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B737 Airspeed Unreliable?
At risk of the usual cadre of crackpot loons answering 10/80% flaps out or 4/75% flaps up targets in the event of Airspeed Unreliable? Boeing are peddling this as a go-to solution to Airspeed Unreliable, it looks reasonable in a takeoff scenario at moderate weights however highly dubious in high altitude scenario with ice related Airspeed issues etc what do you think? |
It’s not designed for level flight, it’s designed to keep the aircraft flying. It will put you in a descent, and into a safe place to get yourself into the QRH and find the QRH pitch and power setting for your phase of flight. A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground. |
Originally Posted by gravityf1ghter
(Post 10589197)
It’s not designed for level flight, it’s designed to keep the aircraft flying. It will put you in a descent, and into a safe place to get yourself into the QRH and find the QRH pitch and power setting for your phase of flight. A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground. |
FFS if only people bothered to open the forbidden manual called FCTM.
Memory items for target pitch and thrust must be accomplished as soon as it is suspected that airspeed indications are incorrect. The intent of having memorized pitch and thrust settings is to quickly put the airplane in a safe regime until the Airspeed Unreliable checklist can be referenced. The following assumptions and requirements were used in developing these memory items: • The memorized settings are calculated to work for all model/engine combinations, at all weights and at all altitudes. • The flaps up settings will be sufficient such that the actual airspeed remains above stick shaker and below overspeed. • The flaps extended settings will be sufficient such that the actual airspeed remains above stick shaker and below the flap placard limit. • The settings are biased toward a higher airspeed as it is better to be at a high energy state than a low energy state. • These memorized settings are to allow time to stabilize the airplane, remain within the flight envelope without overspeed or stall, and then continue with reference to the checklist. • Settings are provided for flight with and without flaps extended. The crew should use the setting for the condition they are in to keep the airplane safe while accessing the checklist. The memorized pitch and thrust setting for the current configuration (flaps extended/flaps up) should be applied immediately with the following considerations: • The flaps extended pitch and thrust settings will result in a climb. • The flaps up pitch and thrust settings will result in a slight climb at light weights and low altitudes, and a slight descent at heavy weights and high altitudes. • At light weight and low altitude, the true airspeed will be higher than normal, but within the flight envelope. At heavy weight and high altitude, the same settings will result in airspeed lower than normal cruise but within the flight envelope. • The goal of these pitch and thrust settings is to maintain the airplane safely within the flight envelope, not to maintain a specific climb or level flight. • The current flap position should be maintained until the memory pitch and thrust settings have been set and the airplane stabilized. If further flap extension/flap retraction is required refer to PI-QRH Airspeed Unreliable table. |
:D High handed follower?
The issue is principally higher altitude and higher weights, in turbulence Do the Boeing Go-To figures propose stand-up to scrutiny? For example 4/75% on the MAX is considerably more thrust Who is Lazy Pilots or Boeing? |
Originally Posted by FlyingStone
(Post 10589533)
FFS if only people bothered to open the forbidden manual called FCTM.
As far as the takeoff scenario, throttling back to 80% is no worse than losing an engine. That being said, in this situation one must be mindful of obstacle clearance, so the pilots should consider following any engine-out routing that might apply for the particular airport. |
Originally Posted by sekmeth
(Post 10589216)
who would change thrust after V1? The A/T is in thrust hold |
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks. |
Originally Posted by Banana Joe
(Post 10589778)
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks. |
Originally Posted by Banana Joe
(Post 10589778)
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks. 10 degrees and 80% isn’t ideal in this situation, but it is safe, which is what Boeing have designed it to be. Back to to my original post, the time to consider not doing this, is a hot and heavy take off from a short field with terrain around, maybe a high flap setting, as I said, I don’t know many about who will rush to set 80% in that situation. |
Originally Posted by Tomaski
(Post 10589620)
Great reference, but not all airlines use the generic Boeing FCTM. My airline has a tailored manual that does not provide these bullet notes on the Airspeed Unreliable NNC.
