The history of the jet engine anti-ice procedure

- according to whistleblower Oluf Husted

Ice on an aircraft can be a dangerous and costly affair. Not only can it degrade the aerodynamic performance, it can also harm or even destroy the engines, when it breaks loose and gets sucked into the engines.
Two main systems are used to keep an aircraft clear of ice: The engine anti-ice system og the de-icing system. Both systems use hot air which is taken from the engine’s compressor section. The de-icing system takes care of the wings, tail, and engine cowlings.
Here we will focus on the Engine anti-ice system which takes care of the jet engine intake section
The air is routed to the fan-cone or spinner via metal tubing, hollow static stator- and inlet guide-vanes in order to keep the temperature on the most ice-prone parts above freezing temperature. When being sucked into the engine again, the air also keeps the back side of the big front fan-blades free of ice.

American certification rules in the 1950s stated:

A jet-engine must be able to stay ice-free with the engine anti-ice system on
- also at idle power!

That meant that the captains just had to remember to turn the system on, when the weather so required. Oddly enough the definition for when "the weather so requires" varies with the type of aircraft carrying the same engine.

Jet engine intake (inlet) icing can occur, according to :

Boeing: When the temperature is below +10 degrees C. in moist air

Airbus: When the temperature is below +8 degrees C. in moist air

McDonnell Douglas: When the temperature is below +6 degrees C. in moist air

- Even in bright sunshine, on cool damp days.

According to an American Airlines explanation from 1991 this is because:

”The reduced pressure occuring, as air is drawn into the engine, causes an assosiated temperature drop which may be sufficient to cause the entrapped moisture to condense and freeze on the engine inlet components. If no engine anti-ice is used, extended ground operations can generate substantial accumulations of inlet ice, which may subsequently break loose and be ingested into the engine during take-off or climb. (copy, American Airline 11-1-1991)
Let’s go back to about 1970, when the engine intake diameter gets wider.
The more powerfull engines means that the idle RPM (revolutions per minute) has to be lowered in order to avoid over heating the brakes during ground operations. However, the lowering of the RPM also results in a lower air-temperature in the engine anti-ice system. This means that turning the system on is no longer enough to keep the engine ice-free.

”Run-up” procedures:

Therefore the term "periodic engine run-ups" begins to emerge in the anti-ice procedures many years later but in very very feeble terms such as: ” The Captain ought to/should/could consider doing a run-up to different power settings (60, 70, or 80 percent RPM or 1.4 EPR) for approximately 15 seconds, if he consideres the weather to be severe icing conditions. But also this term severe icing conditions is vaguely and varyingly defined: Temperatures close to freezing/below +2 degrees C./below +3 degrees C. and with visible moisture/visibility below one mile/visibility below one kilometer.
However the obvious warning is never made, namely that:

Idle power is no longer sufficient to secure ice-free engines

Procedures come from the aircraft manufactors even when it concerns the engines only. McDonnell Douglas recommended the above, until it was taken over by Boeing, but had an odd, yet correct severe icing definition dated december 1st 1985:
"The higher the temperature, the higher the cloud liquid water content, and the more severe will be the icing conditions”.

Still McDonnell Douglas never recomended the obvious, namely that:

Captains shall allways do run-ups, when engine anti-ice system is on!

This procedure was implemented by Scandinavian Airlines Syetem, SAS, on March 17th 1992, following a vigorous internal struggle which ultimately cost me my pilot’s license. The procedure was also recommended by Pratt & Whitney on October 24th 1994. In a so called all operator letter – flight operations with applicability to: All P&W High Bypass Engine Models and with the subject: Engine Icing, it was stated:
“Adherence to these recommended procedures will reduce the chance of engine damage, thus reducing maintenance costs and improving safety margins.”
This P & W letter also stresses the importance of doing run-ups during the taxi-in face, in order to avoid accumulation of ice causing problems for the next crew. The letter is signed by Don Povak & Bob Salva.
But on the February 9th 1996 SAS disregarded the P & W recommendations and reduced run-up procedures to be done only when the temperature is below +2 degrees C. This happened after pressure from Frankfurt Airport, because:
”If all airlines did run-ups like SAS (the final run-up before take off takes approximately 23 seconds on the active runway) the rush-hour capacity of the airport will drop at least 17 percent.” This dangerous reduction stayed with SAS until shortly after:

        "The worst collection of accidents and serious incidents in aviation history.”

These took place at Oslo Gardermoen Airport on December 14th 1998 (click here #37)
The Norwegean Aircraft Accident Investigation Board (HSLB) never published either a temporary or a full report on the events of December 14th. This despite the fact that the event involved 5 different airlines and resulted in 20 damaged or destroyed engines and 5 emergency landings. But in the internal investigations of SAS and Braathens, two of the airlines involved, it was revealed that no run-ups were performed even though this was standing procedure for all 5 airlines. The revelation came with the use of Flight Data Monitoring (FDM).
SAS temporarily changed their procedures but only from run-ups being required at temperatures below +2 degrees C. to below +3 degrees C. The temporary procedures have been valid for six years. Until recently!
On April 9th 2005 a letter from the Danish autorities (SLV) confirmed that SAS never went back to the ideal procedure:

          Captains shall allways do run-ups, when engine anti-ice system is on!

SAS simply made the temporary procedure permanent, and SLV closed the issue for further debate, considering this  a waste of money. (click here to read the latest SAS procedure)

SAS and its Pilots Unions, has refused to promote using Flight Data Monitoring to check that run-ups are done since 1992, because it violates the captain’s privacy (Click here to read the view of the Danish Pilots Union on Flight Data Monitoring). Also some say that it is difficult to know, if, on any particular day, weather required run-ups. This is in fact a valid objection. However, my responce is: ”Select three days each winter, when the weather in all of Scandinavia, unquestionably required engine run-ups and use these to spot-check procedure compliance.

Oluf Husted

13. april 2005