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Fatback

Busting the 250kt Speed Restriction

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I'm having a problem with the AP busting the 250kt under 10k speed restriction. After takeoff and as I'm approaching 250kts I see N1 reducing and pitch going up as the AP tries to meet the 250kt restriction. The rate of speed increase is reducing but many times not fast enough as I bust the limit by 2 or 3kts before it settles back down to 250kts. My VNAV screen shows the restriction and I know that AP is trying to meet it but just can't seem to do it. Or is 2 or 3kts considered an acceptable deviation? 

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My VNAV screen shows the restriction and I know that AP is trying to meet it but just can't seem to do it

 

How heavy are you? If you're really light, perhaps try derating the climb thrust.

 

 

 


Or is 2 or 3kts considered an acceptable deviation?

 

Some airlines have a speed deviation of plus or minus 5 knots that is considered acceptable.

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In the 747, depending on the gross weight, the 250kt restriction might not apply. Often real crews will ask for a high speed departure if the 250kt is below the performance requirements.

 

If you are heavy the clean manuvering speed can be as high as 280kt.

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This is on a light 747 so no need to bust the limit. Maybe derating the climb thrust is the answer when relatively light. 

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5k to 6k, sometimes a bit higher. You can see the AP reducing thrust and raising the pitch to meet the restriction but it just doesn't quite make it. Then at 10k the thrust goes up and pitch comes down to get to climb thrust. 

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Yeah it sounds like your low gross weight is the issue. Derate and cutback the engines i would say, as the autopilot is trying to reach too high of a nose up attitude to maintain the speed (i think the max angle is restricted in the CLIMB page of the FMC). 

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You can see the AP reducing thrust and raising the pitch to meet the restriction but it just doesn't quite make it.

 

I wouldn't really worry about a slight (+/-5 knots for less than a minute) excursion of the 250 kt rule. VNAV is good but it isn't perect.

 

 

 


Then at 10k the thrust goes up and pitch comes down to get to climb thrust.

 

That's normal. You're accelerating to the enroute climb speed.

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5k to 6k, sometimes a bit higher. You can see the AP reducing thrust and raising the pitch to meet the restriction but it just doesn't quite make it. Then at 10k the thrust goes up and pitch comes down to get to climb thrust. 

Even at very light weight the autopilot has no difficulty maintaining 250 kt climb within a few knots either side (I just tried it with very little fuel and no payload).  There's no way the A/T will be reducing thrust to help maintain speed though.  In VNAV it's setting climb thrust and speed is controlled by pitch.  If A/T is reducing thrust in this way there's something wrong with your pre-flight set up.

 

Yeah it sounds like your low gross weight is the issue. Derate and cutback the engines i would say, as the autopilot is trying to reach too high of a nose up attitude to maintain the speed (i think the max angle is restricted in the CLIMB page of the FMC). 

The CLIMB page on the FMC shows airspeed for max climb angle (which at my very light weight case was 229, well below the 250 kt speed constraint).  I don't think there's a pitch angle limit (this is a Boeing, not an Airbus).  But you are right that at low weight you ought to use derated thrust otherwise the climb angle (while realistic) can seem excessive.

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I'm having a problem with the AP busting the 250kt under 10k speed restriction.

 

This is a huge simism.  Of course, this depends on where you are, but in the United States this restriction is horribly misunderstood in the sim hobby (and occasionally real world, as well).

 

Most people stop reading at FAR 91.117a, which most people summarize as "no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots."  Unfortunately for everyone else reading their simple quote, they miss the first part of that sentence - "unless otherwise authorized by the Administrator" - and subparts b, c and d.

 

Subpart b limits speeds around the floor airspace of fields in C and D airspace, subject to ATC approval.  Not often hugely relevant to a 744.

 

Subpart c is a similar limitation under B airspace and VFR corridors.  Not often hugely relevant to a 744.

 

Subpart d, however, is the big hitter:

"If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed."  That's all that needs to be said.  Done.  Unfortunately, being the last line in the reg, people don't often read that far.

 

Now, as I know some will come in here from all walks of life saying "you have to request it from ATC," I'll head it off by saying that in the United States, this is not true.  If you hear it on LiveATC, it is not correct procedure.  Even the rules ATC follow have a note to the controller stating that aircraft complying with any part of 91.117 are expected to comply without reporting.

 

 

 

TL;DR:

The 744 is not subject to the 250/10 "Rule," provided it operates at the published (which will be displayed on the FMC Climb Page) minimum safe airspeed.

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This is a huge simism.  Of course, this depends on where you are, but in the United States this restriction is horribly misunderstood in the sim hobby (and occasionally real world, as well).

 

Most people stop reading at FAR 91.117a, which most people summarize as "no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots."  Unfortunately for everyone else reading their simple quote, they miss the first part of that sentence - "unless otherwise authorized by the Administrator" - and subparts b, c and d.

 

Subpart b limits speeds around the floor airspace of fields in C and D airspace, subject to ATC approval.  Not often hugely relevant to a 744.

 

Subpart c is a similar limitation under B airspace and VFR corridors.  Not often hugely relevant to a 744.

 

Subpart d, however, is the big hitter:

"If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed."  That's all that needs to be said.  Done.  Unfortunately, being the last line in the reg, people don't often read that far.

 

Now, as I know some will come in here from all walks of life saying "you have to request it from ATC," I'll head it off by saying that in the United States, this is not true.  If you hear it on LiveATC, it is not correct procedure.  Even the rules ATC follow have a note to the controller stating that aircraft complying with any part of 91.117 are expected to comply without reporting.

