V2 and obstacle clearance
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V2 and obstacle clearance
To all the performance gurus,
Can anyone explain in simple terms the effect of changing V2 on the performance especially with regard to after takeoff obstacle clearance?
If V2 cannot be less than 1.2Vs (or 1.1vmca) then surely it is a function of configuration and weight.
So, if we increase the weight (all other factors constant) we increase the V2 speed but what happens to our 1st, 2nd, 3rd obstacle clearance performance?
If we use more flap our stall speed reduces so our V2 would be less? What does this do to the climb gradients?
Help
Can anyone explain in simple terms the effect of changing V2 on the performance especially with regard to after takeoff obstacle clearance?
If V2 cannot be less than 1.2Vs (or 1.1vmca) then surely it is a function of configuration and weight.
So, if we increase the weight (all other factors constant) we increase the V2 speed but what happens to our 1st, 2nd, 3rd obstacle clearance performance?
If we use more flap our stall speed reduces so our V2 would be less? What does this do to the climb gradients?
Help
Per Ardua ad Astraeus
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Not quite sure at what level you are asking this, but if you CAN increase V2 - ie fly a higher speed, the climb performance increases. This I know as 'Improved Climb' and is achieved where runway length permits using a higher rotate speed thence into a higher V2, allowing an increased RTOM.
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RTOM or RTOW, Regulated Take-Off Mass or Weight, is assessed as the most limiting of the Structural weight S, WAT weight W (Weight for Altitude and Temperature), Field Length Limited weight F, and Obstacle clearance weight O. Other factors such as Brake Energy limited weight, En-route clearance, and Landing Go-Around and Landing distance might reduce take-off weight.
The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Obstacle clearance is required as 35 ft over the most limiting obstacle, using the Net flight path, which is the gross flight path penalised by prescribed gradients for each segment.
A lower flap setting gives better climb gradient but takes more take-off distance. A higher flap setting gives shortest take-off distance. But if obstacles are present, all these factors have to be evaluated for the altitude, wind and temperature of the day.
V2 is a function of the weight that comes out of the evaluation. So if you are weight limited to a lower value, the V2 is reduced to that required for the lower weight.
All the above will be given as Flight Manual charts, but will normally be computed by RTOW software to give RTOW charts for each individual runway heading, accounting the unique distances and obstacles of each runway heading. The RTOW software can choose the optimum flap setting for the day.
For non limiting runways generic charts might be used. Your own Aircraft Flight Manual AFM and airline SOPS will determine exactly what is done in each type or airline. The above is what is done broadly speaking for Jetstreams, ATP's, 146's and RJ's in many of the airlines that fly them. The pilot may only see the final RTOW chart, or be presented with his RTOW and speeds by a despatcher, depending on airline.
The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Obstacle clearance is required as 35 ft over the most limiting obstacle, using the Net flight path, which is the gross flight path penalised by prescribed gradients for each segment.
A lower flap setting gives better climb gradient but takes more take-off distance. A higher flap setting gives shortest take-off distance. But if obstacles are present, all these factors have to be evaluated for the altitude, wind and temperature of the day.
V2 is a function of the weight that comes out of the evaluation. So if you are weight limited to a lower value, the V2 is reduced to that required for the lower weight.
All the above will be given as Flight Manual charts, but will normally be computed by RTOW software to give RTOW charts for each individual runway heading, accounting the unique distances and obstacles of each runway heading. The RTOW software can choose the optimum flap setting for the day.
For non limiting runways generic charts might be used. Your own Aircraft Flight Manual AFM and airline SOPS will determine exactly what is done in each type or airline. The above is what is done broadly speaking for Jetstreams, ATP's, 146's and RJ's in many of the airlines that fly them. The pilot may only see the final RTOW chart, or be presented with his RTOW and speeds by a despatcher, depending on airline.
This came up during simulator training recently. On this particular B737 simulator a button on the instructor panel causes the leading edge devices to retract 20 knots below V1 causing a flap/LED disagreement light to come on as well as the take off configuration warning to sound. An obvious abort situation.
But occasionally the stunned mullet phenomenon occurs and while the crew mull over noises and lights and the can't believe whats happening look, the aircraft is rapidly accelerating towards VR. Now, regardless of the rights or wrongs let's say the decision is made to continue the take off with this unknown LED problem. Can anyone suggest what speed above VR would you accelerate to before venturing to rotate? For example there is 15 knots difference in VREF between landing a 737 Classic with all flaps up and landing with only trailing edge flaps up, suggesting this 15 knots is the key to a VR 15 knots in excess of the planned VR. I appreciate there should be no question the crew should abort but if for some reason they did not abort, then what would be a safe VR with LED retracted?
