How much time can jet planes make up?

[Guest Howard Hughes]

Wonder what that little kink in the track is for. Maybe a bit of sight seeing on the way-happened to me flying LA to Vancouver. The pilot even wanted to circle Mt St. Helens but ATC said we would loose our slot.

More than likely a delaying sequence (rather than hold) by ATC.

If you need to lose say three minutes, turn right (or left) 60 degrees, hold that heading for three minutes, then turn back 120 degrees to intercept track, hey presto your fix time is now three minutes later! Works like magic every time!

 

 

[facthunter]

Get a clearance first.They usually extend a heading and then bring you back later or turn left onto heading (120 ?)degrees for identification. You know some other buccar who pays more bribe money has been let in front of you. Nev

 

 

[naremman]

Soory Andy what you loose one way you do not make up on the other. The best situation is when there is NO wind. Any other and you total trip time will be more.Nev, as always is entirely correct. Best explained by the 100 knot puddle jumper that embarks on a 100 mile flight, and then returns on the same track with no change of wind.

 

Time taken to cover the two 100 mile legs "over the top to over the top" in nil wind conditions is 120 minutes.

 

In a constant 20 knot wind, the outward flight with a 20 knot tailwind up the backside is 50 minutes. The return flight with a 20 knot headwind on the nose reduces GS to 80 knots, which gives an ET of 75 minutes. Hence you are 5 minutes adrift of the nil wind scenario.

 

The lower the cruise speed the more pronounced the difference. A Tiger Moth in a 20 knot scenario is 67 minutes one way over a 100 miles, and two hours the other way. And I am confident that a good number of DH-82 pilots have looked down to see the cars outpacing them below!

[boingk]

Waiting for my mum... Flight dj437 was 37 mins late leaving Adelaide... And only 18 mins late landing in Sydney ....I wonder how fast they could do it if they went 100%?

100% power can generally only be used for 2 minutes or some other very marginal figure, and is for emergencies only. Maximum cruise is what they set to during flight, and many do not even go above 75% throttle for takeoff as it increases service loads on the engines to a point where maintenance must be increased. Modern runways and wing enhancement technology (ie slats and flaps) are generally very accomodating, and allow even large jets to get away with this.

 

My father recently had a flight delayed taking off by almost 2 and a half hours, from Sydney to Hawaii. They arrived on time! We discussed this and he mentioned seeing speeds over 100km/h faster than he is used to seeing on such trips, displayed via the seatback flight data channel. I mentioned they would have almost certainly delayed the flight to take advantage of those very tailwinds that got them there so quickly.

 

Jets can indeed make up a lot of time, but not using their engines alone.

 

- boingk

 

 

[Gentreau]

Does that mean that all I need to do next time is buy a helicopter and hover and just wait for Sydney to catch up?

If you can hover at about 120,000 ft yes

 

.

 

 

[Sapphire]

If you can hover at about 120,000 ft yes .

 

Please explain

 

 

[Owi]

Very interesting thread this. Thanks, Dave, for the great insight.

 

I flew on a business trip to Auckland from Sydney recently (my first trip to NZ) and the scheduled flight time was 3hrs 15mins. Nice. Actually - we arrived so early, there wasn't even enough time to finish the one in-flight movie which I started as soon as the seat-belt signs went off. Minor annoyance but I was quite confused. I kept thinking they must be arriving at the wrong island, etc. The actual flight time was 2hrs 15mins - a whole hour early!

 

The bigger problem for me was that I had to wait more than that hour for my host (whom I had never met nor seen before) to collect me from the airport. He had timed his arrival based on the scheduled time plus a little more for me to clear passport control and customs.

 

So, this thread has certainly shed some light on the dynamics of scheduled airline times. Thanks!

 

 

[ayavner]

Please explain

I think he was saying if you could hover outside of the atmosphere and wait for the earth to rotate under you until Sydney appears... ?

 

 

[Gentreau]

That was the idea.

 

Not claiming it would necessarily work of course

 

.

 

 

[av8vfr]

At BNE a few years back and an international 757 was told that all runways were closed from midnight due to maintenance and they would have to divert due to the late arrival (headwinds? scheduling?) of 1245am... They gave it a nudge and landed at 1155pm..

 

I expect the alternative would have cost more than the extra fuel burn to the company..

 

 

[PA.]

