B767 Aerodynamics
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B767 Aerodynamics
First post from a newbie, so please redirect me if this should be in another section. I am a 767 line pilot and a sim instructor and I have been unable to get answers to questions about high speed flight characteristics of the 767. My belief from anecdotal information is that the 767 has no bad habits such as mach tuck, but it would be nice to have a better reference than "I have heard" when asked the question.
I am looking for info, books, papers etc. discussing Boeing's flight testing of the 767 in the high speed regime. Specifically, what Mach was it tested to, what is MMO predicated on (structural or aerodynamic limit?), and what are the flight characteristics once MMO is exceeded? My AOM provides little or no information on the topic.
Any Boeing experts on this forum?
I am looking for info, books, papers etc. discussing Boeing's flight testing of the 767 in the high speed regime. Specifically, what Mach was it tested to, what is MMO predicated on (structural or aerodynamic limit?), and what are the flight characteristics once MMO is exceeded? My AOM provides little or no information on the topic.
Any Boeing experts on this forum?
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I don't have any 767 data but it is the norm in UK certification that all airspeed and IMN limits quoted for service use have to be exceeded by 10% in testing.
Well I would have to disagree with that one.
In our AFM for the 75/67 there is no information on what MAch the aircraft was tested to or specific information of flight characteristics above MMO.
The standard 'the aircraft exhibits good handling characteristics throughout the flight envelope' disclaimer is about all there is.
Not that I have any reason to believe anything adverse, but what the poster of the thread is asking is not normally found in most commercial transport flight manuals.
Someone must know though !
In our AFM for the 75/67 there is no information on what MAch the aircraft was tested to or specific information of flight characteristics above MMO.
The standard 'the aircraft exhibits good handling characteristics throughout the flight envelope' disclaimer is about all there is.
Not that I have any reason to believe anything adverse, but what the poster of the thread is asking is not normally found in most commercial transport flight manuals.
Someone must know though !
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It's a couple of years since I last flew the B767 but I seem to remember this topic being discussed before in these forums somewhere. I think the question was something to do with "Does the B767 have/need a Mach Trim system?" References to the MEL and AFM were made which discussed the sort of question you are asking.
I will have a trawl and see if I can find anything for you.
I will have a trawl and see if I can find anything for you.
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From the Boeing 757/767 FCTM:
"The airplane exhibits excellent stability throughout the high altitude/ high Mach range. Mach buffet is not normally encountered at high Mach cruise.The airplane does not have a Mach tuck tendency.
With Mach trim inoperative, the airplane exhibits a neutral trim change when accelerating to speeds approaching MMO. When the Mach trim system is operative, the airplane exhibits a nose up trim change that increases steadily as the airplane accelerates to speeds approaching MMO. The stabilizer motion associated with this nose up trim change is imperceptible without careful scrutiny of the aisle stand indicator.
As speed nears MMO, drag increases rapidly. At high weights, sufficient thrust may not be available to accelerate to MMO in level flight at normal cruising altitudes."
"The airplane exhibits excellent stability throughout the high altitude/ high Mach range. Mach buffet is not normally encountered at high Mach cruise.The airplane does not have a Mach tuck tendency.
With Mach trim inoperative, the airplane exhibits a neutral trim change when accelerating to speeds approaching MMO. When the Mach trim system is operative, the airplane exhibits a nose up trim change that increases steadily as the airplane accelerates to speeds approaching MMO. The stabilizer motion associated with this nose up trim change is imperceptible without careful scrutiny of the aisle stand indicator.
As speed nears MMO, drag increases rapidly. At high weights, sufficient thrust may not be available to accelerate to MMO in level flight at normal cruising altitudes."
Last edited by 763 jock; 15th Jun 2007 at 16:28.
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It most likely will be affected by Mach Tuck. Although I am no "Boeing expert", I just took a look at a "comparable aircraft". The tendency to have adverse pitch stability above a certain Mach number is more or less inevitable for subsonic profiles. Additional effect is that the elevator will lose efficiency quite quick when going over MMO and ultimately MD.
And it actually goes quite quick: the pitch stability switches sign within 0.03 Mach counts.
And it actually goes quite quick: the pitch stability switches sign within 0.03 Mach counts.
