A unit load device, ULD, is the standard container or pallet shape an airline loads into its hold, sized and certified to fit a specific aircraft's cargo bay. A temperature-controlled ULD is one of two things built onto that same standard shape: a passive version, an insulated cover or shell wrapped around a pallet or container with no power source, or an active version, a powered unit with a compressor or thermoelectric core built into the ULD itself. Both fit the same loading slots and tie-down points as an ordinary ULD; the difference is entirely in what happens to the temperature inside once the doors close.
Cold chain shippers moving pallet or bulk quantities by air choose between the two the same way they choose between any passive packaging and powered system: passive covers are cheap and need no charging, active units hold a tighter band for longer but come with a leasing and positioning cost neither the airline nor the shipper can skip.
Passive covers versus active units
A passive ULD cover is a quilted or foam-lined thermal blanket, sometimes reflective-faced, that wraps around an already-palletised load and slows heat gain during ground handling and tarmac exposure, the legs of a flight where a hold is not yet climate-controlled by the aircraft's own systems. It adds no active cooling of its own; it only slows the rate at which ambient heat reaches the pallet underneath.
An active ULD is a fully powered container built to the same external envelope, running its own compressor or thermoelectric plate off an internal battery, holding a set temperature rather than just slowing a drift. It covers the same hold time an active pallet system or an insulated shipper stack would otherwise need, but scaled to the full ULD footprint and built specifically to the loading geometry of one aircraft type.
Airline certification, not universal fit
A temperature-controlled ULD is not automatically accepted onto any aircraft simply because it fits the physical slot. Each airline certifies specific ULD models against its own fleet, checking weight distribution, tie-down compatibility, and, for active units, battery type and charging requirements against its own dangerous-goods and electrical safety rules. A unit approved on one carrier's aircraft type may need a separate certification, or may not be accepted at all, on a different carrier's fleet.
That certification step is why a cold chain lane cannot simply swap airlines at short notice without checking ULD compatibility first. The unit that carried a shipment out on one carrier may have nowhere to go on the return leg or on a connecting flight if the next carrier has not certified that model.
Booking lead times and capacity
Temperature-controlled ULDs, active units especially, are a limited fleet an airline or a specialist provider owns, not stock sitting on a shelf waiting for same-day demand. Capacity gets booked ahead of the flight, often weeks out on long-haul pharma lanes, because the airline has to reserve the loading position, confirm the unit is available at the origin station, and, for active units, confirm a charging point is free at the ground handling stage.
A shipment that turns up without a reservation competes for whatever capacity is left, which on a busy route can mean no temperature-controlled slot at all, forcing a delay or a fallback to a passive insulated shipper stack instead.
Charging, positioning, and empty legs
An active ULD's battery holds charge for a defined stretch, and it has to reach the right airport with enough charge left to cover ground time before the next charging opportunity. Ground handling stations need a charging point rated for the unit, and not every airport has one, so the charging network available along a route is a real constraint on which lanes an active ULD can actually serve, independent of the temperature band it is capable of holding.
Positioning is the other cost nobody sees on the invoice. An empty active ULD has to be flown or trucked to the origin station before it can be loaded, and moved on or returned once it is unloaded, and both legs burn cargo capacity and time without carrying revenue freight. That empty-leg cost is folded into the lease price shippers pay per use.
Availability decides the lane, not the route
Because certified, available ULDs are a finite fleet tied to specific airports and specific airlines, the lane a cold chain shipment can actually fly is often decided by ULD availability before temperature requirements are even considered. A route with the fastest transit time is worthless if no certified temperature-controlled ULD, active or passive, is available at the origin station on the day of departure, or if the connecting carrier has not certified the unit for its own aircraft.
Freight forwarders and pharmaceutical logistics teams plan lanes around this constraint directly: booking capacity months ahead for a predictable, recurring shipment, or building in a buffer day for a one-off move, because a ULD that cannot be booked is not a packaging problem to solve at the last minute, it is a routing decision that has to be made earlier in the process.