For example 4/75% on the MAX is considerably more thrust |
Originally Posted by gravityf1ghter
(Post 10590013)
10 degrees and 80% isn’t ideal in this situation, but it is safe, which is what Boeing have designed it to be. |
Originally Posted by Banana Joe
(Post 10589778)
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks. Bear in mind the information given in the FCTM, quoted in #4 above: The memorized settings are calculated to work for all model/engine combinations, at all weights and at all altitudes. And: The goal of these pitch and thrust settings is to maintain the airplane safely within the flight envelope, not to maintain a specific climb or level flight. MSA could be 7,000 feet or more, and your company will have emergency turn procedures at airports with MSA issues. Pilots should not invent their own procedures - (not suggesting you were). |
Originally Posted by Banana Joe
(Post 10589778)
Right after take off? Isn't it better to set 15° up and TO thrust until above MSA.
I am asking as a relatively inexperienced FO, with airspeed unreliable being an item in my next simulator check in a couple of weeks. |
Originally Posted by Trossie
(Post 10590345)
15° up and TO thrust are what you are going to fly (approximately) until you reduce to climb thrust at 1,500ft. That is a relatively busy period, especially if something like an ASI has gone wrong and the associated 'startle factor'. Best stick to that rather than fiddle with thrust and attitude changes close to the ground and you might find during that time that you have two ASIs that agree. If not, I would say that above 1,500ft you go for you flaps thrust/attitude settings. Don't try to do too much close to the ground, just fly the aeroplane.
1500ft? MSA? Is that from your potentially failed pitot/static? Stick to the procedures. |
It would be nice if Boeing would provide a suggestion in the FCTM of the recommended course of action in the event of Airspeed Unreliable during or after takeoff, but they don’t. This leaves thinking pilots trying to work it out for themselves, hence this thread (and it isn’t the first, this has been discussed on this forum previously). I’ve noticed that it creates two schools of thought: those who would maintain TOGA thrust and TOGA attitude until a safe altitude, and those who would commence the memory items at 400 feet. To my thinking, both options are “safe”. As to what option your airline or your simulator instructor is looking for is probably best discussed before you do it. My airline exposed me to it in a simulator a couple of years ago at 32C, 79.0T (ie MTOW) Flaps 5 at sea level. He made us observe the rate of climb once we got to 10/80% - it was in excess of 1000fpm. Way in excess of an engine out scenario. That was good to experience, because now I know. I didn’t know prior to that and may have had trouble believing it. Reducing thrust to 80% and lowering the nose 5 degrees after taking off at MTOW in a hot climate is not something that would be considered naturally safe unless you had seen it before. One objection to delaying the memory items is that it delays turning F/Ds off. We are accustomed to follow the F/D pitch bar on takeoff, and with Airspeed Unreliable, the F/D might take us somewhere we shouldn’t be. |
Originally Posted by gravityf1ghter
(Post 10589197)
A more interesting one to think about is a take off from a performance limited field, high flap setting, at MTOW with airspeed disagree at V1, I don’t know many who’ll rush to set 10/80 until well clear of the ground. There have been very bad outcomes when pilots have done just that... |
Originally Posted by tdracer
(Post 10590748)
Why in the world would you continue a takeoff if you had an airspeed disagree before V1?
There have been very bad outcomes when pilots have done just that... At my airline, we procedurally do an airspeed crosscheck at 80 KIAS, and if they agree we continue. From that point we are in the "high speed" regime and - procedurally - we only reject the takeoff for a narrow set of circumstances, and airspeed disagreement is not one of them. That being said, if I'm lightweight and rolling down a 12,000 foot or more runway, I still might make the decision to reject past the 80 knot point. However, I fully expect to have that decision questioned by both my Chief Pilot and the FAA. |
Originally Posted by tdracer
(Post 10590748)
Why in the world would you continue a takeoff if you had an airspeed disagree before V1?
