737NG LED's and speed limitations/ops
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However the takeoff with flaps greater then 5 selected allows departures with full extended LEDs.
Retraction failures are possible, mechanical problems or an environmental "impact" could result in a situation where one or more LEDs may not retract to partial, subsequently preventing a complete retraction, something which can be verified on the overhead indicators in LE stay in transit.
On departure, there is thus a risk that there could be one or more fully extended LEDs which you wil not find out about until retracting flaps up as I am sure you will not look during flap retraction on departure at the overhead panel to confirm partial retraction has been achieved...
Thus to prevent an inadvertent acceleration beyond 230kts with the potential that a fully extended LED is present, Boeing decided to prevent VNAV from accelerating above 230kts until LE TRANSIT/EXTENDED lights are extinguished.
On approach, the logic is in reverse.
The LE are retracted and extend to partial with the initial fall selections. They will only be fully extended with selection of flap 10, thus below 210kts, again preventing full extended LED in flight at speeds above 230kts.
Hope this helps
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Hmmm...
What about the SFPs ?
The slats only go full extended beyond flaps 25 !
And his VNAV system logic for departure is just the same of the other versions...
What about the SFPs ?
The slats only go full extended beyond flaps 25 !
And his VNAV system logic for departure is just the same of the other versions...
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But lets just summarise it in the fact that VNAV under no known condition will (autonomously) take the aircraft outside its protected range, or else it would not be certified by FAA without limitations.
So, again I ask, why 230kts? Why not flap-up manoeuvring speed? At least that would be consistent with a non-VNAV take-off, and would actually do more to prevent 230kt overspeed in this LE flap transit event that everyone is panicking about.
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HI everybody for taking the time to reply, I think in particular Skyjob may have hit the (final) nail on the head.
Amazing how quickly you forget - in previous companies on classics there were flap options (1,5,15) for T/O so the potential to T/O with LED fully extended....although F15 was very infrequently used.
In present company on NG only F5 is used at all times.
So either a VNAV or a manual green dot retraction will provide "protection".
On approach VNAV will set the appropriate speed for any flap selected, in MCP operation full LED extended will not occur until flaps 10 selected (210kts max), if you don't bust the flap limitation speeds then approach also has "protection".
All makes sense, great stuff!
NOW the ONLY remaining point: IS THERE, actually, a structural/engineering limitation of 235kts with LEDs fully extended?????????
Or just a Boeing aviation urban myth!!!!!!!!!!
If there IS there would be no need to worry the humble drivers - who already have enough to worry about as (as above) any such limitation would be covered.
Anybody who CAN quote, or better provide, anything from Mr Boeing??
Once again thanks all - greatly appreciated.
Amazing how quickly you forget - in previous companies on classics there were flap options (1,5,15) for T/O so the potential to T/O with LED fully extended....although F15 was very infrequently used.
In present company on NG only F5 is used at all times.
So either a VNAV or a manual green dot retraction will provide "protection".
On approach VNAV will set the appropriate speed for any flap selected, in MCP operation full LED extended will not occur until flaps 10 selected (210kts max), if you don't bust the flap limitation speeds then approach also has "protection".
All makes sense, great stuff!
NOW the ONLY remaining point: IS THERE, actually, a structural/engineering limitation of 235kts with LEDs fully extended?????????
Or just a Boeing aviation urban myth!!!!!!!!!!
If there IS there would be no need to worry the humble drivers - who already have enough to worry about as (as above) any such limitation would be covered.
Anybody who CAN quote, or better provide, anything from Mr Boeing??
Once again thanks all - greatly appreciated.
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Skyboy is correct.
Another poster wondered about VNAV exceeding the flap limitation for a flap 25 takeoff. VNAV commands flap placard -5 knots for take off (so for flap 25 and above acceleration altitude, VNAV commands 185 kts).
Another question was about engine failures and VNAV, the FMC does detect the engine out condition and VNAV commands Vref40+70 above the Engine Out Acceleration Altitude set in Take Off page 2. If there's an emergency turn then SPD INTV is used to prevent the acceleration. The other bonus of 10.8 and VNAV takeoffs is that the autopilot is available with a single engine on takeoff above minimum engagement height (400'). It works well and changes the habits of a lifetime on a 737.
