The heroin was placed within panelling in the toilets of the aircraft, Customs spokesman Qamar Thalho was quoted as saying by The Express Tribune.
The narcotics, wrapped in plastic bags, were found by the Drug Enforcement Cell on Wednesday.
"This has happened a hundred times. About 20 years ago, it used to be gold that we used to find in the toilets of aircraft, up to 140,000 grams of gold at a time, and now we see that it's mostly heroin being smuggled in the toilet compartments," Thalho said.
Sometimes, the drugs were hidden underneath the commode, he said.
"No third party has access to the plane in the hangars or on the runway, which means that the heroin could only be put in by a staff member," he said.
"People who work in maintenance and repairs are the ones with the easiest access and in the past, we've seen even goods being brought in through catering trays," Thalho said.
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A case was registered under the Control of Narcotics Substances Act by the Investigation and Prosecution branch of Customs.
"First we will determine which crew was working in the hangar at that time and identify others who had access to the plane.
"After the suspects are interrogated, we can confirm who was involved... The culprit is most probably an insider," Thalho said.