A/T Speed De-select
 

G’day to all.

Quick question or a debate, but either way I will appreciate your comments.

Why it is not recommended to deselect speed on the A/T system of the 737-300, instead the recommendation is that when the A/P is disconnected the A/T should be disconnected as well.
De-selecting speed keeps the A/T armed for a Go around when TO/GA is pressed? Anyone in the know can perhaps comment.

Thanks
SD

Been several years since I've done the -300 but I'm resonably confident that - as long as you don't deselect A/T on the MCP but only double-click it on the thrust levers - the A/T will engage when you press the TO/GA.

Using the throttle deselect buttons clicks out the MCP A/T switch, I believe. A SINGLE push of the TOGA button gives reduced g/a thrust with 2 engines, or full on one as long as 'speed' is deselected, but A/T left engaged in 'ARM'.

Having just done a 700 convex there seems to be some question over whether there is some extra 'built-in' fail-safe with auto-throttle disengaged. Did not have time to try it in the sim. Does anyone know?

While on the topic, given that min speed reversion is based on 1.3xVs, does anyone have the definitive margins for the 'knots below 1.3xVs' (if any) at which it cuts in and to which number of knots above it jumps?

“Why it is not recommended to deselect speed on the A/T system of the 737-300” .... Sir Donald, could you please expand a bit on deselect? What button is used to deselect?

80/20 I suppose you don't fly the 737. Sir Donald was refering to a manouver discussed here some time ago:you can 'deselect' the speed,during approach,by pushing the speed button,while flying manually.This way ,the A/t remains armed,and ,pushing TOGA it will command go around power.
In order to avoid mixed flight complications (a/t with manual flight) Boeing recomends complete manual or automatic flight.
The problem that can appear in this particular case is,A/t sensing a decrease in speed,just before flare,and minimum speed protection becoming active,the result being power aplied by the a/t (whic was armed), just about when you intend to start the flare.It can complicate your life a little bit.
I use to deselect speed when expecting a late clearance,and an eventual go around,but as soon as I receive clearance to land,and not bellow 200',I disengage the a/t.

Brgds Alex

Just wanted to make sure I understood the question, SD & rest. Boeing is carefully saying that it is normally not recommended because its function can be confusing. Modes may automatically revert and make the bug jump to a higher speed during approaches without A/P. The primary feature of the autothrottle ARM mode is minimum speed protection in the event the airplane slows to minimum maneuvering speed. This recommendation also serves as a disclaimer since other features normally associated with the autothrottle, such as gust protection, are not provided. Many pilots and instructors are use arm mode during approaches.

80/20 - companies vary in philosophy on this. It has acquired the name 'alpha-floor' in Boeing land which I am sure is an AB term and 'alpha-floor' it is not IMO.

Personally I forgo the 'min speed' value of using it and its main benefit for me is in its giving an automatic g/a power application of a REDUCED setting, with the option of a second press to get FULL, whereas the 'rules' (bless 'em) require FULL g/a power for any manual throttle g/a.

Great replies guys/gals, thank you.

The reason that I asked the question is that during the TR course the instructors taught us to deselect speed once we disconnect the A/P because of its advantages, as supposed to completely disengaging the A/T, and being new to jets, the 737, I did not realise the difference until I started my line training, during which some Captains would comment on why I chose to disengage the A/T and why not deselect speed, and others would strictly ask that I did disengage the A/T upon my request to deselect speed. Hence my question.
The manual says one thing, in the sim training you are taught another and on line one gets two different opinions.

Kind regards to you all for taking your time to answer my question, and safe flying.

SD

SG - here is ONE of the previous links mentioned which may be of interest?

Still looking for someone who knows the 'cut-in' and 'cut-out' speed margins for this (other than 1.3xVs) - any 'tecchies' or TRE's?

Out of interest, a previous company encouraged its use on s/e approaches to help with the application of g/a power, engaging it (speed deselected) at around 1000', and REMEMBERING TO CLICK the A/T OUT AFTER the G/A so you do not get caught by the remaining engine whistling back at ALT ACQUIRE:{ . Other companies consider this mode to be 'the work of the devil'. Horses for courses, as they say!

Boeing have already given their advice via the FCTM. You must assume therefore that their test pilots have researched and test flown the procedure very carefully. If Boeing had to explain in full detail all the reasoning behind all of their recommended procedures, the FCTM would be double the thickness.

