Short flights.
 

Hi all.
Just completed a flight with Air Asia from Bangkok to Khon Khaen (sp) in the north of Thailand. Great flight on a Airbus ( not sure which one) .
It was a 55 min flight but I was surprised when the captain announced that we were at 25000 feet.
I know a jets performance/economy is better at high altitude, but is this height normal for such a short distance?
Would it not be more economical to stay at a lower altitude then burn fuel to get higher?
Many thanks.

I remember when BA 757's first starting operating the "Shuttle" routes in the UK. The sectors to Glasgow, Edinburgh and Belfast from Heathrow are all about 1hr (give or take) and the 75' would frequently make it to FL410 on these routes.

In general, it usually makes sense to burn some fuel to get high, where less fuel will be burnt in the cruise, and will be followed by a long, mostly at idle, descent.

Many factors can impact the above, which is a very simplified view.

Perfectly normal. On the classic 737 we frequently went to FL330 (33,000ft) or so for 45-50 minute sectors.

25,000 feet is 4.11 nautical miles. When your horizontal distance is say 350 nm, it's hardly any extra distance to get up there and then down again at the end.

189 nm Ayers Rock (YAYE) to Alice (YBAS) (if I remember correctly), when light F330, if heavy F270 or F290).

Generally as long as TOPC is before TOPD, its good - a slingshot flight. :}

Folks,
Boeing publish "Optimum Cruising Level for Short Sectors" ( or a name to that meaning) and is designed to give minimum block fuel burn on short sectors.
Compared to common airline flight planning practice, the numbers are quite interesting, producing relatively low levels.
On Australia east coast sectors, I regularly used to operated at said levels (as opposed to as flight planned) passenger comfort permitting, and always beat the plan.
The old "theory" but never was correct, even for straight turbo-jets, and progressively became less "right" as by-pass ratio increased.
Tootle pip!!

Another important point to consider tj is while you may spend a large part of a short flight in the climb with a quite short cruise portion you are still covering a lot of ground.


In the 757 after transitioning we climb at .8 mach which is also our normal cruise speed, its not like you are climbing at a much slower speed.

LeadSled,

Would be very much appreciative if you could point me in the right direction to find that document. I can't seem to find it with google.

On 738 fleet we adopted the policy if the flight is more than 1 hour we can climb above 360, less than one hour below FL 360 as the TC and TD result in a very short cruise sector. But to answer the OP, on a 55 min flight we could normally easily achieve 350. in terms of performance, at average take off weights 65-68 T climb rate was between 3500 fpm below FL 320 and 1500 fpm above 320, then 1000 fpm from 320 to TC. In other words 15 mins after take off we were in the cruise at FL 350 if ATC allowed.

Very good rule of thumb for 737 for short flights is that flight level equals the distance (e.g. 360 nm = FL360).

Additionally, FMC OPT cruising level is calculated by using a predetermined minimum cruise time (taking into account common stuff like weight, etc.). I believe default value is 1 minute, but can be easily changed by the operator if required.

This chart was a part of A320 FCOM in 2008. It's been removed since, for some good reasons, but it was never wrong.

http://s19.postimg.org/6rw205qlr/shortrange_OA.jpg

Great info Guys, interesting stuff.
Many thanks.

Check Airman,
I had them for the B767-200/300 and B747 Classic, never had one for B744, a short sector was a rarity.
I got them from Boeing by asking, they are proprietary information for customers, that probably explains not being on the net.
As an example (from memory) a 350m flight would be around FL270 for a B767-200 with JT9-7RE, ---- the others were not much different, all below FL300, the level of max TAS for the M/IAS was significant.
Tootle pip!!

LeadSled & FlightDetent,

Thanks for the info.

Had a sector in an Airbus that despite both the Captain and myself being sick getting over a stinking cold the airline begged us to bring the aircraft back.
We agreed as long as we could keep the aircraft pressurised at ground level to avoid ear problems.
We flew home very low (can't remember exactly how low, but low)

I was amazed at how little the fuel usage deviated from the normal cruise figures.

Thread drift but this is a pet peeve....

"Tourist", what would be your plan of action if you blew an outflow valve off the airplane, or took a bird thru the windscreen? If you were both so sick that you needed to keep the airplane pressurized to "ground level", almost any pressurization issue would have undoubtedly incapacitated you both, either blowing out your eardrums or giving you blinding pain thru a sinus-block.

I have had to place an oxygen mask on a captain who passed out for precisely that reason once in the distant past. Rapid decompression at about 12000 feet in the climb. Which means, went from a cabin altitude of 2000 ish feet to 12000 ish feet in a matter of seconds. NOT a big deal in the sense of blowing a window out at FL450, but he PASSED OUT FROM PAIN, as he was "a little stuffed up". IIRC a couple of passengers in the back, also flying sick, wound up with ruptured eardrums.

After making captain myself, I did not come to work with even the vaguest hint of a cold, and sent people home who came to fly "stuffed up". If that meant delaying the flight for a couple of hours till crew scheduling could scrounge up a reserve F/O, so be it. If people got angry, tough. Say the word "SAFETY" enough times to enough people, and EVERYBODY shuts up.

You guys are lucky, and definitely made a questionable decision to fly that day.

I don't think you actually read my post.

How could we have a pressurisation issue if we were barely pressurised?

FlyingStone - a similar rule of thumb was used on the corporate jet I used to fly, which had no such planning charts.

Cruise level = ground distance adjusted by a fudge factor for especially strong head/tail wind. Worked very nicely.

I have seen LIDO come up with very different answers for the same short route (~300nm) on the 738 in similar wind conditions - some solutions were FL190, others 330-350 with the aforementioned one minute cruise which in practice due to ATC issues always ended up with you stuck very high indeed.

A similar short trip cruise graph to that posted by FlightDetent for the 320 is in the NG simplified flight planning section of the performance manual (2.2.1 for those who have access) with curves for cruise time of 1, 5, 10, 15 and 20 min. The minimum trip fuel line runs from around 5 min cruise at 50nm FL100 to 14 min cruise at FL400 for 260nm.

Tourist, I don't think you read his.

If you were to fly normally at FLs in the high 30's, you'd have a cabin pressure altitude of somewhere less than 8000'.

You elected to fly somewhere about 20,000' so that your cabin could be kept at sea level. Great. Except you are not "barely pressurised". You are at max diff!

No, you also did not read what I wrote.

You guessed that we flew at 20000ft.

We did not.