But occasionally the stunned mullet phenomenon occurs and while the crew mull over noises and lights and the can't believe whats happening look, the aircraft is rapidly accelerating towards VR. Now, regardless of the rights or wrongs let's say the decision is made to continue the take off with this unknown LED problem. Can anyone suggest what speed above VR would you accelerate to before venturing to rotate? For example there is 15 knots difference in VREF between landing a 737 Classic with all flaps up and landing with only trailing edge flaps up, suggesting this 15 knots is the key to a VR 15 knots in excess of the planned VR. I appreciate there should be no question the crew should abort but if for some reason they did not abort, then what would be a safe VR with LED retracted?
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Originally Posted by PIK3141
RTOM or RTOW, Regulated Take-Off Mass or Weight, is assessed as the most limiting of the Structural weight S, WAT weight W (Weight for Altitude and Temperature), Field Length Limited weight F, and Obstacle clearance weight O. Other factors such as Brake Energy limited weight, En-route clearance, and Landing Go-Around and Landing distance might reduce take-off weight.
The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Obstacle clearance is required as 35 ft over the most limiting obstacle, using the Net flight path, which is the gross flight path penalised by prescribed gradients for each segment.
A lower flap setting gives better climb gradient but takes more take-off distance. A higher flap setting gives shortest take-off distance. But if obstacles are present, all these factors have to be evaluated for the altitude, wind and temperature of the day.
V2 is a function of the weight that comes out of the evaluation. So if you are weight limited to a lower value, the V2 is reduced to that required for the lower weight.
The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Obstacle clearance is required as 35 ft over the most limiting obstacle, using the Net flight path, which is the gross flight path penalised by prescribed gradients for each segment.
A lower flap setting gives better climb gradient but takes more take-off distance. A higher flap setting gives shortest take-off distance. But if obstacles are present, all these factors have to be evaluated for the altitude, wind and temperature of the day.
V2 is a function of the weight that comes out of the evaluation. So if you are weight limited to a lower value, the V2 is reduced to that required for the lower weight.
Airplanes on takeoff fly in conditions where drag decreases with speed. Therefore, at a given weight, increasing speed would allow better climb gradient... therefore, where the climb gradient is the minimum allowed by regulations, increasing the speeds in each segment would allow increasing weight?
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Attempting here to stay strictly within the framework of the original post, which asks - "Can anyone explain in simple terms the effect of changing V2 on the performance especially with regard to after takeoff obstacle clearance?"
If V2 cannot be less than 1.2Vs (or 1.1vmca) then surely it is a function of configuration and weight.
Yes it is a function of configuration and weight. Up to a certain weight (for a given configuration), V2 will be constant, being constrained by the 1.1 Vmc requirement. Above that weight, V2 is governed by the 'minimum of' 1.2 Vs rule, and will increase with increasing weight. It NEED NOT be equal to 1.2 Vs, there is no upper limit specified to the people doing the performance certification, although practically, it rarely is seen higher than ABOUT 1.35 Vs. 1.2 Vs is inevitably on the low side of Vmd, thus available gradient will be less than optimum, but a good trade-off against runway length. Obviously a V2 of 1.35 Vs will require more runway than 1.2 Vs. If increased runway lengths are available, then the operator may avail himself of the "Increased Speeds" V2, but only if AFM approved. (It is for this reason that operations using V2min are often permitted to carry out 2nd segment Climb at speeds such as V2+10 or V2+15 or so, provided that the aircraft has already reached that speed at the time of the failure).
"So, if we increase the weight (all other factors constant) we increase the V2 speed but what happens to our 1st, 2nd, 3rd obstacle clearance performance?"
Given that all other factors are indeed constant, the 1st segment gradient will be less (but not necessarily longer), the 2nd segment gradient will be less, and the 3rd segment will be longer. This is normal. If we are seeking the Maximum Permissible Takeoff Weight, we will steadily examine increasing weights, with their associated increasing speeds and declining performance, UNTIL the Takeoff is limited (either by runway length or minimum obstacle clearance). Under the same environmental conditions and configuration, but at a lesser weight than the limiting weight, performance will EXCEED the runway length and obstacle clearance requirements. In other words, you're better off. This process of steadily increasing the weight until limiting is normal procedure in establishing RTOWs.
"If we use more flap our stall speed reduces so our V2 would be less? What does this do to the climb gradients?"
Yes, V2 will be less (provided that you are above 1.1 Vmc), and the climb gradient will also be less. against this, the required Runway length will be less, and this may be the more limiting factor. In fact, one of several good reasons for using a greater Flap settings (there are others) are that provided obstacle clearance is not compromised by the reduced 1st, 2nd, and 3rd segment performance, improved runway performance is possible. In short, operation from a short runway may be possible, whereas it is not possible or not viable at the lower Flap setting.