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hi, i'm not the greatest expert in this field but the question of mach tuck came up at work the other day staring at the hori-stab.
An old LAME was with me while we where performing elevator free-play checks (i'm an AME at Qantas Brisbane Heavy Maintenance), i asked him why the hori stab has a percieved wash out, he told me it was for mach-tuck alleviation, i had to check wikipedia first to see what that ment. Anyway as he explained it, as the aircraft approaches critical mach, the hori stab produces a lifting moment which inturn induces an elevator control input the opposite direction. Feel free to bag me but this is the information i have, anyway no stones can hurt me as i leave oz in three days time for my first ever oshkosh .
An old LAME was with me while we where performing elevator free-play checks (i'm an AME at Qantas Brisbane Heavy Maintenance), i asked him why the hori stab has a percieved wash out, he told me it was for mach-tuck alleviation, i had to check wikipedia first to see what that ment. Anyway as he explained it, as the aircraft approaches critical mach, the hori stab produces a lifting moment which inturn induces an elevator control input the opposite direction. Feel free to bag me but this is the information i have, anyway no stones can hurt me as i leave oz in three days time for my first ever oshkosh .
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I cannot comment specifically about the B767, but most aircraft experience a raise of pitch stability and then a sudden drop. Hence, an aircraft would experience a slight pitch-up before going beyond critical Mach. However, today with dozens of computers in the loop the establishment of an automatic system seems also appropriate.
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"I have heard" that the US registered 767's have a higher Vmo/Mmo than the UK reg ones, due to different buffet margins in certification. I would love to know if this is true or not. The ones I fly are limited to Mach 0.84, but the link below says Mach 0.86.
Certainly at high altitude the speed doesn't seem that stable, but I put that down to the autothrottle. Without it, at a constant (correct) thrust setting, the speed stays very stable indeed.
This place
http://www.smartcockpit.com/plane/boeing/B767/
seems to have some useful information - although I would treat it with the usual prudence.
Certainly at high altitude the speed doesn't seem that stable, but I put that down to the autothrottle. Without it, at a constant (correct) thrust setting, the speed stays very stable indeed.
This place
http://www.smartcockpit.com/plane/boeing/B767/
seems to have some useful information - although I would treat it with the usual prudence.
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The 767 isn't fitted with a Mach Trim system, unlike the 757.
As far as I am aware, the Boeing certification team dived the 767 up to .96mach with no adverse mach Tuck tendency, hence no Mach trim system.
As far as I am aware, the Boeing certification team dived the 767 up to .96mach with no adverse mach Tuck tendency, hence no Mach trim system.
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Are you sure about that "no Mach trim" statement? The NTSb report on the Egyptair B767 specifically mentions Mach trim in its discussion of the pitch axis controls....
The 767 autoflight systems include the autopilot/flight director, yaw damper, automatic stabilizer trim, Mach trim, maintenance monitoring, instrument landing system deviation monitor, and thrust management systems. The thrust management system includes autothrottle control.
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Are you sure about that "no Mach trim" statement? The NTSb report on the Egyptair B767 specifically mentions Mach trim in its discussion of the pitch axis controls....
There are certainly no QRH or failure items for it in the FCTM (flight crew training manual)
A non Boeing Link
767 Autoflight Systems Information
The 767 autoflight systems include the autopilot/flight director, yaw damper,
automatic stabilizer trim, Mach trim, maintenance monitoring, instrument landing system
deviation monitor, and thrust management systems. The thrust management system
includes autothrottle control.
Straight out of the NTSB report...although there is no Mach Trim light on the 767 overhead.
Last edited by haughtney1; 3rd Aug 2007 at 21:05. Reason: getting my facts right!
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IIRC (and it's been nearly 25 years since I worked at Everett) Mach Trim is a sub-function of the AP on the 767 like it is on a lot of other aircraft. 757s have a seperate Elevator & Pitch Trim (it's the only 757-only EICAS Caution I beleive like the Aileron Lockout Failure Caution is the only 767-only one - otherwise the EICAS list is identical). I remember going on to the first GE-powered '67 with one of the EICAS design team and seeing the Elevator Pitch Trim message - that's how I know the difference. Back to the drawing board as it wasn't on the PW version. BTW the '67/'57 EICAS Lead Engineer, "Ed" Farhner, just retired from BFTC today - only about the third time 
- we're betting he'll get bored again....
