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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. |
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