View Full Version : Alpha Floor/Min Speed Reversion (MSR)
From a discussion on the AMS 737 crash thread I have been shown a clip from a 737 Tech Manual which refers to the 'Alpha Floor' mode and makes no mention of it not being active on a coupled GS.
I have, since 737 early days, and on the NG, been told that MSR was quite different to AF and the two are not to be confused. Can anyone confirm whether it is correct to use 'Alpha Floor' in a Boeing publication? I understood the phrase to be virtually an AI 'copyright'?
Can anyone also CONFIRM whether MSR is active on a coupled GS or not? My tech manual is for NG but dated 2002.
To quote from our manual in Normal Procedures, Approach, ILS Approach, CAT I:
Prior disengaging the A/P, the A/T should be left in the ARM mode by deselecting the SPEED-Mode on the MCP. This allows manual throttle control without interference from the A/T system. However, minimum speed reversion protection and automatic A/T GA are provided when TOGA is pressed.
From personal experience that is true both on the classic and the NG (and the OM B quoted covers both variants), however that only covers flight director coupled approaches, i have no idea if that works on autopilot coupled approaches with manual thrust nor if it works on both autopilot and A/T coupled approaches.
Sorry if that isn't the answer you were looking for.
c100driver
7th Mar 2009, 17:50
I would love to know where the "alpha floor" statement in a Boeing B737 manual came from? I am picking Boeing would like to know as well!
All my manuals only mention Min Speed Reversion!
Minimum Speed Reversion can in no way be classed as ALPHA Floor or even thought of as similar.
Vol 2
Minimum speed reversion is not available when the A/T is off, and the AFDS is in ALT HOLD, ALT ACQ, or after G/S capture. Minimum speed reversion is not available when in VNAV PATH and capturing or flying a level segment.
Actually, i digged a little in our different manuals and found the following excerpt in our OM B III NG:
Autothrottle Engagement
Moving the A/T Arm switch to ARM, arms the A/T for engagement in the N1,
MCP SPD or FMC SPD mode. The A/T Arm switch is magnetically held at ARM
and releases to OFF when the A/T becomes disengaged.
A general summary of A/T mode engagement is as follows:
...
• alpha floor automatically engages the A/T when armed.
PEI_3721
7th Mar 2009, 18:18
One way of remembering the mode logic is to understanding what the elevators are controlling. In GS the task is to maintain the flight path (path on elevator - speed controlled by the AT); after a speed reversion the elevator adjusts pitch attitude to maintain speed according to the thrust level (speed on elevator – power rating).
From GS mode a speed reversion without power change could be hazardous, and if power is correctly selected the end result is similar to TOGA … so from GS why not go to TOGA. This is the design logic.
I also suspect that where Airbus uses alpha as the switching parameter (maintains an alpha floor), speed reversion still uses airspeed as the dominant input even if alpha is introduced somewhere … hence the mistaken terminology – blame the salesmen.
A possible confusion is that pilots think of speed reversion as speed protection. The Airbus concept can be considered as a full protection system (but not totally), whereas speed reversion only applies to some modes – the difference between semi automatic (occasionally/mode dependent) and ‘fully’ automatic. These differences are aspects of critical safety knowledge when operating highly automated aircraft and the issue also applies to systems other than AP/AT.
Tee Emm
8th Mar 2009, 09:21
To quote from our manual in Normal Procedures, Approach, ILS Approach, CAT I:
Quote:
Prior disengaging the A/P, the A/T should be left in the ARM mode by deselecting the SPEED-Mode on the MCP.
The instructions in your company operations manual directly contradict the Boeing philosophy published in the B737 Flight Crew Training Manual which states: The autothrottle ARM mode is not normally recommended because its function can be confusing. The primary feature the autothrottle ARM mode provides is minimim speed protection in the event the airplane slows to minimum maneuvering speed. Other features normally associated with the autothrottle, such as gust protection, are not provided. The autothrottle ARM mode should not be used with Non-Normal checklists. Some malfunctions that affect maneuvering speed cause the autothrottle to maintain a speed above approach speed.
In a previous statement, the Boeing advice is that autothrottle use is recommended during take off and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged.
Having read all that, there is no doubt that many 737 operators deliberately choose to disregard the Boeing advice and with the autopilot disengaged, leave the autothrottle always in the ARM mode by de-selecting SPEED. As Sir Humphrey Appleby would say "that is a courageous decision"
...Now about that class action after a prang..