Regards,
Old Smokey
If V2 cannot be less than 1.2Vs (or 1.1vmca) then surely it is a function of configuration and weight.
Yes it is a function of configuration and weight. Up to a certain weight (for a given configuration), V2 will be constant, being constrained by the 1.1 Vmc requirement. Above that weight, V2 is governed by the 'minimum of' 1.2 Vs rule, and will increase with increasing weight. It NEED NOT be equal to 1.2 Vs, there is no upper limit specified to the people doing the performance certification, although practically, it rarely is seen higher than ABOUT 1.35 Vs. 1.2 Vs is inevitably on the low side of Vmd, thus available gradient will be less than optimum, but a good trade-off against runway length. Obviously a V2 of 1.35 Vs will require more runway than 1.2 Vs. If increased runway lengths are available, then the operator may avail himself of the "Increased Speeds" V2, but only if AFM approved. (It is for this reason that operations using V2min are often permitted to carry out 2nd segment Climb at speeds such as V2+10 or V2+15 or so, provided that the aircraft has already reached that speed at the time of the failure).
"So, if we increase the weight (all other factors constant) we increase the V2 speed but what happens to our 1st, 2nd, 3rd obstacle clearance performance?"
Given that all other factors are indeed constant, the 1st segment gradient will be less (but not necessarily longer), the 2nd segment gradient will be less, and the 3rd segment will be longer. This is normal. If we are seeking the Maximum Permissible Takeoff Weight, we will steadily examine increasing weights, with their associated increasing speeds and declining performance, UNTIL the Takeoff is limited (either by runway length or minimum obstacle clearance). Under the same environmental conditions and configuration, but at a lesser weight than the limiting weight, performance will EXCEED the runway length and obstacle clearance requirements. In other words, you're better off. This process of steadily increasing the weight until limiting is normal procedure in establishing RTOWs.
"If we use more flap our stall speed reduces so our V2 would be less? What does this do to the climb gradients?"
Yes, V2 will be less (provided that you are above 1.1 Vmc), and the climb gradient will also be less. against this, the required Runway length will be less, and this may be the more limiting factor. In fact, one of several good reasons for using a greater Flap settings (there are others) are that provided obstacle clearance is not compromised by the reduced 1st, 2nd, and 3rd segment performance, improved runway performance is possible. In short, operation from a short runway may be possible, whereas it is not possible or not viable at the lower Flap setting.
Regards,
Old Smokey
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The WAT weight is prescribed in BCAR, JAR, FAR etc as minimum gradients of climb in the 1st, 2nd and 4th segments of the take-off gross flight path. eg 2.4% gradient for a twin in the 2nd segment.
Mutt
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There is no required 3rd segment gradient, since the 3rd segment is level acceleration. If I recall correctly, Boeing FD's command a 100fpm climb in order to ensure no sink during flap retraction.
Best V2 performance is found at Vimd (minimum drag) which allows max excess thrust for steepest climb angle.
V2 is not normally the BEST speed, but more of a minimum speed to meet required gradients.
If you have a simple "total drag curve" you'll see that with an increase in weight, the graph moves up and to the right, resulting in Vimd occuring at a higher speed.
Large swept wing aircraft have lower coefficients of lift at low speed and therefore have to accelerate to higher speeds for a given takeoff performance. Sometimes runways don't allow the time/room to accelerate to these most favourable speeds, so a slower compromise is made.
If however, you have excess runway, then you can hold the aircraft down, accelerate longer and rotate at a higher speed, allowing a steeping climb angle and therefore increased takeoff performance. This is called V2 overspeed.
V2 is not normally the BEST speed, but more of a minimum speed to meet required gradients.
If you have a simple "total drag curve" you'll see that with an increase in weight, the graph moves up and to the right, resulting in Vimd occuring at a higher speed.
Large swept wing aircraft have lower coefficients of lift at low speed and therefore have to accelerate to higher speeds for a given takeoff performance. Sometimes runways don't allow the time/room to accelerate to these most favourable speeds, so a slower compromise is made.
If however, you have excess runway, then you can hold the aircraft down, accelerate longer and rotate at a higher speed, allowing a steeping climb angle and therefore increased takeoff performance. This is called V2 overspeed.
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There is no required 3rd segment gradient
Not quite the case .. the AFM level acceleration is reduced by an amount equivalent to the notional gross to net climb gradient delta ..
Not quite the case .. the AFM level acceleration is reduced by an amount equivalent to the notional gross to net climb gradient delta ..
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J_T
Did I understand you correctly that the level acceleration phase is actually a slight descent?