There have been very bad outcomes when pilots have done just that... At this point, do not be lulled into trying to diagnose the reliable airspeed, as per the Boeing checklist and FCTM, you are not in a position to be doing this until you are sat stable and in trim, at a pre determined pitch and power for the phase of flight from the QRH. |
As Boeing say there are 4 reliable indications:
- attitude - N1 - groundspeed - radio altimeter. If I have a suspected Airspeed Unreliable why would you do anything except maintain standard takeoff pitch - 15 degrees - and maybe tweak power back to MCT, sit fat dumb and happy until MSA THEN start screwing around with things? You can do anything you want - as long as you can justify it. To me achieving MSA then doing the Airspeed Unreliable checklist increases the safety of the aircraft, that's what I'd explain to those lined up against me at the enquiry - just remember the warm and fuzzy feelings Sully and the F/O received! - for them to chew on. Ultimately the passengers are safe, the aircraft's secure on the ground and I've written/am writing the report(s)....what's your problem?? Cheers. |
Originally Posted by galdian
(Post 10591858)
If I have a suspected Airspeed Unreliable why would you do anything except maintain standard takeoff pitch - 15 degrees - and maybe tweak power back to MCT, sit fat dumb and happy until MSA THEN start screwing around with things? You can do anything you want - as long as you can justify it. To me achieving MSA then doing the Airspeed Unreliable checklist increases the safety of the aircraft, that's what I'd explain to those lined up against me at the enquiry - just remember the warm and fuzzy feelings Sully and the F/O received! - for them to chew on. Ultimately the passengers are safe, the aircraft's secure on the ground and I've written/am writing the report(s)....what's your problem?? Cheers. Keep in mind that once the initial steps have been completed, the crew should then move on to the expanded NNC. Step 7 of this checklist directs the crew to the tab data to obtain the appropriated pitch and power settings for their aircraft and conditions. If executed correctly and promptly, then it shouldn't take long before the aircraft is configured for a safe climb to MSA. I would caution against intentionally disregarding the steps of an established NNC procedure. It's one thing to decide that a certain NNC does not fit a certain situation. It is entirely another to make up your own procedure. Don't expect your flight department or certificate authority to take such a casual attitude when they review your actions. That said, I have three concerns with the current Airspeed Unreliable NNC procedures as they apply to takeoff (particularly when accompanied by a false stall warning). First, there is no clear guidance on when to retract the flaps. Second, there is only limited information in the NNC regarding erroneous system effects at either the low-speed (stick shaker, EFSM, STS stall ID, autoslats) or high-speed (overspeed clacker, flap load relief) regimes. Third, flight crews are not being exposed to this malfunction on a regular basis in the sim. It can be challenging situation to handle, but really not much more than an takeoff engine failure. The difference is that we practice one all the time, the other not so much. |
The settings are not the same for all aircraft.
The 777 is 10/85 and 4/70. |
Originally Posted by Tomaski
(Post 10592003)
A significant challenge for creating this NNC is to come up with "memory item" pitch and power settings that work for all aircraft and all flight regimes. Presumably, MCT and 15 degrees does not work for all aircraft and all conditions. The tab data is limited, but the go-around chart suggests that for certain aircraft at heavier weights and high density altitudes (e.g. Mexico City), 15 degrees pitch would be too high. I suspect it might also be unsuitable for a heavy-weight, Flaps 25 takeoff. At the other end of the spectrum, a light-weight low density altitude scenario may result in a flap overspeed.
Keep in mind that once the initial steps have been completed, the crew should then move on to the expanded NNC. Step 7 of this checklist directs the crew to the tab data to obtain the appropriated pitch and power settings for their aircraft and conditions. If executed correctly and promptly, then it shouldn't take long before the aircraft is configured for a safe climb to MSA. I would caution against intentionally disregarding the steps of an established NNC procedure. It's one thing to decide that a certain NNC does not fit a certain situation. It is entirely another to make up your own procedure. Don't expect your flight department or certificate authority to take such a casual attitude when they review your actions. That said, I have three concerns with the current Airspeed Unreliable NNC procedures as they apply to takeoff (particularly when accompanied by a false stall warning). First, there is no clear guidance on when to retract the flaps. Second, there is only limited information in the NNC regarding erroneous system effects at either the low-speed (stick shaker, EFSM, STS stall ID, autoslats) or high-speed (overspeed clacker, flap load relief) regimes. Third, flight crews are not being exposed to this malfunction on a regular basis in the sim. It can be challenging situation to handle, but really not much more than an takeoff engine failure. The difference is that we practice one all the time, the other not so much. |
Originally Posted by gearlever
(Post 10592108)
Will the stick shaker on 737-800 be activated if CPT or FO pitot blocked?
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Originally Posted by Tomaski
(Post 10592129)
Yes. A faulty airspeed or AOA input will cause the bad side Stall Management Yaw Damper (SMYD) computer to generate a stall signal. This stall signal will then initiate four system responses:
I always thougt the airbus is a complex beast..... Hopefully all 737-800 jocks will know about the stall warning design of their bird. Days are obviouly gone when a stick shaker was a stall warning just by an AoA vane (B727, A300), no computers which may get it wrong. |
Originally Posted by gearlever
(Post 10592141)
Thx Tomaski.
