Back at flight school, most pilots were introduced to air data malfunctions at some point. Unfortunately, this topic is often reduced to memorizing the effects of a blocked pitot tube or a blocked static port, rather than a thorough understanding of the matter. Especially when dealing with transonic aircraft, the topic of air data measurement becomes much more involved, and the oversimplified mnemonics no longer hold. In this article, we shall look at the subject with an emphasis on Static Source Error Correction (SSEC) and see real-world examples where an advanced understanding would have improved to situation.
The background: Why it matters.
Accurate measurement of total and static pressure is essential for modern transport aircraft. Especially in Reduced Vertical Separation Minima (RVSM) airspace, the margins are small. So much so, that aircraft are routinely monitored by external agencies while in flight:
See here: RVSM monitoring
As we shall see, the accuracy of the air data can be altered during certain malfunctions and a clear understanding of these effects will help in dealing with the situation.
Measuring static pressure
It can be shown by analysis and test that the accuracy of airspeed and altitude largely depend on the error of the static pressure measurement [1] [2]. The total pressure measurement in the pitot tube is usually far less critical [3]. Therefore, we will focus on the former. It turns out, the static pressure is quite difficult to measure accurately, as the presence of any object in the flow will affect the static pressure.
The depiction above shows the static position error pressure coefficient (pl-p∞)/qc along the dashed line [3]. This quantity can be described to be the ratio of the difference between local pressure (pl) and freestream static pressure (p∞) to the impact pressure (qc). It equates to 1 at the stagnation point and reaches 0 when there is no static pressure error. Locations 1 to 6 are regions of minimum static pressure error for the fuselage with wing & tail curve (magenta) [3]. These locations are usually selected for positioning the static pressure sensors.
For transonic aircraft, this distribution tends to “shift” approaching high Mach numbers, and a static error correction is required [1]. Similarly, at high angles of attack (AOA) the static pressure error can change significantly [3]. It is therefore typical for transonic RVSM aircraft to incorporate some form of SSEC [1].
In order to get a grasp on this, we will look at data from a typical nose-boom installation. It becomes evident that the AOA and Mach number are critical variables when we look at the figure below. The effect of AOA is represented by n (load factor) and W (weight) in the graph.
The figure above holds two key messages:
At lower speeds (< M0.6) the error largely depends on n (load factor), W (weight) and δs (pressure ratio). For aircraft with little AOA and weight variation this may even be approximated by one curve [3].
At higher speeds (> M0.8) the Mach number becomes the dominant factor.
This data set is usually established during certification flight testing and is then stored in look-up tables. In this manner, the air data modules can accurately compensate for the effects.
So, how bad can it get if left uncorrected?
Until now, we have only looked at static pressure errors, but not at the ultimate errors in altitude or airspeed that will result. This is what really matters.
To get an idea, here is a plot of the induced altitude error for a static position error pressure ratio of 0.01:
Looking at these numbers, it becomes immediately obvious that for RVSM compliance SSEC is indispensable. Bear in mind that the static position error ratio might be larger than 0.01.
This context also manifests, that an incorrect total pressure can induce an altimeter error through SSEC!
Similarly, the effects on airspeed can be plotted:
With this in mind, it will become clear that the inter-relationships between air data measurements are not always straightforward. We will now look at two incidents to get an idea how SSEC may induce unanticipated effects in a malfunction scenario.
Incident 1: Pitot blockage causing incorrect altitude indication [5]
A Cessna Citation encountered an airspeed discrepancy during climb into RVSM airspace. After changing the autopilot pitch mode from IAS to V/S, the crew realized that they were reading different airspeed and altitude values. After querying ATC, they were mistakenly led to believe that the left (nr. 1) system was showing the correct flight level.
Obviously, a cross-check with ATC regarding barometric altitude is futile since the transponder simply uses the same air data information as presented to the crew.
It did not take long for a close call to occur. From the report:
“En route, after having observed that the left and right altimeters were giving different indications, the crew informed the controller of the onboard altimeter fault. The latter then informed the crew of converging traffic (the Embraer 170 registered F- HBXG) at a distance of 2 NM, in theory 1,000 ft higher than them. In reality, the traffic was lower than them (the minimum separation was estimated at 665 ft and 1.5 NM). No collision avoidance system warning, whether it be on the ground or onboard the Embraer 170 was emitted, as the systems had analysed erroneous data from the Cessna 525.”
What had happened?
The report concluded that the aircraft encountered several (!) air data malfunctions during the previous years that were never completely understood. It appears that a “low point” in the pitot tubing of system nr. 1 had allowed moisture to accumulate and led to a temporary partial pitot blockage. Crucially:
The total pressure information was used by the altimeter for static error correction and thus the incorrect total pressure information led to an incorrect altitude being displayed.
