Air freight rates on a lane can move sharply in a week when nothing about demand has changed. A large share of the world's air cargo travels in the lower holds of passenger aircraft rather than on dedicated freighters, and that capacity is a by-product of passenger network decisions. When an airline retimes a route, downgauges an aircraft or shifts a frequency to a different market, cargo supply on your lane changes with no reference to cargo at all.
Capacity moves in steps rather than curves. Airline schedules change at the end of March and the end of October, and a route served by a widebody in one season may be served by something smaller, or not served, in the next. A narrowbody substitution is close to a total loss for containerised cargo: bulk hold only, small door openings, and on many sectors no cargo sold whatsoever. Those two dates belong in the planning calendar.
On a passenger flight, cargo gets the payload left over after passengers, baggage and fuel, and that residual is not a constant. A long sector, a hot or high departure airport, strong headwinds or a full cabin all reduce it, so the same aircraft type on the same route can offer materially different capacity from one day to the next. When a flight is payload-restricted at closeout, the lowest-rated and last-booked consignments are offloaded first.
Space and usable space are different things. Lower holds take contoured lower-deck units with restricted door heights, so a consignment taller than a standard lower-deck container has to travel on a freighter's main deck regardless of how much tonnage is nominally on offer. Long single pieces and concentrated point loads run into the same wall. This is why a carrier can genuinely have space on a flight and still be unable to accept your shipment.
Regulation removes some cargo from belly capacity altogether. Under the IATA Dangerous Goods Regulations, certain classes and packing instructions are cargo aircraft only and cannot travel on a passenger aircraft under any circumstances, with lithium batteries shipped on their own the most widely encountered case. For that cargo the relevant supply is freighter capacity on the lane, not total air capacity, and the two can move in opposite directions in the same week.
Density decides who wins when capacity is short. Air freight is charged on the greater of actual and volumetric weight, and a lower hold frequently fills by volume before it fills by weight. Carriers manage the mix of light and dense cargo to use both limits, which means a bulky low-density consignment competes poorly for space in a tight week and pays more for the privilege. Accurate dimensions supplied early beat an early booking built on estimates.
Freighter capacity is the slower-moving half of the market. Aircraft are deployed against expected yields, and repositioning them takes crew, maintenance slots and traffic rights, so a shortage in one region is often relieved by pulling capacity out of another, which tightens the second region and moves rates there as well. Rates track the ratio of tendered tonnage to offered capacity on a specific lane in a specific week, and that ratio has two moving parts.
Practically, this argues for a handful of habits. Ask whether a quoted rate is tied to a named flight or is a market rate that will be re-confirmed at booking. Give real weights and dimensions before asking for a price. Accept that confirmed space on a passenger flight stays conditional on payload at closeout, and agree in advance whether splitting a consignment across two flights is acceptable. For restricted cargo, ask which freighter service is relied on.
None of this makes air freight unreliable; it makes it a market with a supply side you can actually read. We quote air alongside sea, rail, road and multimodal options and state what the price depends on, including whether the routing needs a freighter and what the alternative costs if belly capacity on the lane tightens. Send us the weights, dimensions and required arrival date, and we will come back within one business day.