For ages the Boeing 737-200 did not have autothrottle nor alpha floor protection (or whatever you wish to call it). Flying the 737 was not labelled dangerous because of this.

Why on earth cannot people accept that Boeing do know what they are on about when they explain that operating the autothrottle in ARM while flying manually is not recommended unless during take off and climb. During all other phases of flight autothrottle use is recommended only when the autopilot is engaged. Accept this very sound advice. Live with it. Stop reinventing the wheel etc etc.

Yes, Hudson - frustrating isn't it:D

A reminder that it is best, however, to follow YOUR company SOPs whatever the FCTM says.

Hudson, chill out bud.

Nobody is trying to re-invent the wheel, hence my question on here.
Sorry to frustrate you, but if you have an explanation please expand on it.

I have even bought a maintenance training manual among a host of other books just to understand the reasoning behind it all. The FCTM unfortunately does not expand enough on a lot of matters-CLEAR?
Only trying to collect info in order to broaden my knowledge base, if that is ok with you of course.
As for the rest, thank you for your input.
G’Day and safe flying,
SD

Sir Donald. Apologies - point taken. I can understand your dilemma when faced with different points of view from different authorities (check pilots, simulator instructors and so on).

Been there done that and it gets frustrating when you would prefer to believe Boeing via their FCTM but are faced with convincing counter points of view from all of the above chiefs of the flight deck.

The best reply you can offer when faced with these varied authorities is to gently ask whether or not they have actually had the opportunity to carry out the measured flight tests that would be needed to prove that they correct. I doubt if they would have.

The next suggestion that you could put to these gentlemen is that it would be a splendid idea if they could share their views with the Boeing Flight test department and perhaps get Boeing to revisit the FCTM advice in order to incorporate the changes suggested by the proponents. I am sure the Boeing reply (if any)would be illuminating.

Meanwhile to be on the safe side there would be no harm in erring on the safe side and accept the Boeing FCTM advice as authoritive. Full marks to you for delving deeply into the subject, all the same.

Sir D,

Here is one little gotcha which I believe that Alexban may have been referring to. If you are at around 50' and the A/T enters the Alpha Floor mode (or underspeed limiting as my tech manual refers to it), at 27' the Autothrottle will then transition to the Retard mode (aka an AutoLand). This (depending on how you handle it) can lead to a pretty firm touchdown.

Me? Well I prefer the A/T in 'Arm' to touchdown. Should the Alpha floor engage below 100ft, I disconnect it.

Hi guys,

I fly the 737 classics and the NGs. Now, our Instructors and Examiners preach the use of AT in the arm mode for all approaches except a visual traffic pattern. As variations in attitudes/speed/configuration happen very quickly in a traffic pattern, the AT is best kept out of picture. Now, recently we had this Instructor guy from Boeing and he was travelling OBS. on couple of our flights. He happened to notice this and on one of my flights, when he pointed out this, I tried to get the Boeing point of view. Now he said a lot of operators in the world flying the 737s were deselecting the AT and others disconnecting it. According to him, if complete overview was taken, there were more operators reporting AT "Kick-ins" which somewhat destabilized the approaches and a minority reported having used the TOGA feature available with "deselecting". So as he put it, Boeing sees the entire world scenario and based on that still recommends "Disconnecting" the AT. As far as I am concerned, I'm happy "de-selecting" the AT following SOPs and disconnecting will take some time getting used to. BTW, based on his observations, my company has already started re-valuating the AT modes in approaches.:D
Happy flying and keep it safe out there, :ok:

I rest my case....

arm mode
 

I have to agree with Hudson. I fly the classic and our company un offically used the arm mode, but it never got in to our S.O.P's. Later on the company forbid using the arm mode as per the FCTM. We have in addition our "own" ordered a/c, planes from all over the place, and I have noticed on maint. test flights that not all planes have the "alpha floor" set the same. Some start adding power when you get five knots slower than whats in the window, and others wait 'till just below a shaker event!!! I think de-selecting the A/T has its place if you are on a non prescision and you are in level flight waiting for the PDP/VDP, if you go missed just press toga and you get the A/T's. I dont use speed mode. It's kind of a grey area, so I stick with the FCTM.

Bratko,you do some very intelligent posting....how is life down in yugoslavia?