 

 

 

TL;DR:

The 744 is not subject to the 250/10 "Rule," provided it operates at the published (which will be displayed on the FMC Climb Page) minimum safe airspeed.

One of the best worded answers to the rule book I have ever seen posted. Good job my man you have done your homework. Well worded by the way and easy to understand. Well done. Cpt out.

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One of the best worded answers to the rule book I have ever seen posted. Good job my man you have done your homework. Well worded by the way and easy to understand. Well done. Cpt out.

 

Thanks!

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Kyle,

Again, clear and concise. I believe that European regs are the same.

 

Thanks!

 

I think the letter of the regs is similar, but I know I've heard a couple people mention that ATC handles speeds differently in some way.  I can't remember what they said exactly, but I believe they're given more latitude to approve speed deviations in cases where it isn't excepted specifically by the reg.  Something along those lines...

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This is a huge simism.  Of course, this depends on where you are, but in the United States this restriction is horribly misunderstood in the sim hobby (and occasionally real world, as well).

Kyle,

 

I knew you'd make this reply as you've often commented on it, quite correctly.  I agree with you, it's not a rule that can never be broken.  But in this case this is not what the OP was asking.  He is flying a 744 at light weight and the FMC has set the speed restriction by default in the flight plan.  So it's actually an FMS speed restriction he's asking about, and the sim's attempt to maintain it, not the regulatory one.

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He is flying a 744 at light weight and the FMC has set the speed restriction by default in the flight plan.  So it's actually an FMS speed restriction he's asking about, and the sim's attempt to maintain it, not the regulatory one.

 

It wasn't really clear either way, to me, and the next post down also referenced crews "asking for it," so I figured I'd chime in just in case.  I can see how you're interpreting it now.  Makes sense.

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This was all duplicated over on the main FSX forum. Bottom line, don't sweat over a few knots either side of your target IAS, especially when you are flying through virtual air.

 

http://forum.avsim.net/topic/407141-250kt-under-10k-restriction-any-rw-controllers-out-there/#entry2661619

 

Kyle, I wish every flight simmer (and so called hardcore Virtual airline) could read your post.

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The controller  also knows your aircraft type, in 10 years of flying the DC10 i've never had to request permission in any airspace world wide. Pilot wise, you should intervene any time the aircraft is doing something you don't like. This happens in the GV sometimes, more so when slowing before 10,000.  Vert speed and speed hold is a good fix to keep a light jet from getting away from you. + 5 knots is not a big deal as on most planes the controller doesn't know your indicated. Routinely im asked my current indicated or mach when they are dealing with spacing issues. when you are alot faster than everyone else, they will figure it out. Keep in mind that ADS can send your speed and other info to the controller.

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Kyle, I wish every flight simmer (and so called hardcore Virtual airline) could read your post.

 

Thanks!  Yeah, there's definitely a reason I shy away from VAs.  Heck, I even got fussed at by a controller, and had to explain to him the concept of the effects of wind on groundspeed (and the fact that the reg references indicated, while he sees ground), on a particularly windy day coming into MEM.

 

Hopefully the few people who see the posts here will correct others, and it'll slowly become more common knowledge.

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Here endeth the lesson & well put!

 

I wish this could be tattooed onto the inside of every new online ATC Controllers skull (especially those with the squeaky hoarse voices & acne spots)

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Hopefully the few people who see the posts here will correct others, and it'll slowly become more common knowledge.

Seeing as how you are so modest, I took the liberty of making your most excellent summary into a new "Tutorial" article:

http://forum.avsim.net/tutorials/article/92-busting-the-250kt-speed-restriction/

:ph34r:

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Seeing as how you are so modest

 

Haha - I think modest might be slightly marring the term when used to describe me, but thanks!  That's actually really cool.

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I took the liberty of making your most excellent summary into a new "Tutorial" article

 

Great move!  Wish that would happen more often here on Avsim.  

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This is a huge simism.  Of course, this depends on where you are, but in the United States this restriction is horribly misunderstood in the sim hobby (and occasionally real world, as well).

 

Most people stop reading at FAR 91.117a, which most people summarize as "no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots."  Unfortunately for everyone else reading their simple quote, they miss the first part of that sentence - "unless otherwise authorized by the Administrator" - and subparts b, c and d.

 

Subpart b limits speeds around the floor airspace of fields in C and D airspace, subject to ATC approval.  Not often hugely relevant to a 744.

 

Subpart c is a similar limitation under B airspace and VFR corridors.  Not often hugely relevant to a 744.

 

Subpart d, however, is the big hitter:

"If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed."  That's all that needs to be said.  Done.  Unfortunately, being the last line in the reg, people don't often read that far.

 

Now, as I know some will come in here from all walks of life saying "you have to request it from ATC," I'll head it off by saying that in the United States, this is not true.  If you hear it on LiveATC, it is not correct procedure.  Even the rules ATC follow have a note to the controller stating that aircraft complying with any part of 91.117 are expected to comply without reporting.

 

 

 

TL;DR:

The 744 is not subject to the 250/10 "Rule," provided it operates at the published (which will be displayed on the FMC Climb Page) minimum safe airspeed.

Very well worded and thought out post. Thank you. Just a little historical perspective. This rule was implemented following a mid-air collision on Dec 17 1960 between a TWA Constellation and a UAL DC-8 over Brooklyn NY and was a logical step in allowing safe operations in crowded air corridors between the piston airliners of the day and the newly introduced jetliners, the two having obviously completely different flight parameters. Regards

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