Ben Tong 發問於 汽車及交通飛機 · 1 十年前

# 請問點解飛機在高過大約FL240之後飛機愈高飛行速度就愈慢?

thx!

Lool: so the reason is not enough air to burn by engine at high altitude due to flame out (the engine)?

### 3 個解答

• 1 十年前
最愛解答

Again, wilson knows shxt about aircraft and engine......

FL 240 ~ 8000 meter AGL This mean the atmospheric pressure are about 1/3 of 1 atm. (Which mean only 1/3 air at 24000 fts than on ground)

As you may or may not know, jet engine work by burning fuel and air with turbine to pressurize the final product (and reburn if afterburner present), that give thrust to propell the aircraft.

So, in a medium only 1/3 air than atm, you would need 2/3 less fuel to burn with in the jet engine (to achieve the ratio of 1/3:1/3), otherwise engine will flames out (When you burn more fuel than air, the engine will choke and not enough air to completely burn with fuel, meaning the engine will simply stall.)

And when you have to restrict fuel into engine, you go slower, well this is just common sense.

2010-08-29 09:29:57 補充：

Well, just to shut somebody up, I KNOW FL240 not exactly = TA 24,000 fts, depend on QNH, but since i wouldn't want to waste time explain more than i have to, let's say FL240 is 24,000 for the sake of the question. In reality, it can be +- 2000 fts AGL

2010-08-29 19:14:06 補充：

LOL Wilson said i didn't answer the question, but just look at his answer, Brilliant!!!!!

The question is about why a plane would slow at FL240, the answer is at FL 240, you need to lower your engine output if you don't want flame out.

EXACTLY what i said, general or not

2010-08-29 19:15:11 補充：

IF he is asking on Fl180, then i will answer the answer correspondent to FL180, but he didn't, so my answer is correct. By the way, aircraft can only fly full speed at FL100 (Or in US 10,000 AGL as TA is set at 130 in US System )

2010-08-29 19:15:58 補充：

At FL 180, it still the same situation as air is 1/2 the content of atm. So you need to fly half slower not 1/3 of the speed as you would in FL240

2010-08-29 19:16:21 補充：

2010-08-29 19:20:19 補充：

Oh, By the way,this theory work NOT only at jet engine, it work at Piston Engine (old WW2 aircraft), Turbo-prop and jet engine, well, that's the whole she-bang.

2010-08-29 19:21:36 補充：

on not at*

2010-08-31 00:12:09 補充：

Well, yeah adolf is pretty much to the point here, as Airspeed sensor is detected by getting atmospheric pressure thru pitot tubing, which give you a corrected result.

As air is thinner, the actual airspeed related to the aircraft remain unchange.

2010-08-31 00:13:12 補充：

as the actual airspeed is"relative" to the environment, the speed remain unchanged.

Again, i didn't say that there cuz

b.) I am just too lazy hehe

2010-09-01 09:28:19 補充：

Lol Wilson........

2010-09-01 09:29:23 補充：

I don't even want to agrue with wilson anymore. AS HE JUST DID NOT USE HIS BRAIN or rather, he didn't HAVE a brain.

If he would have used it, he would have seen the lisk i posted.

2010-09-01 09:30:41 補充：

At FL180 air content is about 1/2 thinner

At FL240 air content is about 1/3 thinner

At FL520 air content is about 1/10 thinner

2010-09-01 09:32:03 補充：

Now, for people who have half a brain will know, even if the asker rephase the question to this

為什麼飛機在超過大約FL520之後,飛機愈高飛行速度就愈慢?

2010-09-01 09:32:37 補充：

IT WOULD BE THE SAME ANSWER AS IF HE IS ASKING FOR FL240, you just change the FL240 to FL520. Cos the reason behind is exactly the same one.

God, how i hate people with no brain.

2010-09-01 09:33:58 補充：

And for those of you who even care about his last comment.

This is absurd, unless you are an airport groupie, plane spotter or work for that airline, no one will care which flight leave where in WHAT time, pilot have his own thing to do rather than sitting in front of the departure information

2010-09-01 09:35:33 補充：

If you are that good wilson, can you tell me who long it take for SAS to make a trip from Gothenburg-Landvetter Airport to Paris Charles De Gaulle Airport and return?

2010-09-01 09:36:13 補充：

Cos No one want to know what you think, especiallywhen it's not about the question anymore

2010-09-01 09:39:55 補充：

Not SAS, i meant Air France.....

2010-09-01 09:43:02 補充：

Well, or you can change to Stockholm Arlanda Airport, it's up to you.

資料來源： have a pilot license and just started CPL training part time, last week.
• 1 十年前

問者可能問indicated speed

實際上Airspped 沒有減少

這跟indicated speed測量方法及空氣密度有關'

Wilson以他錯誤知識回答

害人甚深

2010-09-06 13:02:40 補充：

wilson 收檔啦

用你低能中學知識答呢D問題

笑死人

• wilson
Lv 7
1 十年前

i would say the plane should be flying at the most "cost-effective" speed.

飛機是用最高速飛行的

**** not necessarily. because flying at full speed "waste fuel" and will be implemented when there's a need (catching up a connection flight etc) after assessing other factors like cost etc.

2010-08-28 18:11:01 補充：

oh... i may have misunderstood your question, sorry.

飛機是用最高速飛行的

--- do you mean Ground speed?

2010-08-29 18:28:30 補充：

<<超過>>大約FL240之後,飛機愈高飛行速度就愈慢

**** the answer given by Lool is just a general rule. but doesn't answer why specifically FL240.

Would that be the speed be higher at FL180?

and the info (info only, not an answer) given regarding jet engine, even if it is correct, doesn't answer the question.

Haaa...

2010-08-31 20:38:03 補充：

the asker was asking there was no significant drop in speed. but why PARTICULARLY at FL240?

2010-08-31 20:38:12 補充：

(everyone can "claim" he is doing PPL, CPL, ATPL. ha.... incl someone who has no knowledge NZ flying from AKL to HKG and then onward to LHR at ~1hr 30mins in between. and claim aircraft must rest after long hauls)

2010-09-02 20:30:52 補充：

2010-09-02 20:32:39 補充：

poor both chinese and english.

為什麼飛機在超過大約FL240之後,飛機愈高飛行速度就愈慢?

This mean below FL240, the speed is fairly the same. and the threshold is FL240. Since the density of atm is a continuum. Telling 1/3 density at FL240 doesn' answer the question.

2010-09-06 16:07:26 補充：

wilson 收檔啦

用你低能中學知識答呢D問題

笑死人

**** If foundation of High School Physic is not well built, how to do CPL (full time / Part time)?

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