- we're betting he'll get bored again....
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Hi guys.
Found this in my notes.
http://www.aerospaceweb.org/question...cs/q0137.shtml
Maybe that can help....
Greetings,
PW2040
Found this in my notes.
http://www.aerospaceweb.org/question...cs/q0137.shtml
Maybe that can help....
Greetings,
PW2040
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Does this help?
Applicable to 767.
Applicable to 767.
MACH TRIM MODE
General Description
The mach trim mode controls the stabilizer trim during flight when
no autopilot is selected (no FCC engaged) and no other stabilizer
trim command exists. The controlling SAM commands the stabilizer
trim as a function of change in mach number to enhance the
longitudinal stability of the airplane. A speed increase results in an
airplane nose up trim while a speed decrease results in an airplane
nose down trim.
NOTE:
When not engaged, the mach trim control law is referenced
to stabilizer position. After engagement, the control law
commands the stabilizer trim system to reposition the
stabilizer in response to variations in Mc (computed mach).
The mach trim mode controls the stabilizer trim during flight when
no autopilot is selected (no FCC engaged) and no other stabilizer
trim command exists. The controlling SAM commands the stabilizer
trim as a function of change in mach number to enhance the
longitudinal stability of the airplane. A speed increase results in an
airplane nose up trim while a speed decrease results in an airplane
nose down trim.
NOTE:
When not engaged, the mach trim control law is referenced
to stabilizer position. After engagement, the control law
commands the stabilizer trim system to reposition the
stabilizer in response to variations in Mc (computed mach).
Operation
Both SAM’s contain the mach trim circuits and receive mach data
from the two ADC’s.
After initialization the controlling SAM automatically engages the
mach trim mode and commands the stabilizer to move at half rate of
speed when:
• The airplane is in the air (20 sec delay).
• No manual electric trim switches input is being made.
• No alternate electric trim switches input is being made.
• No FCC is engaged (no autotrim)
• Flaps and slats are retracted.
• The airplane speed changes.
• No SAM fault is present.
The mach trim schedule provides for a greater stabilizer trim
correction as the mach number increases.
Movement of the elevator control column in an opposite direction to
the airplane longitudinal trim causes the elevator control column
cutoff switches to stop the mach trim mode.
Both SAM’s contain the mach trim circuits and receive mach data
from the two ADC’s.
After initialization the controlling SAM automatically engages the
mach trim mode and commands the stabilizer to move at half rate of
speed when:
• The airplane is in the air (20 sec delay).
• No manual electric trim switches input is being made.
• No alternate electric trim switches input is being made.
• No FCC is engaged (no autotrim)
• Flaps and slats are retracted.
• The airplane speed changes.
• No SAM fault is present.
The mach trim schedule provides for a greater stabilizer trim
correction as the mach number increases.
Movement of the elevator control column in an opposite direction to
the airplane longitudinal trim causes the elevator control column
cutoff switches to stop the mach trim mode.
Maintenance Practices
A failure of the controlling SAM to perform in mach trim mode
results in the other SAM taking over control. Depending upon the
cause of the failure, faultballs are set on the failed SAM. Failure of
both SAM ’s to perform the mach trim function is not annunciated.
Mc (mach) data which is identified as failed by the ARINC 429
status matrix is discarded and the previous valid value is used. If
four or more invalid values of a parameter are received within eight
successive samples, the failure is set as an ADC fault.
A failure of the controlling SAM to perform in mach trim mode
results in the other SAM taking over control. Depending upon the
cause of the failure, faultballs are set on the failed SAM. Failure of
both SAM ’s to perform the mach trim function is not annunciated.
Mc (mach) data which is identified as failed by the ARINC 429
status matrix is discarded and the previous valid value is used. If
four or more invalid values of a parameter are received within eight
successive samples, the failure is set as an ADC fault.
757s have a seperate Elevator & Pitch Trim (it's the only 757-only EICAS Caution I beleive
This tallies with the highlighted text above which is from the 767.
Last edited by Spanner Turner; 24th Aug 2007 at 12:15.
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In addition, this thread may possibly be of interest to you.
http://www.pprune.org/forums/showthread.php?t=289112
http://www.pprune.org/forums/showthread.php?t=289112