It used to take 55 minutes to fly from Adelaide to Melbourne, now it takes 75 minutes. They have slowed down to save fuel so they can bury the boot and make up time.

 

 

[Guest Howard Hughes]

The best situation is when there is NO wind.

Too right, nothing seems to stuff up ATC more than a good tailwind! I don't see why it should matter, the tail/head wind applies to all aircraft!

 

It used to take 55 minutes to fly from Adelaide to Melbourne, now it takes 75 minutes. They have slowed down to save fuel so they can bury the boot and make up time.

Flight time is not much different to what it was years ago, just the stuffing around on the ground has increased, oh and the extra 80+ track miles required to get to runway 16!

 

 

[mAgNeToDrOp]

Went on a recent trip from perth to Kalgoorlie and back on the same day. Took these screenshots out of ozrunways on the iPad. Perth airport was congested, sat on the Tarmac for 40 minutes till we got a turn on the runway. So they had to crank it a bit. We barely had time to drink our coffee and we were in Kalgoorlie, ground speed was around 500 knots.

 

 

On the return trip we left on time but we were told perth airspace was congested again and had to slow down, would be 40 minutes late, banked left ( south) and ground speed was down to about 280 knots. Slower flight back.

 

 

Tuesday and Thursday seem to be the busiest in perth due to FIFO traffic

 

 

[Mark11]

That's because the earth was rotating towards you so Sydney arrived quicker than if the world stood still.I always thought that is correct- especially for long flights from say Denmark

 

[boingk]

That's because the earth was rotating towards you so Sydney arrived quicker than if the world stood still.

I believe this effect is also true for long range shooters and snipers - they must take into account the apparent drift of the bullet due to the rotation of the earth during its flight time.

 

Normally it is not of any significant effect, but if you start stretching ranges out over a kilometer, and especially to those extreme ranges (1500~2000m) you sometimes hear for military snipers, then the effect becomes more pronounced. Believe it or not, at 2000m you must compensate around 10" or so!

 

Cheers - boingk

 

 

[dazza 38]

I believe this effect is also true for long range shooters and snipers - they must take into account the apparent drift of the bullet due to the rotation of the earth during its flight time.Normally it is not of any significant effect, but if you start stretching ranges out over a kilometer, and especially to those extreme ranges (1500~2000m) you sometimes hear for military snipers, then the effect becomes more pronounced. Believe it or not, at 2000m you must compensate around 10" or so!

 

Cheers - boingk

Snipers have to also calculate other effects on the projectile at long ranges.

 

Gyroscopic drift -also known as Spin drift- depending on which way the bullet spins, depends on which way the drift is. clock wise or counter clock wise.

 

I will give some examples thses are all at a 1000 yards.

 

.223 - (5.56 mm) 55 gr projectile drifts side ways 23 inches

 

.308 (7.62 mm) 173 gr projectile drifts side ways 11.5 inches

 

.338 (8.59 mm) 300 gr projectile drifts side ways 6.5 inches

 

This is in NIL wind.

 

The faster the projectile spins the more gyroscopic drift.

 

other calculations have to be made for -

 

Magnus effect

 

Poisson effect &

 

Corolis effect. (which is what I beleive you have mentioned above.)

 

PS- I have know idea if this helps a airliner make up time though.

 

 

[boingk]

PM sent.

 

- boingk

 

 

[Phil Perry]

The biggest factor is winds at cruise level. A Jet aircraft at the optimim cruise altitude cannot go faster or slower because it will get to a high or low speed stall with a speed variation of about 15 knots as a percentage of about 480 knots Average TAS, this is not significant as a consideration. Nev

I had my first ( and only ) flight in a Learjet some years ago, we were flying at FL 420,. . . .Following a long discussion with the crew, I was told by the handling pilot that it was not adviseable to "Hand fly" the aeroplane at this level as the curve between VNE and stall was very narrow; and that autopilot was the only sensible option at these levels. Dunno your name Facthunter, but there is a good aerodynamic reason for this, have you got the formula in basic terms ? This isn't a "wynd up" by the way, as I don't know you since I'm a forum Newbie.

 

 

[Sapphire]

facthunter will probably respond to you as soon as he puts on his cape and eats his spinach.

 

 

[Gentreau]

The phenomonem you're referring to Phil is the narow speed range at high altitude between aerodynamic stall speed and critical mach number.

 

As altitude increases, given a constant True Airspeed (TAS), Indicated Airspeed (IAS) reduces.

