B737-300 V Speeds and FD correlation

[Centaurus]

My understanding is that when the V speeds for take off are entered into the FMC they have a direct relationship with the flight director commands following an engine failure at V1. However that relationship assumes correct rotation rate.

My question is this: Providing the rotation rate is correct after engine failure at V1, the aircraft should attain V2 and the subsequent FD commands are to maintain V2 speed until level acceleration. But what happens if the pilot is slow to rotate or once airborne fails to continue rotation to the desired attitude and subsequently picks up speed beyond V2 during initial climb.

In that case does the FD demand a higher body angle in order to pull back the speed to V2? I know the FCTM states that the FD will demand a body angle commensurate with the speed attained at the actual time of engine failure. In other words if engine failure occurred at V2 plus 15 then the FD will provide you with the correct body angle to hold V2 plus 15.

But what happens to the FD if due pilot error a slow rotation following engine failure on the ground at V1 occurs. Does it demand a pitch up back to V2 or does it demand a new body angle at some point. And a further question: If V speeds are not entered in the FMC, then does this omission somehow affect the FD demanded body angle in the case of an engine failure at V1?

[TAC inop.]

Commanded speed will be V2 to V2 +15

If your current IAS is > V2 +15, it will draw you back to V2 + 15 as a maximum value

Speeds are ALWAYS entered in the FMC...to wonder what happens if not, is like wondering whether Shrodinger's cat is still kickin'.

[OPEN DES]

I am sure you know. But on a typical day, 15 knots above V2 will give you a better climb gradient than at V2. Coming back to V2 at this stage will give you the benefit of a short 'zoom' climb, but overall V2+15 would still give you a better average gradient. The FD applies the same logic and it makes sense.

[RobinR200]

Centaurus, Q1: the FD will indicate V2. Q2: If u don´t put in your own speeds, the fmc will take its own QRH speeds. The FD function remains the same.

[Cough]

Small point,our tech manual quotes that the AFDS gets its V2 speed from the MCP window (ie not the FMC). Seems sensible to me as you can depart with the FMC inop....

Errrrrrm. Many moons ago when I was new to this FMC thingy (-200 lad) we took off without any perf data in the FMC. We had an aircraft with an aspirated TAT probe so the FMC came up with a full power take off based on that temp. Our speed bugs were set correctly so the takeoff and flight directors were normal (well a little excess thrust thats all-V2 was set in the window!). No drama!

On another line, we regularly do improved V2 takeoffs, with the improved V2 up to 25kt greater than the normal V2. So, wihilst in the sim, I had a go at an engine failure at V2 and V2+15 (ie. in the second case, normal V2+40!). We then worked out the climb gradient and the V2+40 case exceeded the V2+25 case. Keeping V2+15 IS going to help you!

[john_tullamarine]

We then worked out the climb gradient and the V2+40 case exceeded the V2+25 case

Minor point to note ... the use of V2 + 15, or thereabouts, does not reflect the maximum climb performance .. rather a reasonable compromise between better climb and worse takeoff distance ... the curve tends to be a bit flattish ... the absolute best climb is not of much use due to the need for half a continent's worth of runway to get to the requisite speed ...

[RAT 5]

My impression has been, and trying to keep it simple in answer to Centaurus's question, is this:

F.D. minimum speed on SE departure is V2.
maximum V2 + 15

Speed IN = Speed Out. By that I mean if the engine failure occurs between
V2 - V2+15 the F.D. will hold that speed.

If you rotate late/slow and speed is high F.D. will command a pitch up then stabalise at V2 +15. If you rotate too fast/high then F.D will command an attitude to give a minimum V/S to accelerate to V2 and then pitch up to maintain.

Stand correction if not quite so.


Another question:

Does anyone know why on NG's Boeing have altered the white bug to set at
V2 +15 for departure and Vref +20 for approach? This softwear change appeared with no explanation and no-one in our outfit knows why.

[A37575]

Does anyone know why on NG's Boeing have altered the white bug to set at
V2 +15 for departure and Vref +20 for approach? This softwear change appeared with no explanation and no-one in our outfit knows why

My guess is because the FCTM advises that in event of engine failure on final approach (737-800/900) and it is decided to continue approach and sufficient thrust is not available for landing flaps, speed should be increased to 20 knots over the previously set flaps 30 or 40 Vref. This is equal to at least VREF for flaps 15. Hence white bug at Vref + 20.

On other 737 versions, including the Classics and -600 and -700 under similar scenario, the speed is increased to +15 knots above previously set flaps 30 or 40 VREF and this policy is reflected by setting a white bug at VREF +15. That said, the FCTM advice for the 737-400 situation is that too should have the speed increased to +20 knots