As for the 230 kts limitation... It's not in the limitations since it applies to a non-normal condition. Hence, it's in the QRH, as mentioned, for a LE flap transit light illuminated with trailing edge flaps up, i.e. leading edge device fails to retract. VNAV's acceleration speed logic, makes logical sense: flap placard limit -5 and leading edge failing to retract protection. The leading edge flaps will always be fully extended and the leading edge slats will be partially or fully extended depending on the flavour of 737 for every takeoff. So the system is programmed to ensure that the leading edge devices are fully retracted before commanding an acceleration above 230kts. Watch the LED lights on the overhead panel and ask your colleague when the VNAV command bug moves from 230 to 250 kts.
Outwith VNAV, what you do with the MCP SPD window is what's limited by your fingers and your head. Before VNAV on takeoff and 10.8 logic, VNAV wasn't selected after takeoff until the TE flaps and LEDs were fully retracted for LE speed protection and UP speed was never more than 230 kts for even the heaviest variant, hence VNAV continues to demonstrates LED speed protection.
In respect of the approach, aiming or having to extend flaps at the limit speed points to a mis-managed approach. The limit speeds are limits, not targets, as previously stated. Extending flaps as the appropriate manoeuvering speed is approached is better practice and demonstrates better planning ahead - rather than the descent and approach anxiety commonly seen in some less experienced pilots.
As asked before, why would you set 240 kts before the flaps were retracted? What does it gain and aren't you contravening SOPs as well as the Boeing's directions in the FCTM? They designed it.
Mentioned previously: 240kts in FMC below FL100 is only pre-programmed for descent and it has 250/FL100 for climb on the 737. Look at instrument approach design criteria in ICAO 8168. The range of speeds for initial approach is 160-240kts for a CAT C aircraft. Cat D is 185-250. If a Boeing Cat D aircraft FMC also has 240/FL100 in the descent page then logically it's a buffer for the speed limit. Why is that "buffer" not applied in the climb? It's been a while and I can't recall for Cat D. Add to this that most Boeing Cat D aircraft using the MTOW logic for flaps up manoeuvering speed will have a speed of more than 250 kts when clean after takeoff. If that logic isn't used for Cat D then at most long-haul weights, the actual clean speed will still be >250.
Structural limit for -800 fully extended leading edge devices? The TE limit would apply and so the relevant limit would be the F30 limiting speed (175kts) for SFP and Flap 10 (210kts) for non SFP. For the purist, with no trailing edge flaps and fully extended leading edge devices, it would be another non-normal condition and the QRH for Trailing Edge Flaps Up Landing has a 230 kts maximum when the alternate flaps system is used to extend the leading edges.
Another poster wondered about VNAV exceeding the flap limitation for a flap 25 takeoff. VNAV commands flap placard -5 knots for take off (so for flap 25 and above acceleration altitude, VNAV commands 185 kts).
Another question was about engine failures and VNAV, the FMC does detect the engine out condition and VNAV commands Vref40+70 above the Engine Out Acceleration Altitude set in Take Off page 2. If there's an emergency turn then SPD INTV is used to prevent the acceleration. The other bonus of 10.8 and VNAV takeoffs is that the autopilot is available with a single engine on takeoff above minimum engagement height (400'). It works well and changes the habits of a lifetime on a 737.
As for the 230 kts limitation... It's not in the limitations since it applies to a non-normal condition. Hence, it's in the QRH, as mentioned, for a LE flap transit light illuminated with trailing edge flaps up, i.e. leading edge device fails to retract. VNAV's acceleration speed logic, makes logical sense: flap placard limit -5 and leading edge failing to retract protection. The leading edge flaps will always be fully extended and the leading edge slats will be partially or fully extended depending on the flavour of 737 for every takeoff. So the system is programmed to ensure that the leading edge devices are fully retracted before commanding an acceleration above 230kts. Watch the LED lights on the overhead panel and ask your colleague when the VNAV command bug moves from 230 to 250 kts.
Outwith VNAV, what you do with the MCP SPD window is what's limited by your fingers and your head. Before VNAV on takeoff and 10.8 logic, VNAV wasn't selected after takeoff until the TE flaps and LEDs were fully retracted for LE speed protection and UP speed was never more than 230 kts for even the heaviest variant, hence VNAV continues to demonstrates LED speed protection.
In respect of the approach, aiming or having to extend flaps at the limit speed points to a mis-managed approach. The limit speeds are limits, not targets, as previously stated. Extending flaps as the appropriate manoeuvering speed is approached is better practice and demonstrates better planning ahead - rather than the descent and approach anxiety commonly seen in some less experienced pilots.
As asked before, why would you set 240 kts before the flaps were retracted? What does it gain and aren't you contravening SOPs as well as the Boeing's directions in the FCTM? They designed it.
