Primary sources · 4
- [1] ICAO Annex 2 — Rules of the Air — Defines flight levels and the reduced-vertical-separation-minimum (RVSM) airspace · ICAO · Current edition https://www.icao.int/safety/airnavigation/Pages/standard.aspx
- [2] Boeing 787 performance manual extract — Service ceiling 43,000 ft; optimum cruise band 35,000–41,000 ft · Boeing flight crew operations manual · Current https://www.boeing.com/commercial/787/
- [3] FAA AC 91-85 — Authorisation of Aircraft for RVSM Operations — Background on FL290–FL410 standard cruise altitudes · U.S. Federal Aviation Administration · Current https://www.faa.gov/regulations_policies/advisory_circulars
- [4] International Standard Atmosphere (ISA) — Reference for temperature, pressure, and density vs altitude — underlies all cruise-altitude analysis · ISO 2533:1975 · 1975, current standard https://www.iso.org/standard/7472.html
Cruise altitude is a fuel-burn problem. Drag depends on air density, so flying higher cuts fuel — but engines also lose thrust at altitude, and every aircraft has a structural ceiling. The result is a narrow band where everything flies: FL350 to FL410 for most commercial operations.
Flight levels, not altitudes
Above the transition altitude (typically 18,000 ft in the US, 6,000 ft in much of Europe), all altitude reporting switches to "flight levels" — nominal altitudes above the 1013.25 hPa pressure datum, written as FLxxx where xxx is the altitude in hundreds of feet. FL370 means 37,000 feet pressure-altitude. This convention means every aircraft above the transition altitude is measuring height against the same reference, so vertical separation in dense airspace is consistent regardless of local sea-level pressure.
| Flight Level | Feet | Metres | Common use |
|---|---|---|---|
| FL290 | 29,000 | 8,839 | Lower long-haul, off-track |
| FL310 | 31,000 | 9,449 | Heavy aircraft early-cruise |
| FL330 | 33,000 | 10,058 | Mid long-haul |
| FL350 | 35,000 | 10,668 | Standard long-haul |
| FL370 | 37,000 | 11,277 | Optimal for most twins |
| FL390 | 39,000 | 11,887 | Late-cruise step-up |
| FL410 | 41,000 | 12,497 | Max for most narrow-body |
| FL430 | 43,000 | 13,106 | Service ceiling, some wide-body twins |
Why higher is more efficient
At FL350, the air density is about 32 % of sea level. Drag (the force that opposes motion) is proportional to density × velocity² × area, so holding a fixed Mach number at altitude requires roughly one-third of the thrust required at sea level for the same true airspeed. The engines' fuel burn falls almost in proportion. The result: every 4,000 ft of climb roughly cuts cruise fuel burn by 10–15 %.
Why not higher still
Two limits cap cruise altitude. The structural service ceiling is the altitude beyond which the cabin pressurisation system cannot maintain a safe cabin altitude — most current jets are certified to 41,000 to 43,000 ft. The engine flameout margin falls with altitude — high-altitude flameouts are harder to relight, and required margins above engine restart limits constrain real cruise to a band below the absolute ceiling.
Cruise altitude affects the contrail problem
Contrail formation depends on relative humidity over ice and the temperature at cruise altitude. Flights that pass through ice- supersaturated regions tend to leave persistent contrails; those that fly slightly above or below avoid them. Recent research (Teoh et al. 2024) shows that 2 % of flights produce 80 % of the contrail warming effect — and a small altitude adjustment could remove most of that forcing without significant fuel cost.