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B737 automatic and navigaton question

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B737 automatic and navigaton question

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Old 12th Jun 2018, 00:24
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B737 automatic and navigaton question

Hi All,

Automatic Flight Takeoff: When you press TOGA on the Classic and NG the FD bar roll command maintains wings level till 400 ft RA.

F/D Go–Around & A/P Go–Around: F/D roll commands hold current ground track at or below 50 feet AGL for NG and above Above 400 feet RA for the classic.

1. Why does the track guidance differ for a go-around and take off? (I think the case of a big crosswind etc…)

2. Two engine F/D Go–Around & two engine A/P Go–Around: When you press TOGA, the command airspeed cursor automatically moves to a target airspeed for the existing flap position based on maximum takeoff weight calculations, why is this logic used for the classic and NG flap retraction based on a max takeoff calculation?

3. On the classic, the EGPWS contains a internal GPS, is this the same for the NG?

Thank you.

Last edited by downwind; 12th Jun 2018 at 01:41.
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Old 12th Jun 2018, 10:50
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2. The principle is, during flap retraction, you accelerate towards the magenta speed bug. Therefore it needs to be at a higher speed than current. If it drives to the F speed for the elected flap at MTOW it will guarantee that it is at a higher speed and satisfy the philosophy. IMHO.

Why, after all the previous types did it in simple fashion, was the flap lever designed to control the speed bug in only this one manoeuvre is another topic. During observations in the sim, and reading reports of mess ups, a simple all engine GA seems one that is a common candidate for more practice. Why? Is there a startle factor? Shouldn't be on a bad Wx day, and even on an ATC GA there is no need to rush. It can be a calm manoeuvre. I wonder if the LPC sim training may have some blame in that it is always done close to the ground with sink height being a pass/fail criteria. And then some SOP's have lots of mouth music and config changes low down. Why? Let's climb away, adjust config, gear up and navigate. As to acceleration why not do it the same as every takeoff, the same as the engine failure case and the same as the SE GA case? i.e. at e.g. 1000' manually increase MCP speed. Why design a fancy never used at any other time system to accelerate and cause confusion?
Is it like this on B777 & B787 and thus was it thought let's bring advanced technology back to the dinosaur? I've no idea. There were cross overs from B744 back to B767's, but that was moe in EFIS displays; nothing like this. I've always been curious where Boeing saw the benefit in this method of acceleration for a single uncommon manoeuvre.

Last edited by RAT 5; 12th Jun 2018 at 11:00.
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Old 13th Jun 2018, 15:52
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Originally Posted by downwind View Post
Hi All,

Automatic Flight Takeoff: When you press TOGA on the Classic and NG the FD bar roll command maintains wings level till 400 ft RA.

F/D GoľAround & A/P GoľAround: F/D roll commands hold current ground track at or below 50 feet AGL for NG and above Above 400 feet RA for the classic.

1. Why does the track guidance differ for a go-around and take off? (I think the case of a big crosswind etců)

2. Two engine F/D GoľAround & two engine A/P GoľAround: When you press TOGA, the command airspeed cursor automatically moves to a target airspeed for the existing flap position based on maximum takeoff weight calculations, why is this logic used for the classic and NG flap retraction based on a max takeoff calculation?

3. On the classic, the EGPWS contains a internal GPS, is this the same for the NG?

Thank you.
1. It actually depends on MSNs and if You have preselected LNAV before takeoff for example. In that case LNAV will become engaged at 50ft AGL on takeoff. During go around You also get LNAV above 50 ft AGL for certain MSNs. The latter is a requirement to qualify the aircraft for RNP (AR) approaches.

3. The EGPWS terrain display is correlated to the aircraft GPS position.
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