The report could not completely exclude additional effects in the static line. While the crew had doubts about their air data system, they only began to realize the true gravity of the situation after the flight.
It is worth noting that ACAS systems (e.g. TCAS) use barometric information and therefore will not provide any alert if an aircraft is subject to erroneous air data.
This underlines the need for an independent RVSM monitoring system (link above).
Incident 2: AOA probe malfunction leading to IAS/ALT disagree [6]
A B737 crew was performing a ferry flight after maintenance. During the take-off run the “80 kts” call was “slightly off” according to the pilots. When the captain rotated the aircraft, the IAS disagree caution was displayed, shortly afterwards followed by AOA disagree and ALT disagree. The crew was climbing into busy airspace (London TMA) and decided to postpone the application of the relevant checklists. Once the checklists were completed, the flight crew elected to continue towards their destination (Paris Orly).
Notably: During cruise at FL200, they observed differences of 2000 ft in altitude and up to 40 KIAS in airspeed between the left and right PFD!
Once the correct indications were identified, the crew used the autopilot and auto-throttle for the remainder of the flight and carried out an uneventful landing. No comment to ATC was made.
The events were entered in the tech log and the aircraft was subsequently checked by a maintenance technician, who did not find any fault.
It is therefore not surprising that the same show would go on the next day:
This time, it was a passenger flight with a captain in training on the left seat and an instructor on the right seat. The trainee captain was pilot flying. Again, the “80 kts” call was slightly off and this time the IAS disagree message appeared around 90 KIAS. The instructor took over the controls and performed the remainder of the take-off. Once airborne, the AOA disagree and ALT disagree cautions appeared, and the instructor determined that the data on the left PFD was correct thus he handed the controls back to the left seat pilot. After applying the memory items and checklists, the crew returned for a landing in Paris Orly. A PAN PAN call was made to ATC.
It was only after this event that maintenance found the right AOA vane “hard” to rotate and therefore removed it from the aircraft.
After disassembly of the AOA resolver, some contamination was discovered, which most likely entered the resolver during the manufacturing process. The epoxy resin was contaminated with Tetra Hydro Furfuryl Alcohol (THFA), a substance used in the assembly line building.
This contamination led to an angular difference between the actual resolver angle and the reported resolver angle of more than 100°. It is noteworthy that despite being obviously incorrect, this value continued to be used for the SSEC of the right-side air data. Furthermore, the incidents highlight that an inadequate troubleshooting can result, if the root cause is not identified before/during maintenance.
The crews involved in the high-profile B737 MAX crashes were also presented with IAS and ALT disagree cautions caused by incorrect AOA information [7].
What about your aircraft?
The above incidents are type-specific and serve as a stark reminder that system knowledge of the aircraft is essential. It is not always easy to extract this kind of information from the basic documentation and sometimes a technical request to the manufacturer can shed light on this topic.
In any case, when faced with air data problems, fly the aircraft first with a focus on pitch/N1. Or as captain Warren “Van” Vanderburgh used to say:
“Do not get clever early.”
Is there an alternative to conventional air data measurement?
If you are wondering if there is another way to measure an aircraft’s speed, check out this article:
References
[1] | E. Haering Jr., «Airdata Measurment and Calibration NASA TM104316,» NASA, California, 1995. |
[2] | Wuest, «Pressure and flow measurement,» NATO AGARD FTI Series, Bd. vol. 11, 1980. |
[3] | US Air Force Test Pilot School, «Pitot-Statics and the standard atmosphere,» in Performance flight testing, Edwards AFB, US Air Force, 1996. |
[4] | B. de Go Mars, «Pression statique au flanc d'un fuselage,» [Online]. Available: https://commons.wikimedia.org/wiki/File:Pression_statique_au_flanc_d%27un_fuselage.png. [Zugriff am 05 01 2025]. |
[5] | BEA, «Serious incident between the Cessna Citation 525 CJ registered F-HGPG operated by Valljet and the Embraer 170 registered F-HBXG operated by HOP!,» 2023. |
[6] | BEA, «Incidents to the Boeing 737 - 800 registered F-GZHO operated by Transavia France,» Bureau d'Enquêtes et d'Analyses pour la sécurité de l'aviation civile, 2020. |
[7] | KOMITE NASIONAL KESELAMATAN TRANSPORTASI REPUBLIC OF INDONESIA, «Aircraft Accident Investigation Report PT. Lion Mentari Airlines Boeing 737-8 (MAX); PK-LQP,» REPUBLIC OF INDONESIA, 2018. |