three-week business trip, including clothing, footwear, business attire, and personal items. The bag was eventually returned, but only after one month had passed since the flight.
Under the Montreal Convention, baggage delayed for more than 21 days is officially considered lost. The passenger's bag was returned outside this window, meaning the disruption met both the threshold for delayed baggage and crossed into the territory normally treated as loss.
During the month-long deprivation, the passenger purchased essential replacement items, including basic clothing, footwear, and toiletries. The purchases were modest and consistent with the requirement to mitigate expenses where possible.
What the airline did
Swiss Airlines' response moved through several stages.
The initial position was that no reimbursement was due because the passenger's journey was homebound. According to the airline, when a passenger arrives at their home destination, essential replacement items are assumed to be readily available, and interim expenses are therefore not warranted.
When this position was challenged with a detailed explanation that the entire contents of a three-week business trip had been in the missing bag, the airline shifted. It offered CHF 254.35 as interim compensation, expressly framed as without prejudice to a full claim should the baggage remain unrecovered.
Once the baggage was eventually returned, Swiss treated the interim payment as final and declined any further evaluation. The position was that the matter had been resolved.
This position did not hold up to scrutiny.
Why this matters under the Montreal Convention
The Montreal Convention governs liability for delayed, damaged, or lost baggage in international air carriage. It provides for compensation up to a fixed liability limit, calculated in Special Drawing Rights (SDRs), to cover the damages a passenger actually suffers.
Reasonable interim expenses incurred as a direct result of baggage delay fall squarely within the scope of compensable damages. The duty to mitigate exists, but it does not require passengers to do without essential items for indefinite periods. A month-long deprivation of business attire, footwear, and personal items during an active working period is not something the Convention expects passengers to absorb without recourse.
The legal framework here is clear at the level of principle. What complicates the picture is what happens when airlines invoke domestic law to argue against it.
See: How Airline Compensation Works
Where the case became more complex
The matter proceeded to litigation in Zurich. In court, Swiss Airlines' counsel introduced a defense based on Swiss domestic law. Specifically, the airline invoked the doctrine of unjust enrichment, arguing that because the bag had eventually been returned, reimbursing the passenger for the interim purchases would result in unjust enrichment under Swiss civil law. The position was that the Convention's SDR liability should yield to this domestic-law principle.
This is where the case raises a broader question about how international conventions actually function in practice.
Can national law override the Montreal Convention?
In principle, no. The Montreal Convention is explicit on its own supremacy. Article 29 of the Convention states that in carriage of passengers, baggage, and cargo, any action for damages can only be brought subject to the conditions and limits of the Convention itself. The Convention is exclusive in the matters it governs. Domestic law cannot supplement, modify, or override it where its provisions apply.
This principle has been confirmed at the highest judicial levels in multiple jurisdictions. The Court of Justice of the European Union has held that the Convention's terms must be interpreted uniformly and autonomously, not according to national legal concepts. Supreme courts in the United Kingdom, the United States, and elsewhere have reached similar conclusions. The Convention preempts conflicting national law in its scope of application.
If the Swiss Airlines case had been escalated through every level of appeal, the unjust enrichment argument would almost certainly have failed. Convention supremacy is well-established at the level of higher courts. A national civil law doctrine cannot lawfully be used to defeat compensation that the Convention provides for.
Why this still matters in practice
The problem is not whether the Convention has supremacy. It does. The problem is what happens between the Convention's principle and the individual case.
In practice, most baggage cases are heard in lower courts that do not specialize in international air law. The amounts involved are usually modest, often in the low thousands of Swiss francs, euros, or dollars. Litigating a case all the way through to a final appellate ruling on the supremacy question would cost far more than the underlying claim is worth.
Airlines understand this. Domestic-law arguments are introduced not because they are likely to win at the highest level, but because they create friction at the level where most cases are actually decided. A judge weighing a settlement in mediation is not adjudicating whether Swiss unjust enrichment doctrine preempts the Montreal Convention. The judge is trying to broker a resolution. The airline's domestic-law argument becomes leverage, even when it would not survive scrutiny in a higher court.
This is the gap between legal principle and case-level enforcement. The Convention works at the level of doctrine. It is partially undermined at the level of practical application.
See: Why Escalation Is Sometimes Required, How Claim Catalyst Handles Airline Resistance
What we did
The case was litigated in Zurich. Swiss Airlines' unjust enrichment defense was challenged directly. The position taken was that the interim purchases were necessary and proportionate, consistent with the duty to mitigate, and well within the scope of damages compensable under the Convention.
The choice was then between continuing to litigate the supremacy question through further appeals, which would have been expensive and time-consuming, or accepting a court-mediated settlement that recovered the majority of the actual expenses incurred. The pragmatic decision was to settle.
Outcome
The case was resolved through court-mediated settlement. Just over CHF 1,000 was awarded, covering the majority of the interim expenses. A small number of items were excluded on the basis that they could have been returned to the retailer once the baggage was eventually recovered, even though the recovery fell outside the standard 21-day return window for most retailers.
Lesson
The Montreal Convention provides clear entitlement. National law cannot lawfully override it within its scope of application. In principle, the Convention works.
In practice, airlines exploit the gap between principle and enforcement. Domestic-law defenses are deployed because they create cost and friction at the case level, where most disputes are actually resolved. Even when these arguments would fail at the highest courts, they often succeed in shaping settlements at lower instances.
The honest answer is that the Convention works partially. It establishes rights that cannot be overridden by national law, but those rights are only fully realized when claims can be pursued through every level of escalation. For most individual passengers and most individual claims, that level of pursuit is impractical. The result is a system where the Convention is respected in theory and partially eroded in practice.
Why most passengers lose here
Most passengers never reach the point where domestic-law arguments are even raised against them. Their claims are resolved or abandoned at earlier stages, on the basis of arguments like the homebound defense or the framing of interim payments as final settlements.
For passengers who do push further, the next obstacle is the legal complexity of arguments designed not to win at the top, but to extract concessions in mediation. Recognizing that a domestic-law defense is structurally weaker than it appears, but that pursuing it to full vindication is impractical, is not something individual passengers can reasonably be expected to do alone.
Bigger picture
The Montreal Convention is one of the strongest international consumer protection frameworks in any industry. Its supremacy is established, its scope is clear, and its provisions are enforceable.
What erodes its effectiveness is not the law itself but the practical economics of enforcement. Airlines deploy arguments calibrated to the level of court most likely to hear the case, knowing that full vindication requires escalation few passengers can afford. The result is a system that respects the Convention at the level of doctrine while routinely diluting it at the level of outcome.
This is the structural reality behind the question of whether international conventions actually work. They work to the extent that they are enforced. Without enforcement, they function more as guidelines than as binding rights.