I always thougt the airbus is a complex beast..... Hopefully all 737-800 jocks will know about the stall warning design of their bird. Days are obviouly gone when a stick shaker was a stall warning just by an AoA vane (B727, A300), no computers which may get it wrong. |
Tomaski
You reaised some interesting points in your original response, suppose I was thinking a departure in a "normal" environment where why change something that's still working OK at 15 degrees/MCT until MSA. Obviously hot and high altitude/mountainous departures have their own considerations. We very occasionally do flap 25 takeoffs and have 800' acceleration height - I wonder when I'd start doing the Airspeed Unreliable NN checklist, whether you'd retract flaps - and which is the more reliable airspeed indication(s) to do so? - or are the flap speed limitations factored in so you'd just leave the flaps at 25? Points to consider, suppose that's why we're paid the (not so) big bucks!! Cheers. |
Originally Posted by galdian
(Post 10591858)
..................If I have a suspected Airspeed Unreliable why would you do anything except maintain standard takeoff pitch - 15 degrees - and maybe tweak power back to MCT, sit fat dumb and happy until MSA THEN start screwing around with things?.............
To me achieving MSA then doing the Airspeed Unreliable checklist increases the safety of the aircraft............... Boeing designed their NNC to work at all weights, all engine variants and all altitudes. Does your procedure work for all such variations? Will your procedure work at, for example, Las Vegas during the summer? (very high MSAs and very high ISA deviation). Or Geneva? (LSGG). Best climb is with a clean aircraft, but your proposal is to climb dirty? :) |
Uplinker
You little devil you! ;) OK - based on which Airspeed Unreliable airspeed indicator(s) are you going to retract the flaps? And in fairness I DID previously acknowledge hot and high/mountainous etc does throw more considerations into the mix. Cheers. |
If you were in need of retracting the flaps, you would make a note of your power and pitch setting for the desired stage of flight you wish to be at with lesser or no flaps from your ‘Performance Inflight’ source (company), and then, as per FCTM, set said power and pitch, and change configuration. |
Yes, fair enough.
The question I am asking - at what height will you be taking the time to consult manuals etc to do the same. 800' AGL - my companies acceleration height? 1500'AGL - another standard used by many? XXXX AGL? I have no qualms doing the NNC, just a matter of when, in many cases suggest it can be delayed whislt everything's going OK with 15 degrees/MCT to MSA as an example. Retracting flaps (if required/desired) - that's the next stage for consideration. Cheers. |
Originally Posted by galdian
(Post 10592605)
Yes, fair enough.
The question I am asking - at what height will you be taking the time to consult manuals etc to do the same. 800' AGL - my companies acceleration height? 1500'AGL - another standard used by many? XXXX AGL? I have no qualms doing the NNC, just a matter of when, in many cases suggest it can be delayed whislt everything's going OK with 15 degrees/MCT to MSA as an example. Retracting flaps (if required/desired) - that's the next stage for consideration. Cheers. I'll say again, this can be a very challenging procedure, and the crew does not have the luxury of figuring these things out on the fly. We really need to treat this as seriously as an takeoff engine failure and know (and have hopefully practiced) what we are going to do and when we are going to do it long before it happens. Based on the scanty guidance, this is my recommendation on how to execute the existing Airspeed Unreliable NNC during takeoff.
