View Full Version : Cruising speed of jet aircrafts

6th Jan 2017, 12:17
When i was studying the performance atpl theory book i read that the best range speed for a jet aircraft is 1.32Vmd while its max endurance speed is Vmd .

What are approximately these speeds for a twin engine jet aircraft like an a320 ,and do they actually fly at 1.32Vmd at cruise ? I heard and i do not know if its correct that such aircrafts cruise at a speed near there Mmo , so if they do so is it better to fly at high speeds over flying on best range speed , and why

Thank you .

6th Jan 2017, 17:51
1.32Vmd is a ballpark figure based on two not necessarily valid approximations, rather than being a fundamental physical constant.


Mad (Flt) Scientist
6th Jan 2017, 20:00
Also, cruise speeds are picked for things in addition to simple range considerations - time of flight is an important consideration, both in terms of sticking to a schedule and in terms of the fact that many maintenance tasks on an aircraft are figured on the number of flight hours. So it can make sense to fly faster, burn more fuel, but rack up fewer hours for maintenance.

There's something called "cost index" which is a simple way to balance the cost of fuel versus the cost of time, and airlines will pick a desirable number which gives them the balance they find most economically effective overall. (There's bound to be multiple threads about cost index around here)

6th Jan 2017, 20:21
The maximum duration speed uses the minimum amount of fuel necessary to keep an aircraft flying steadily at a given altitude, i.e. without gaining or losing height.

The maximum range speed uses increased fuel burn to fly faster and generally higher, allowing the aircraft to travel as fast as possible while trying to keep drag or air resistance at a reasonable level. The higher you go, the less air resistance you encounter, but this costs fuel.

So you have a trade off between time in the air and total distance covered. It seems counter intuitive that a slow flying aircraft that can stay in the air longer doesn't have the same range as one flying faster and burning more fuel, but that is indeed what happens.

6th Jan 2017, 20:51
Thank you all for your help

6th Jan 2017, 21:58
...and then the wind comes into the equation. Generally, in a headwind, itīs reasonable to fly faster for maximum range as well as for minimum time, while in a tailwind, reducing IAS is advisable.

Assume a Piper Cub, cruising at economic 60kts against a headwind of the same 60kts - it will burn lots of fuel but will not make a knot of groundspeed. Opening the throttle a notch and increasing speed to a theoretically less economic 70kts will cost more fuel per hour but make the aircraft slowly gain miles and eventually reach its destination. Now assume the 60kts of wind coming from the rear and pushing the aircraft: in this case, flying at normal cruise speed will see You at the destination in no time, but you can save tremendous amounts of fuel by just throttling back, reducing to maximum endurance airspeed and letting the wind do the work for you. And with an airliner, itīs not really different, a smart choice of flight level and climb/descent schedule vs. expected wind situation can result in saving much cost and time.

7th Jan 2017, 02:11
Our jets go much faster on the way home.

Metro man
7th Jan 2017, 12:49
Modern jets with a flight management system use cost index rather than speed and pilots simply enter a value, typically 20-50 when loading the flight plan. The lower the value the less fuel used but more time taken.

The planning department take a number of factors into consideration to come up with a number such a fuel price, lease rate per hour of the aircraft etc. a couple of minutes either way per flight over a large fleet makes a big difference.