Mentioned previously: 240kts in FMC below FL100 is only pre-programmed for descent and it has 250/FL100 for climb on the 737. Look at instrument approach design criteria in ICAO 8168. The range of speeds for initial approach is 160-240kts for a CAT C aircraft. Cat D is 185-250. If a Boeing Cat D aircraft FMC also has 240/FL100 in the descent page then logically it's a buffer for the speed limit. Why is that "buffer" not applied in the climb? It's been a while and I can't recall for Cat D. Add to this that most Boeing Cat D aircraft using the MTOW logic for flaps up manoeuvering speed will have a speed of more than 250 kts when clean after takeoff. If that logic isn't used for Cat D then at most long-haul weights, the actual clean speed will still be >250.
Structural limit for -800 fully extended leading edge devices? The TE limit would apply and so the relevant limit would be the F30 limiting speed (175kts) for SFP and Flap 10 (210kts) for non SFP. For the purist, with no trailing edge flaps and fully extended leading edge devices, it would be another non-normal condition and the QRH for Trailing Edge Flaps Up Landing has a 230 kts maximum when the alternate flaps system is used to extend the leading edges.
Last edited by shlittlenellie; 19th Nov 2015 at 11:18.
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Extract from Systems manual:
Some people are are adjusting this speed in the DES PATH page to 250 its, in which case the monitoring will be 250-260 its.. not recommend!
If the aircraft cannot decelerate it will fly level until the speed is achieved and the path will slide down.
The "structural limit" for ONE leading edge device extended is 300 its, but Boeing allow for the worse case of multiple devices and thus limit to 230 kts. The limit is not prescribed by the trailing edge devices as if the alternate system is used to extend ONLY the trailing edge devices can be retracted and in the event of a go-around you would need to be above the trailing edge device speed with the LEDs still extended.
Normally, the target speed is economy speed above the airspeed restriction altitude and 240 knots below that altitude, until deceleration for approach. VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots. The start and end of the airport speed restriction deceleration segment is shown on the map as a green open circles with no labels.
If the aircraft cannot decelerate it will fly level until the speed is achieved and the path will slide down.
The "structural limit" for ONE leading edge device extended is 300 its, but Boeing allow for the worse case of multiple devices and thus limit to 230 kts. The limit is not prescribed by the trailing edge devices as if the alternate system is used to extend ONLY the trailing edge devices can be retracted and in the event of a go-around you would need to be above the trailing edge device speed with the LEDs still extended.
And framer: yes VNAV would have kept you safe, no need for LVL CHG or V/S
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Wow, you've obviously never flown a B737 in VNAV. You're saying that leaving it in VNAV at 230kts, the aircraft will not exceed 230kts? Yet you also say it selects 240kts to provide a margin for 250kt speed limit below 10000'.
"**** in, **** out" comes to mind when you refer to VANV not maintaining 230kts in a DESC, as VNAV can only be as good as the data you put in the forecast pages for it to calculate a CDA. Incorrect winds are the most common problem when experiencing this, every second FO does that when starting on the line, not realising... An accurate forecast model allows for an accurate CDA with the VANV PTH speed as defined by CI.
In climbout, VNAV PTH acts similar to LVL CHG and pitches for the FMC commanded speed, that speed being defined by a virtual point 180nm after takeoff, based on the track direction at this position, the wind direction and speed as per TOC WIND, FL entered, ISA Dev and CI.
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In climbout, VNAV PTH acts similar to LVL CHG and pitches for the FMC commanded speed, that speed being defined by a virtual point 180nm after takeoff, based on the track direction at this position, the wind direction and speed as per TOC WIND, FL entered, ISA Dev and CI.
19th Nov 2015 13:04
19th Nov 2015 13:04
Agreed if you don't put in the forecast winds in the DES FORECAST page or forget engine anti-ice ON/OFF, to allow for increase in N1, the path is not calculated correctly and VNAV will not retain the DES SPD as shown with Msg " drag required" in scratch pad. If VNAV cannot manage the path then VNAV SPD will appear in FMA and possibly " unable next altitude" in scratch pad. A quick look at LEGS route DATA will help you to see the times predicted at each waypoint. Its not rocket science to figure if the aircraft can loose 100 its and 10000 feet in 2mins.. well it is to some;(
CLB PAGE:
The page title displays the type of climb. Normally, the title displays ECON for the economy
climb mode. Fixed speed climbs modify the title.
XXX LIM CLB indicates the limit speed, XXX, is based on leading or trailing edge flaps:
• target speed is 5 knots below trailing edge flap placard speed
• speed is limited to 230 kts if leading edge devices are not completely retracted
ECON indicates the speed is based on a cost index.