The latest revision to our Brazilian jungle jet has included a 1.2% positive gradient in the acceleration phase with quite severe payload penalties.
Mutt
Did I understand you correctly that the level acceleration phase is actually a slight descent?
The latest revision to our Brazilian jungle jet has included a 1.2% positive gradient in the acceleration phase with quite severe payload penalties.
Mutt
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The level acceleration phase will be carried out at a gross level acceleration height so far as the pilot is concerned, and he will be presented with that height. The engineer computes the obstacle clearance on the net acceleration height. eg for a simple example on the JAR25 Jetstream 41 the AFM states the pilot flies gross at 500 ft minimum, the engineer computes the obstacle clearance at 400 ft net. Hence a gross to net margin or safety margin is established in the AFM which on most other types will not be a fixed value, eg the 146 / RJ.
Surely that 1.2% gradient mentioned for the jungle jet is the minimum required 4th segment climb gradient as per JAR25, FAR25 ? (Not that I have first hand knowledge of that jet.)
Surely that 1.2% gradient mentioned for the jungle jet is the minimum required 4th segment climb gradient as per JAR25, FAR25 ? (Not that I have first hand knowledge of that jet.)
Moderator
Did I understand you correctly that the level acceleration phase is actually a slight descent?
No, my good friend .. however, there is a gross to net delta built into the AFM sums similar to the other segments .. ie, the net accel segment extends further than the gross ..
No, my good friend .. however, there is a gross to net delta built into the AFM sums similar to the other segments .. ie, the net accel segment extends further than the gross ..
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Originally Posted by mutt
The latest revision to our Brazilian jungle jet has included a 1.2% positive gradient in the acceleration phase with quite severe payload penalties.
(3) At each point along the takeoff path, starting at the point at which the airplane reaches 400 feet above the takeoff surface, the available gradient of climb may not be less than—
(i) 1.2 percent for two-engine airplanes;
(ii) 1.5 percent for three-engine airplanes; and
(iii) 1.7 percent for four-engine airplanes; and
(i) 1.2 percent for two-engine airplanes;
(ii) 1.5 percent for three-engine airplanes; and
(iii) 1.7 percent for four-engine airplanes; and
7.3.1 An aeroplane may be accelerated in level flight from V2 speed to final take-off climb speed at a height above the take-off surface that is the greater of:
(a) 400 feet; or
(b) the height necessary to achieve obstacle clearance in accordance with
paragraphs 12.1 and 12.2.
7.3.2 During any such level flight acceleration manoeuvre, an aeroplane with the critical engine inoperative must have an available gross gradient of climb of at least:
(a) for a twin-engined aeroplane — 1.2%; or
(b) for a 3-engined aeroplane — 1.4%; or
(c) for a 4-engined aeroplane — 1.5%.
(a) 400 feet; or
(b) the height necessary to achieve obstacle clearance in accordance with
paragraphs 12.1 and 12.2.
7.3.2 During any such level flight acceleration manoeuvre, an aeroplane with the critical engine inoperative must have an available gross gradient of climb of at least:
(a) for a twin-engined aeroplane — 1.2%; or
(b) for a 3-engined aeroplane — 1.4%; or
(c) for a 4-engined aeroplane — 1.5%.
It is the excess available climb performance that is used to accelerate to final climb speed.
Last edited by Ex Douglas Driver; 25th Aug 2006 at 00:20.
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Originally Posted by Ex Douglas Driver
FAR 25.111
Aussie rules (CAO 20.7.1b) say the following:
Note that the 4 engine gross gradient is 1.5% for both the third and fourth segment - not 1.7% as in FAR/JARs. Anyone know why the difference?
Aussie rules (CAO 20.7.1b) say the following:
Note that the 4 engine gross gradient is 1.5% for both the third and fourth segment - not 1.7% as in FAR/JARs. Anyone know why the difference?
FAA and JAA require 1,5 % from trijets and 1,7 % from quads. CAO is satisfied with 1,4 % for trijets and 1,5 % for quads.
Trijets and quadjets can afford to meet the higher one-engine-out performance standards because of their extra engines. But is it really any less safe to operate a trijet or quad with equal engine-out performance to twin, with the same obstacles ahead?
You could argue that the requirement for extra engine-out climb gradient for trijets and quads fails to make best use of the inherent redundancy of having extra engines, imposes unduly heavy restrictions on their payload/fuel capacity and favours inherently less safe twinjet designs.
Do CAO standards favour trijet and quadjet (B747, A340...) use in Australia rather than twins (A330, B777, B787, A350)?
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Originally Posted by chornedsnorkack
But is it really any less safe to operate a trijet or quad with equal engine-out performance to twin, with the same obstacles ahead?
