# High-bypass turbofan engines: thrust vs. speed & altitude? **URL:** \ **Category:** Factual Questions\ **Created:** [June 20, 2024, 12:13pm UTC](https://boards.straightdope.com/t/high-bypass-turbofan-engines-thrust-vs-speed-altitude/1003108 "2024-06-20T12:13:24Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 20, 2024, 12:13pm UTC](https://boards.straightdope.com/t/high-bypass-turbofan-engines-thrust-vs-speed-altitude/1003108/1 "2024-06-20T12:13:24Z")
How does the max available thrust from a passenger jet engine vary with air speed and altitude? I’m aware of the concept of thrust lapse, in which the thrust produced by a jet engine tends to decrease as airspeed and/or altitude increase. But ISTM that the lapse rate can perhaps be offset, partially or completely, by adjusting operating parameters. For example, an engine may be thrust-limited at sea level due to temperature and pressure limits inside the engine, but those limits are harder to violate when operating in the cold, thin air at cruise altitude. So suppose I have **[an engine from a Boeing 787](https://en.wikipedia.org/wiki/General_Electric_GEnx#Specifications)** that makes ~75,000 lbf of thrust during takeoff. what’s the max available thrust when cruising at 40K feet? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 20, 2024, 2:10pm UTC](https://boards.straightdope.com/t/high-bypass-turbofan-engines-thrust-vs-speed-altitude/1003108/2 "2024-06-20T14:10:53Z")
Very rough rule of thumb is that max thrust at high cruise altitude is 10% of max sea level thrust. So ballpark 7,500lbf for your example. I don’t know of a good source for a more precise chart for any particular modern engine; they’ve systematically removed all those interesting engineering details from all the pilot manuals I have access to. I didn’t Google, so you might have good success with that. In general your point is well-taken that at low altitude there’s enough mass flow available such that given enough fuel flow you’d over-temp or over-rpm or over-pressurize something. And that at high altitude, there just isn’t. A separate factor though is that so-called max takeoff thrust is time limited to e.g. 5 minutes. For sustained operations there are various different lower limits for climb, cruise, and emergency cruise. Setting aside any computer-enforced refusal to exceed a limit, the underlying turbomachinery and fuel supply systems may well be able to exceed one or another long-term limit even at cruise altitude. This latter concern was real back in the days of non-smart engines with purely hydromechanical engine management. Even at full cruise altitude, mindlessly stuffing the throttles to the forward stop could exceed a limitation. Probably not to an immediately catastrophic degree though due to the reduced mass airflow available at altitude. Nowadays at low or high altitude, the various computers have our backs and pushing throttles to the stops will produce all the thrust that fits under the various limitations, and not a bit more. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 22, 2024, 12:17am UTC](https://boards.straightdope.com/t/high-bypass-turbofan-engines-thrust-vs-speed-altitude/1003108/3 "2024-06-22T00:17:29Z")
I found these charts: [https://www.airliners.net/forum/viewtopic.php?t=1453913](https://www.airliners.net/forum/viewtopic.php?t=1453913) The GE-90 was the highest bypass ratio engine among those shared: [![](https://i.ibb.co/pR4NBsf/HBP-Thrust-1.jpg) ](https://i.ibb.co/pR4NBsf/HBP-Thrust-1.jpg) 70 kN/388.8 kN is 18%. Engines with a lower bypass ratio aren’t quite as bad.