Busting the 250kt Speed Restriction

[Fatback 84]

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? 

[PHXMD-11 86]

 

 

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.

[James Bennett 52]

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.

[Fatback 84]

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

[James Bennett 52]

When you say after takeoff how soon after takeoff are we talking?

[Fatback 84]

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. 

[James Bennett 52]

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). 

[PHXMD-11 86]

 

 

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.

[kevinh 409]

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.

[scandinavian13 5,192]

 

 

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.

[Don t moderate me this way 882]

Though it may be 'safe' to operate with flaps extended under 10.000 ft, the 'minimum safe airspeed' is still related to a 'clean' configuration ...

[rjmcgill 15]

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.

[scandinavian13 5,192]

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!

[Richard McDonald Woods 168]

Kyle,

 

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

 

I have found that FAR is an interesting read. They are available via http://en.wikipedia.org/wiki/Federal_Aviation_Regulations.

 

Regards, R

[scandinavian13 5,192]

 

 

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...