Actually, that is a Boeing approved SOP for normal operation. Considerations for non normal use are in the relevant checklists. And it is there specificly to provide low speed protection on instrument approaches during manual flight, the throttle is still used manually. Only if the speed drops below Vref you will get autothrust engagement to make sure the speed does not drop further. Once the speed is safely above Vref it will automaticly switch back to ARM.
It can be confusing if you are not aware of that feature and if you are not trained to use it. If you are trained to used it however it is just an additional safety net.
You are right however that in many non-normal situations auto thrust usage should be avoided and that is mentioned in the relevant non normal checklists, usually the first or second step is: Autothrottle ... disengage.
100% agreement there, TeeEmm, althought the g/a ability of reduced power is a nice one to have. BA even USED to teach selecting ARM in the last stages of a manual throttle s/e approach to make the g/a 'easier' in strict contravention of Boeing's rec. Lawyers' benefit as you say.
In my time RHS I have seen a Captain flare too high with ARM and the TOGA function kick in (with humerous results), and also you will find an ACCURATELY flown Vref+5 at some F40 settings (a rare event!) will trigger the TOGA reversion as it can be just below 1.3Vs at some weights..
Any input on the original query?
Denti - you must have a later Training manual than me!
Mine says (my bolding)
Autothrottle Use
To simplify thrust setting procedures, autothrottle use is recommended during
takeoff and climb in either automatic or manual flight. During all other phases of
flight, autothrottle use is recommended only when the autopilot is engaged.
Autothrottle ARM Mode
The autothrottle ARM mode is normally not recommended because its function
can be confusing. The primary feature the autothrottle ARM mode provides is minimum
speed protection in the event the airplane slows to minimum
maneuvering speed. Other features normally associated with the autothrottle, such
as gust protection, are not provided. The autothrottle ARM mode should not be
used with Non-Normal checklists. Some malfunctions that affect maneuvering
speeds cause the autothrottle to maintain a speed above approach speed.
No, it is not later BOAC, it is just different. However we do clear all SOPs first with Boeing and then of course with the relevant authority before they are put to use. And yes, the usage of ARM is indeed one of the very few things we still have from the BA heritage as part of our company once was owned and audited by them (same as DODAR, however that we had to abandon in favor of FORDEC after a recent IOSA audit).
From what i can get out of our technicians and our manuals (although the latter do not say much about it) MSR is not available once APR mode has kicked in, however the A/T should provide low speed protection unless you enter RETARD mode because RA1 is below 27ft, in that case there is no more low speed protection (same for ARM operation by the way, low speed protection: yes, MSR: no, low speed protection during RETARD because auf RA1 < 27ft: no).
Thanks Denti (I sympathise with you.....:)) It's a jungle out there, for real.same for ARM operation by the way, low speed protection. - I'm pretty sure I have seen the A/T cut in ARM
lederhosen
8th Mar 2009, 14:10
We originally used the Boeing approved method and then thanks to some cross pollination from other airlines tried the disarm speed mode. Initially I did find it could be confusing. But then when I had got used to it we changed back, due to some of the reasons already mentioned.
As with everything in life there are advantages and disadvantages. The min speed protection and simpler go-around are obvious. I seem to remember in gusty weather it messing with the MCP selected speed, which was irritating.
Denti's airline was pure 737-300 at the time as I remember. We were predominantly NG. I wonder if both aircraft behave the same in this respect and whether that might have affected preferences.
BOAC-
I believe the alpha-floor term came around for 2 reasons - It is derrived from Bus speak (getting into that now) and the flashing 'A' symbol that appears on the MCP when active. Do you remember the good old girl (-200ADV) which actually had an alpha symbol arrive in the speed window when active! You put the clues things together and end up with Boeing pilots stealing an Airbus term. However the MSR on the 737 is completely different to Alpha-Prot/Floor on the Airbus!
I don't believe the MSR would have helped in AMS - Having giggled at several colleagues when MSR has activated at 60' - Only to have the MSR revert to retard passing 27'. Which leads me to think that the retard has priority over MSR hence it wouldn't have helped in AMS. That is if the NG behaves like a classic in that way....
Cheers, Cough
Hi mucka! getting into that now - oomigawd.
Having giggled at several colleagues when MSR has activated at 60' - I'd asked you not to talk about that.......................
I have come to the same conclusion on MSR at AMS (how's that for TLAs):)