Boeing vs Airbus and Speed Brakes

[luke3]

This is kind of a nerdy question but as a former engineer it keeps popping up in my head. I've asked a few captains that have flown both but am still not satisfied with the answer

During my brief time on the 320 one of the things I had to get used to was slowing down and getting down. With the speedbrakes out Vls would shoot way up to the point of having to get Flaps 1 out, often well above Green Dot so that I could even select the speed I needed. Now in the *cough* 7ER, *cough* i mean 757 and 767, you can be at clean speed or whatever flap maneuvering speed, with full boards, and not have to worry. Talking to my 737 friends it's not an issue there either. So my question is, why such a big difference?

Are the wings so different to have dramatically different performance? Does Airbus calculate Green Dot differently from Boeing's clean speed? Do they just add a much higher safety margin? Or is Boeing simply a case of "what you don't know can't hurt you?"

 

[Bandit_Driver]

If you are able, keeping the speed up and getting down and then slowing down works better than slowing and then trying to go down.

Vls Represented by the top of an amber strip along the airspeed scale on the PFD. Computed by the FAC based on FMS weight data, and on aerodynamic data as a backup, and corresponds to 1.13 VS1g during takeoff or following a touch and go.

Becomes 1.23 VS1g, after retraction of one step of flaps.

Becomes 1.28 VS1g, when in clean configuration

Above 20,000 ft, VLS is corrected for Mach effect to maintain a buffet margin of 0.2 g. In
addition, VLS increases when the speedbrakes are extended

 

[Minuteman]

I'm curious: is the green dot speed an operating limitation for you?

 

[derg]

This is kind of a nerdy question but as a former engineer it keeps popping up in my head. I've asked a few captains that have flown both but am still not satisfied with the answer

During my brief time on the 320 one of the things I had to get used to was slowing down and getting down. With the speedbrakes out Vls would shoot way up to the point of having to get Flaps 1 out, often well above Green Dot so that I could even select the speed I needed. Now in the *cough* 7ER, *cough* i mean 757 and 767, you can be at clean speed or whatever flap maneuvering speed, with full boards, and not have to worry. Talking to my 737 friends it's not an issue there either. So my question is, why such a big difference?

Are the wings so different to have dramatically different performance? Does Airbus calculate Green Dot differently from Boeing's clean speed? Do they just add a much higher safety margin? Or is Boeing simply a case of "what you don't know can't hurt you?"

Well, having flown both, it's like this if you're on a round dial 757/767: That clean speed you see has a lot of caveats. Load, trend, whatever and it's just a "best estimate". Many times on a heavy 767, you're above clean speed, bringing up the flaps and turning BRLBRBLBRBLRBLRBL there goes your stick shaker because the aircraft had a load on it.

The bus is constantly computing VLS depending on load, atmospheric conditions, trend, configuration, etc so it has a much more realistic (honeset?) representation of VLS.

 

[Bandit_Driver]

I'm curious: is the green dot speed an operating limitation for you?

Airbus will always keep you between GD, S, or F speeds and 340/.80/VFE/VLE

 

[luke3]

Well, having flown both, it's like this if you're on a round dial 757/767: That clean speed you see has a lot of caveats. Load, trend, whatever and it's just a "best estimate". Many times on a heavy 767, you're above clean speed, bringing up the flaps and turning BRLBRBLBRBLRBLRBL there goes your stick shaker because the aircraft had a load on it.

The bus is constantly computing VLS depending on load, atmospheric conditions, trend, configuration, etc so it has a much more realistic (honeset?) representation of VLS.

Thanks this is exactly the explanation I was looking for. It's always eye opening going from Flaps 1 to Up on a heavy 767 and seeing where the PLIs drop for just a moment before disappearing. At times it's awfully close to your current pitch attitude. It's a good reminder especially after weeks of flying relatively light 757s down to Florida.

 

[DE727UPS]

I flew those things for years with round dials and classic PFD. It was an eye opener once with got the upgraded flat panel displays with the PLI. Can't say I didn't hit the stick shaker a time or two when heavy or with the boards out in a turn.

 

[roundout]

I flew those things for years with round dials and classic PFD. It was an eye opener once with got the upgraded flat panel displays with the PLI. Can't say I didn't hit the stick shaker a time or two when heavy or with the boards out in a turn.