MAX RATE indicates the speed is based on the maximum altitude over the shortest period of time.
MAX ANGLE indicates the speed is based on the maximum altitude over the shortest horizontal distance.

Flight Management, Navigation Chapter : 12-11 FMC Takeoff and Climb Section : 41
Fixed climb speeds display XXXKT for a fixed CAS climb speed or M.XXX for a fixed Mach climb speed profile. Reasons for fixed speeds are:
• takeoff/climb acceleration segment constraints
• waypoint speed constraints
• an altitude constraint associated with a speed constraint
• a speed restriction
• a crew entered speed.
Displays ACT when the climb phase is active.
Last edited by Avenger; 20th Nov 2015 at 08:16.
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Chesty Morgan, quotes from FMC Supplemantary Data Document explain this as well as the 240/FL100 in descent:
Avenger, please provide references to your quotes regarding "VNAV makes 3 basic calculations based on the initial inputs to the FMC. up to 5000 ( terminal area) 5000-10000 (speed limited) above 15000", its areas of applicability, etc.
We all know a speed restriction in VNAV CLM is limiting acceleration, thus my post stating VNAV CLM acts as LVL CHG did not refer to this, merely to full climb scenario. In fact such speed limits are not adhered to as minimum speeds based on Flap position are observed by VNAV, protecting you from flying too slow. On your next departure try entering 180kts at the third or so waypoint, engage VNAV and treract flaps, it will fly Flap Up speeds to your surprise.
Furthermore, I cannot find any references to VNAV modes or calcuations being different below 5000'.
The thrust reduction below 15,000' as well as 250/FL100 equally have nothing to do with VNAV as it is an autothrottle/FMC function, as such it is avaialble in ALL thrust modes with a closed IAS window, not just VNAV CLM.
The FMC defaults to the economy speed modes for VNAV operation. In addition, by default, a transition climb speed limit is observed below the speed transition altitude during climb (250 knots below 10,000 feet in the USA), and a similar limit, but 10 knots less, during descent (240 knots below 10,000 feet in the USA). The 10 knot margin was added to reduce the probability of exceeding the transition speed limit in turbulence. The FMC increases the transition climb speed at heavier gross weights to equal the flaps-up full maneuvering speed of VREF40 + 70 knots (minimum transition climb speed).
FMC generates CAS/Mach speed schedule based on a fixed speed throughout the climb, as a function of cost index and initial climb weight. The speeds are calculated using a reference point 180nm downpath of origin, at which point the TOC wind component and predicted OAT are used to make corrections to the fixed speed. CAS is increased up to 40kts with 150kts headwind, decreased up to -20kts with 150kts tailwind, increased up to 4kts with ISA -40C and decreased up to -30kts with ISA +40C [no effect between ISA 0C to +10C]. Econ CLB Mach is set equal to ECON CRZ at the estimated initial cruise conditions. Thus to derive the Econ CLB Mach, estimate TOC Weight, Altitude, Wind and Cost Index are required. PERF INIT provides input area for all calculations.
We all know a speed restriction in VNAV CLM is limiting acceleration, thus my post stating VNAV CLM acts as LVL CHG did not refer to this, merely to full climb scenario. In fact such speed limits are not adhered to as minimum speeds based on Flap position are observed by VNAV, protecting you from flying too slow. On your next departure try entering 180kts at the third or so waypoint, engage VNAV and treract flaps, it will fly Flap Up speeds to your surprise.
Furthermore, I cannot find any references to VNAV modes or calcuations being different below 5000'.
The thrust reduction below 15,000' as well as 250/FL100 equally have nothing to do with VNAV as it is an autothrottle/FMC function, as such it is avaialble in ALL thrust modes with a closed IAS window, not just VNAV CLM.
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Hi all
Just to finish off my original question - info from the maintenance dept shows that 230kts is the point after which (if leading edge devices are fully extended) inspection(s) by maintenance are required so obviously comes down to a load limit/airworthiness question.
Phase 1 or 2 inspection (or a mix) depends on speed over 230kts - but that's another thing altogether.
Thanks for playing!
Cheers
galdian
Just to finish off my original question - info from the maintenance dept shows that 230kts is the point after which (if leading edge devices are fully extended) inspection(s) by maintenance are required so obviously comes down to a load limit/airworthiness question.
Phase 1 or 2 inspection (or a mix) depends on speed over 230kts - but that's another thing altogether.
Thanks for playing!
Cheers
galdian