If the unreliable airspeed is generating an overspeed, you will have an overspeed clacker. There is no way to silence this alert short of shutting off the bad side ADIRU, and you probably don't want to do that. If the left side is faulty, that may trigger the flap load relief function (flaps will not extend fully). This function varies by aircraft, so check your FCOM for detail. Load relief only retracts flaps to the next higher setting, so you can still get to Flaps 30 by selecting Flaps 40. Depending on how far off the airspeed is, you may still be able to find a usable landing flap setting (Flaps 15) that does not cause the overspeed clacker to come on. As I said, a lot of moving parts to this one. It is unfortunate it has not been given the attention that it deserves. |
Question regarding the procedure as I found myself in a discussion with our instructor the other day:
we experienced airspeed unreliable just short before minimums on a cat1 ILS, and had to make a go around. after ap/at off and FDs off, and askint the PM to set the power to 80% Question regarding the procedure as I found myself in a discussion with our instructor the other day: we experienced airspeed unreliable just short before minimums on a cat1 ILS, and had to make a go around. after ap/at off and FDs off, and asking the PM to finetune the power to 80%, I asked for gear up. the instructor asked me during the debrief, where I found the gear in the memory items. According to him it isn’t written anywhere. I tend to disagree, as the values given specifically say “gear up pitch attitude and thrust “. The instructor said that if Boeing found the gear that important they would have made it a separate memory item, which is fair enough. But when I asked/said that the values given in the QRH are only valid for gear up, and without asking for gear up I was also not flying the procedure, his response was that I was being a smartass and I should listen to him. I agree this being an academic discussion but I was wondering how you are looking at this. Would you raise the gear or not? fyi: we were training in Innsbruck, so terrain is always an issue |
You won't get any decent rate of climb with F40, 80% and 10deg pitch and gear down. Terrain or no terrain, the gear has to come up for the memory item pitch/thrust values to be valid.
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Quote
The instructor said that if Boeing found the gear that important they would have made it a separate memory item, which is fair enough. But when I asked/said that the values given in the QRH are only valid for gear up, and without asking for gear up I was also not flying the procedure, his response was that I was being a smartass and I should listen to him. The memory items for a V1 cut with fire or severe damage don’t include an item for gear up either. But it’s covered in the emergency brief much the same as gear coming up is covered in the go around. Sounds like a very egotistical examiner, still around unfortunately and even more surprising they seem to be the younger ones. Agree that it’s not a good idea to make up you’re own procedures, but it’s still a good idea to have some contingencies ready. The scenario of birds on approach taking out a radome and pitot could potentially also take out an engine. So you now have one engine and no reliable airspeed.Now Try going around with gear down and 10/80 set.... |
...his response was that I was being a smartass and I should listen to him. As I suggested in another thread, the response to these types of scenarios that appears to work best is to nod, pretend to make a note of it somewhere, then move on. Afterwards you can ask someone else that you respect to give you an answer/explanation. I think the logic used to raise the gear is difficult to disagree with in this situation. Why would you want to leave it down and invalidate the figures in the QRH by doing so? Especially when terrain constrained. It doesn’t say to put the speedbrakes away either, should you be using them, but it makes a lot of sense... |
Sometimes I have to shake my head a little.
Back when, we had to commit to memory the data relevant to the current Type; ie pitch, thrust for altitude and configuration to achieve a speed which was associated with a climb or descent rate. This didn't require memorising books full of numbers but one did need to commit a few useful combinations to memory. For newbies (eg cadet pilots at around the 200 hour mark converting onto the 737), and knowing that they probably would never have the exposure opportunity again, I used to get them to fly a single pilot circuit towards the end of the endorsement program - raw data, failed pitot statics, low cloud and vis for a recovery via an ILS. I can't recall that anyone ever had a major problem - OK many were not pretty to watch but they got the aircraft back onto the runway in a reasonably satisfactory manner - some did superbly proficient jobs of the task. This was just a skills exercise which probably would never arise on the line. However, the student confidence building value and confirmation to me as the instructor that they had a reasonably sound grip of the I/F basics at that stage were sufficient justification to me for the time spent. I always thought it unfortunate that, other than for very low time pilots coming onto their first airline jet, the endorsement program hours/sessions were too tightly constrained to permit such sideline exercises. SOP procedures aside, is it too difficult to commit a few number combinations to memory to save your tail in extremis ? |
Refer to the Flight Crew Training Manual for Airspeed Unreliable on approach.
Normal go-around using go-around thrust and 15 deg pitch until reaching a safe altitude, then do the Airspeed Unreliable checklist. |
great thread!
One has to remember that there were what, 7 revisions to the 737 NG manual, and in the 3rd revision to the 737 NG/MAX manual, and after the joint technical review board brought it up, they will finally correct the spelling and terminology issues! |
SOP procedures aside, is it too difficult to commit a few number combinations to memory to save your tail in extremis ? In terms of crew coordination, it works better IMO. As soon as someone says “Airspeed Unreliable” you know what to do and also what should be done by the other pilot, rather than seeing someone else set a performance attitude that is not quite what you expected. |
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