 

One interesting effect of this is that you can cruise at high altitude with a TAS in excess of VNE without damaging your aircraft, since VNE is given as IAS.

 

As we all know, the critical angle of attack of a wing flying straight and level can be translated into a fixed stall speed givan as an IAS.

 

Now, when you get silly high, like commercials and Learjets, the IAS at maximum cruise speed (TAS) starts to get quite close to the stall speed.

 

Eventually you reach a height at which the smallest reduction of speed stalls the wing.

 

Now there is another figure which is important and that is the critical mach number of the wing. Try to fly the wing beyond this speed and the air flowing over it starts to go supersonic in some areas and cause massive drag. Since the speed of sound reduces as air gets thinner (as we go higher) the equivalent mach number for the max cruise speed gets closer to the critical mach number.

 

So, at high altitudes, you have a small speed range to play with. Go a bit slower and you get aerodynamic stall, go a bit faster and you exceed the CMN and stall again !

 

 

[Sapphire]

gentreau said: Now, when you get silly high, like commercials and Learjets, the IAS at maximum cruise speed (TAS) starts to get quite close to the stall speed.

 

Interesting, so when flying in that situation you are flying at an angle of attack close to the stall speed which is the high drag, slow speed end of the drag curve. That is inconsistant with wanting to fly at best lift/drag for most economical performance.

 

 

[Gentreau]

However my understanding is that with jets the lift/drag profile is not the only consideration for economy.

 

High altitude gives economy benefits which are presumably greater than the losses caused by flying the wings at a relatively high AoA.

 

Those benefits must be substantial as even the shortest comercial jet flights climb to high altitude for the cruise.

 

 

[facthunter]

Gentreau has the facts there. The air gets very thin and most aircraft at high altitude use the autopilot as it's simply very tiring to fly the plane by hand . It can easily lose many hundreds of feet quite rapidly if you con't concentrate . Above FL 310 an aircraft without a serviceable autopilot has to have more vertical separation from other aircraft, by law.

 

At the LIMIT cruise altitude the plane gets to a situation where the aircraft can be almost stalling in the normal way, (AoA), but has a problem if the speed is increased because the wing has areas of airflow where the airspeed becomes supersonic and shock waves form. You can actually see them on the upper surface of the wing. The wing has reached M crit. A hard rapid buffetting occurs at that point. The plane is not designed to be there. You are in a place called "Coffin Corner". Imagine if you hit turbulence or what would happen if you have "ALT. Hold" engaged and an engine lost power.

 

Sensible pilots stay away from that situation and fly a bit lower to provide a margin from both stall, scenario's. It may still only be a range of 10 knots indicated, so it's not much of a margin. You have a Mach/ Airspeed indicator in one instrument so you are aware of the situation. If you have an "upset" ie fall out of a a high level, you may lose over 10,000 feet before you recover, even if you do everything right.. Nev

 

 

[Sapphire]

Jet engines only work effeciently at high altitude and guzzle lots of gas at lower altitudes. A jet engine I saw at Oshkosh used for ultralights consumed 40 US gallons per hour flying below 10000 feet. If I put one on my phantom Weed Hopper, I will consider making a pressureized flight deck for it and put myself where the airliners fly.

 

FH said: If you have an "upset" ie fall out of a a high level, you may lose over 10,000 feet before you recover, even if you do everything right.. Nev

 

What flying configeration would the a/c have after having stalled. Would it constantly want to spin with you constantly kicking in rudder and "easing the stick forward" What do you have to do right to recover in 10000 feet.

 

 

[facthunter]

Sometimes they wing drop and stall/spiral. You have to stay in the airspeed range where you should be, but as you descend this becomes better. ( larger margin of speed available.) You initially have not enough extra lift to pull out of the spiral, because if you pull the stick back too soon it is immediately stalled. You are also building up a high rate of descent and your trajectory becomes more towards vertical. so the pull-out has to be more carefully executed. You have little extra power available too, because it is not there to be had. The extra drag with the extra lift required can only be provided by height loss. (Potential energy) The minimum height loss with a stall and immediate application of the (little ) extra power is usually 3,000 feet, but with a proper "upset" you have a bit of work to do. Some have fallen to FL150 or thereabouts, often exceeding speed and load limits on the way. The Airfrance Airbus fell flat all the way to the sea, only falling apart at contact. Nev