Surprise!

 

[derg]

I flew those things for years with round dials and classic PFD. It was an eye opener once with got the upgraded flat panel displays with the PLI. Can't say I didn't hit the stick shaker a time or two when heavy or with the boards out in a turn.

Going to the dark side to the Airbus, seeing how the relationship between configuration, wing load and atmospheric conditions interacted and then thinking back to my round-dial days on the 767, I didn't realize how close I was, often, to the edges of the envelope.

 

[jhugz]

Going to the dark side to the Airbus, seeing how the relationship between configuration, wing load and atmospheric conditions interacted and then thinking back to my round-dial days on the 767, I didn't realize how close I was, often, to the edges of the envelope.

I'm not so sure if this is more of an apples to oranges comparison. Different aircraft, different wings, different aerodynamics...no? I flew the 76 with both the round dials and the upgraded ones with the new glass cockpit that had the low speed awareness built into the speed tape. I never noticed an issue with the 76 and getting close to the low speed envelop unless you were bringing in the brakes at a really obscenely high altitude.

 

[Richman]

Compared to Boeing speed brakes, I always thought the ones on the ‘bus were just there for show, and I could get more drag by sticking my hand out the window.

 

[ERfly]

Compared to Boeing speed brakes, I always thought the ones on the ‘bus were just there for show, and I could get more drag by sticking my hand out the window.

Um...that's the 737. I was always told Idle Descent at 250 knots you'll get 1700 feet per minute. Add speed brakes and you get 1700 feet per minute and vibration.

 

[Richman]

Um...that's the 737. I was always told Idle Descent at 250 knots you'll get 1700 feet per minute. Add speed brakes and you get 1700 feet per minute and vibration.

The brakes on the 727, 757 and DC-9 all actually work. You can come down like a chrome plated manhole cover.

Now get off my lawn.

 

[ZapBrannigan]

Slats and flaps 5 on the DC9 and it would come down like tossing a telephone booth off a cliff.

(Note for kids: Telephone booths were tiny metal rooms where one could put a coin into a telephone and make a call before cellphones were a thing)

 

[SteveC]

...
(Note for kids: Telephone booths were tiny metal rooms where one could put a coin into a telephone and make a call before cellphones were a thing)

Seen in Panama recently:

 

[Wardogg]

Do the kids not know of Dr Who? I thought he was a thing.

 

[ERfly]

The brakes on the 727, 757 and DC-9 all actually work. You can come down like a chrome plated manhole cover.

Now get off my lawn.

The 757 speed brakes actually did a pretty good job. You just had to not be shy using them, especially with winglets.
Old 757 adage:
"Never be afraid to use the spoilers. If you think you need the gear, drop it. If you KNOW you need the gear, get ready to go around."

Never flew the 727 or the 9. The MD-11 spoilers work pretty good, too (said as I'm leaning in my seat because we're now in a slight back because not all of them came up symmetrically).

 

[///AMG]

I did the CTP sims in the 757. They gave me the old dual engine failure to dead stick into SLC scenario. I think i just subconsciously flew the high/low key numbers i knew from the F-16 SFO pattern, and while the boards did help, it wasn’t enough. I was able to do a full chandelle (I guess this is why we learned those in commercial training maneuvers eh?) after doing an unpowered go around with all my excess energy, and land on the opposite direction parallel. I have no recollection of when i configured, dropped gear, whatever. But i do recall thinking “well that one had a whole lot more wing than I’m used to!”

end really cool simulator story

 

[CoffeeIcePapers]

I'm not so sure if this is more of an apples to oranges comparison. Different aircraft, different wings, different aerodynamics...no? I flew the 76 with both the round dials and the upgraded ones with the new glass cockpit that had the low speed awareness built into the speed tape. I never noticed an issue with the 76 and getting close to the low speed envelop unless you were bringing in the brakes at a really obscenely high altitude.

VLS is more akin to an AOA indicator in how it works. It will change as you load the wing, add or subtract spoilers, etc. you probably just never saw the true effect of the speed brake. It’s also way more conservative than a low speed cue.

 

[Bandit_Driver]

The brakes on the 727, 757 and DC-9 all actually work. You can come down like a chrome plated manhole cover.

Now get off my lawn.

I remember the 727 Gear down full boards would just fall out of the air if you needed to get down asap.