Flaps – 737 vs 747 #1

Flap usage in Boeing 737 NG and 747-400. Similarities and differences between two PMDG aircraft I fly.

I sourced this piece on the FCTM for Boeing 737 and 747-400.

If you like images – they were all captured with PMDG 737 and 747 in P3D. 

737 and 747 – similarities and differences – introducing new series on C-Aviation.net

I alternately fly PMDG 737 and 747 and it helps me recognize how different they are in some respects, and how related in other. When designing 747-400 Boeing overhauled the whole cockpit getting rid of the traditional analog indicators replacing them with six big CTR screens (subsequently replaced by LCDs). With the automation -400 introduced, Flight Engineer was no longer required. Pilots took his duties – augmented by computers. These modifications were like a clean cut – allowing for introducing all-new avionics. 

It was somewhat different with 737 NG. Earlier versions of the 737 came into service in the mid-80s. With early 80s avionics on board 737 Classic wasn’t exactly outdated. Airlines welcomed the advance from analog gauges to screens, but some retained the old layout of gauges by displaying them on new screens as nothing has changed – an attempt to lower crew retraining costs. Nearly 2000 were 737 Classics served around the world. Switching to a new type, with entirely new systems seemed burdensome. And the established systems functioned. And so Boeing kept them (most of them). 737 NG, which arrived after the 747-400 and after the 777 was, in terms of its systems, far behind both bigger aircraft. But it made sense – the Flight Crew Training Manual is common for all 737s from -300 to -900. A considerable advantage to any airline that uses a mixed fleet of Classics and NGs.

At first glance 737 and 747 show different design philosophies. Making them different in how they handle. They are also different in terms of size. 747 has twice the number of 737s engines. That translates to differences in speed, flap construction and so on…

When I fly both in Prepar3d, alternating between 737 and 747 I see I make mistakes by doing something “the 747 way” in a 737. That is why I keep notes on the differences. Which come especially practical for me – a virtual pilot who spends most of his time in general aviation aircraft flying low and slow. When I find time to fly airliners – my notes come in handy. Sharing them seemed like a good idea.

Different but similar

I do not want to sound like suggesting that both planes being utterly different – switching between them is entertaining. And not that tough. I also fly (although much less) the MD-80 and this is a different beast. Both Boeing planes are, in comparison, similar.

737 and 747 by PMDG

For this series, I use 737 NGX (with -600 expansion) and 747 Queen of the Sky II (with -8 expansion). PMDG has built such a market position that I will not try reviewing their work. Instead, I will present you with some screenshots so you can see how the planes look. I will not judge the excellence of their programming – on this, the only argument I want to present is this – for both 737 and 747, I use the real world manuals. And I did not find a discrepancy. 

Flaps in 737 and 747

The distinction one recognizes immediately is in the flaps range. 1, 2, 5, 10, 15, 25, 30, 40 in a 737. To reduce speed pilot can use flaps 10 with gear up. 

1, 5, 10, 20, 25, 30 in 747. Flaps 20 available when reducing speed with gear up.

FCTM recommends flaps 30 for landing in a 737 and flaps 25 in a 747. Which leaves full flaps for low-speed landings in both aircraft.

To decrease takeoff distance use larger flaps (within the approved range). Larger flaps increase tail clearance on takeoff in a 737. This does not matter that much in a 747, with only a 0.1-degree difference in liftoff attitude.

Flaps retraction – differences

This is what I find most confusing when alternating between 747 and 737. Flap retraction schedule is different.

737: „During flap retraction, selection of the next flap position is initiated when reaching the maneuver speed for the existing flap position.”
747: „During flap retraction, selection of the next position should be initiated when at and accelerating above the recommended flap speed for the new position.”

In a 737 select flaps 5 when the speed reaches “10” mark on the PFD or HUD. In a 747 – select flaps 5 when speed reaches “5” displayed on the PFD.

There is no difference in flap extension schedule. Extend flaps before speed drops below current position indication.

Flaps indicators

737 NG still uses the old analog gauge for flaps. In the 747 you will find flap position displayed on EICAS.

Takeoff configuration

737: flaps positions 1, 5, 10, 15, 25. Larger flaps (in that range) shorten takeoff run. Smaller flaps reduce drag and so fuel burn and noise on takeoff. Tail clearance is a considerable factor when choosing flaps for takeoff – Tail Strike Pitch Attitude of 737-900 is 10 degrees. Liftoff attitude varies from 8 degrees with flaps 1 to 7 degrees with flaps 25. That converts to 33 centimeters of tail clearance at flaps 1 and 64 centimeters at flaps 25. This is not a factor for 737-600 – even with flaps 1, there is a 71-centimeter tail clearance.

FCTM says: “Consider using a larger flap setting for takeoffs at light gross weights. Because of the short fuselage, aft fuselage contact is unlikely in the 737-600”.

From what I have found – airlines often use flaps 5, unless there are reasons for other settings (noise abatement for smaller flaps, the terrain in front of the runway – for larger).

In a 747-400 use flaps 10 or 20 for takeoff. With a 10-degree Liftoff Attitude and 12.5-degree Tail Strike Pitch
Attitude -400 translates to a 1-meter tail clearance. Difference between flaps 10 and 20 in this respect is negligible (3 cm). 

Landing configuration

In both planes set flaps 1 on arrival (STAR or vectors) and flaps 5 before turning to intercept the localizer. Then…

…in a 747 – set flaps 10 (optional) when intercepting heading, flaps 20 when glide slope is alive…

…in a 737 – set flaps 15 when glide slope is alive…

…then set landing flaps when intercepting glide slope.

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