With the 737 there is a 80 knot call and with Airbus there is a 100 knot call. The principle as far as I know is a check that both ASI's are within a bull's roar of each other. In other words guarding against a gross error. A wise man will know the wind velocity before the start of the take off run and by association the expected ground speed read-out at the 80 or 100 knot call for the aircraft type.
As a general observation it is probable that an experienced simulator instructor will see more events or incidents in one week while training various ranks of pilots than most line pilots would see in 20 years. So the opinions following are the result of watching years of "events" with erroneous airspeed during type rating and general simulator training.
It takes less than one second to glance at the ground speed indicator during the take off roll at the support call of 80/100 knots. That is of course if you know exactly where the ground speed indicator is located on the instrument panel.
You can never know if a hidden defect is present giving erroneous airspeed. It could be on the take off roll which is critical - or some other time. Indeed, a reminder of this fact may be found in the B737 FCTM where there is a statement "ground speed information is available from the FMC and on the instrument displays (as installed). These indications can be used as a cross-check."
A year or so ago ATSB published an incident report to an A330 on take off at Brisbane with the F/O as PF. I don't recall the exact details but I think there was an insect stuck in the captain's ASI which had the effect of causing his ASI to under-read by a significant amount. Passing 100 knots the F/O expected a support call of 100 from the captain but didn't get one. He said nothing assuming the captain had been distracted and forgot to make the call. Approaching VR on the copilots side and as the copilot was preparing to rotate, the captain called "100 knots". There was immediate confusion as you can imagine. After a very brief moment the captain took control and made high speed abort. Back at the gate the fusible plugs let go as the brakes were hot.
In the Boeing FCTM there is a statement under the general heading of "Callouts" and says "The PM makes callouts based upon instrument indications or observations for the appropriate condition. The PF should verify the condition/location from the flight instruments and acknowledge. If the PM does not make the required callout, the PF should make it."
That is why it is good airmanship for the PF to call something like "95 knots my side" if he didn't get the call of "80 Knots" from the PM. The high speed of the abort by the Brisbane A320 might have been avoided if the PM (the F/O in this case) had called "110 my side" during the take off run instead of saying nothing. While 100 knots was the SOP call here by the time the PF has reacted to no support call by the PM the aircraft would likely have been 10 knots faster - hence the theoretical "110 my side" call by the PF. That would have alerted the captain to a potential problem and he would have to decide to take over and abort or tell the PF to keep going especially as an erroneous airspeed defect is not normally a reason to make a high speed abort.
This then leads us to the value of the ground speed glance where in this case the PF could have quickly noted his ground speed as he passed 100 knots. Having already had in his mind what the ground speed should read approximately as he saw his own IAS going through 100 knots he would be in a good position to know his own ASI was good in event of an erroneous airspeed rare event. In fact there was an erroneous airspeed event on the captain's side - so these things can happen albeit at an unexpected time and very rarely. But that is what good airmanship is all about.
In the simulator an instructor may select a 25 knot airspeed over-read or under-read on one of the two pilots ASI's. Example follows: Captain as PF has the over-reading error but doesn't know about it of course as he starts the take off run. His ASI comes off the stops early and soon rockets past 80 knots. The PM will not know of course. The captain doesn't receive the expected support call from the PM of "80 knots" (Boeing). He quickly calls
"90 my side". The astonished PM mentally thinks "Bull****" as his ASI is around 55-60 knots. He says something out aloud about airspeed disagreement and by this time the aircraft is rapidly accelerating through 100 knots real airspeed.
The captain realises there is an erroneous airspeed problem but there is no time to glance at the standby ASI and evaluate the umpire. Now he could have picked up the problem earlier if he was in the habit of glancing at his ground speed as he passed 80 knots IAS. With his over-reading airspeed he would have been a bit shocked to see his ground speed was something like 50 knots. Providing as a good captain he knew what the W/V was before the start of the roll, he knew the airspeed of 80+ and the GS of 50+ didn't compute and it would then be up to him to continue or stop. He could rotate at the scheduled VR speed on his ground speed reading (allowing for wind component) and at a safe altitude sort out the problem in the air.
This technique taught in the simulator is invaluable and once the crew get into the habit of ground speed awareness because of simulator practice then it becomes second nature for all future take offs. Honestly it is no big deal. But it is a real big deal if a premature action is taken to make a high speed stop simply because of a perceived airspeed difference between the two main ASI's in the middle of the takeof run.
One Pprune contributor in this thread scorned use of the ground speed indication on very short final as useless. In fact those pilots who have flown into some of the atoll runways like Nauru, Tarawa, Truk, Pohnpei and the like will remember the unreliability of local anemometers and at night the windsocks may be unlit. In fact years ago, we had the situation at one island airport the flight service operator announced the wind as calm by simply looking at the forecast wind. His anemometer was u/s and some vandals had shot out the windsock lights. We were on final approach at night at 5 miles and kept a wary eye on the ground speed which showed a 15 knot tailwind from 10,000 ft down. "Wind calm - runway clear" said the local in the flight service tower. That didn't tie in with on course on the VASIS rate of descent. At 500 ft the ground speed was now 10 knots tailwind. The runway was just 5600 ft long and no way were we going to land with any TW so we went around and landed from the opposite direction. It was good to see a 10 knot HW component on that runway.
Morale of that story? Be fully aware of the ground speed indications that may alert you to unforecast or actual tailwinds. Nothing wrong with a quick glance at the ground speed at 200 ft on short final. Especially with varying winds. Similarly on the take off run be aware of the expected ground speed indication as the PM makes his IAS support call for the aircraft type. This could prevent an unwarranted high speed abort because of perceived airspeed reading difference.
Please note: The above comments are personal opinion only. Arguments against should preferably be based on technical disagreements - not playing the man!
Last edited by Centaurus; 25th Jan 2013 at 12:26.