Continuing the theme of the difference between what is commonly assumed in Aviation and the facts, let’s just think about cruise altitude. Everyone knows that aircraft fly high for reasons of efficiency, not to mention community noise. But is this always right? For years I cringed as colleagues harangued controllers, insisting that they simply had to maintain their level when ATC wanted them to descend and slow down in anticipation of holding. They didn’t need to insist. ATCOs, who to be fair have got their own problems to sort out without worrying about the flight crews’, are also largely unaware of the physics. So when is it necessary to fly high, and when doesn’t it matter?
There is no argument that for best range, aircraft need to fly high. The simplest way to look at it, is that as the altitude increases, air density decreases, and you can fly faster for the same drag. That means you get there in less time, and as jet engines’ fuel consumption is roughly proportional to thrust and time, for a given thrust, the quicker you get there the less fuel you will burn. So what stops aircraft cruising at say, FL 600? Well, this previous statement means that as the aircraft climbs into less dense air, the True Airspeed is increases, and eventually it reaches a percentage of the speed of sound (i.e. Mach Number) beyond which nasty things happen to the airflow around the aircraft.
Additionally, for most aircraft the available thrust falls off as the density, and amount of oxygen, fall and eventually even full power won’t sustain the cruise speed. Which of these two limits cuts in first depends on the aircraft design, but most modern airliners are pretty similar in terms of wing loading, sweep and span/chord ratio, so they all top out at FL 370 – 400. The biz jets, such as the G5, as you can see at once, tend to have a lot more wing relative to their size, and fly correspondingly higher (up to FL 500). To cruise higher than that you have to look distinctly odd, like the U2 or the Avro Vulcan. Incidentally, the recent rash of little electric aircraft don’t lose power as they climb (their engines don’t need oxygen), so can be expect to cruise higher than their internal combustion predecessors if their batteries last long enough to do so; their crew, on the other hand still need to breathe!
So far, nothing new, so what am I getting at? The previous paragraph was concerned with range. What about endurance? This is where it gets counter-intuitive. To first order best endurance is obtained with the aircraft flying pretty close to its minimum drag speed, and this min indicated speed doesn’t vary much with altitude. For a given indicated speed, the drag is roughly constant, and as the engines still consume at a rate which depends primarily on the thrust (= drag), the fuel consumption at Min drag speed varies hardly at all with altitude. The modern fan jet aircraft have a rather flat curve of holding fuel consumption with a very broad minimum between FL150 and 250. But the penalty of holding at 5000ft versus 15000 is only about 5% (see fig 1 taken from an RAE report of 1973 so there is nothing new in this insight).
for a typical jet aircraft
To put this into simple language, if you are flying A to B , you go as high as you can, but if you aren’t going anywhere , then the altitude is not critical.
The next important conclusion is that it doesn’t make sense to fly an ultra efficient profile straight into a holding pattern. You would consume less fuel if you slowed down earlier; minutes wasted in cruise are minutes saved in the hold, and the fuel ‘wasted’ in the inefficient slower cruise is less than that wasted going in circles. To see how this works out, fig 2 shows what can happen if you know well in advance that there is holding at the destination (of course it would be even better to wait on the ground, but that is for another discussion; let’s just say that for very long haul flights, the suggestion that one should delay in anticipation of delays up to 24 hours away can only ever be made by someone who hasn’t stuck is head out of the lab window for a long time…). Nearly everyone knows that it is worth slowing down early if there is holding ahead, but not everyone realises that to get full value, it is worth descending too. Fig 2 is a rather messy graph which shows what happens if you decide to absorb a significant delay in the air, and you have a long distance to do it in. It was produced in anticipation of a linear holding trial for early morning arrivals into Heathrow and reflects the fact that there about 500 NM between the oceanic Boundary and London. It shows that it is possible to lose 20 mins in that distance… by descending to FL240, and slowing to M 0.55. Doing so burns more fuel to the hold fix, but, crucially, saves about 90% of the fuel that would have been burned in the hold to the same exit time. These are numbers that cannot be ignored in the current climate of heightened environmental awareness, and both ATCOs and pilots are going to have to get their minds round these issues.
